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Old 09-30-2012, 12:58 PM   #1
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Persistant Undiagnosable Miss

My 95 4cyl. 2.7L Tacoma 4x4 188,000mi has a problem that no one can seem to find. When first start the truck the engine runs smooth, then in about two miles it begins to slightly miss approx. every 8 10 seconds and as I continue to drive the problem becomes worse- it will run smooth up to 1800 rpm, then it starts missing, but otherwise in between misses it runs okay. I have replaced the cap, rotor,coil, condenser, and wires two times, and the problem still exists. Brought the truck to a Toyota dealer and they performed a compression test, computer diagnostic test, and fuel pressure check-- all were systems and results showed no abnormalities- the machanic suggested another set of plug wires, which I did replace and even replaced the new NGK plus with Bosch- the plugs I removed and they looked really clean, so its not running rich and even replaced the condenser again, but the problem still persists!! I did just notice that fuel injector 1 and four appear to loose as I can rotate them back and forth. As a test I unplugged each injector one at time and the engine ran with the same consistant roughness each time , but the miss still persisted, also the check engine light only comes on during start up then shuts off. Any suggestions would be greatly appreciated.
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Old 09-30-2012, 09:11 PM   #2
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The fact that your injectors can rotate in not an issue. It is normal for that. The fact that it runs fine cold, but misfires as it warms up suggests that as a most likely cause, an ignition component is failing slightly after it gets hot. However that is not exclusive. Considering all the components you have replaced, you have pretty much covered the secondary ignition system. I would first suggest getting the system checked for codes. As I have mentioned in several other threads, you can have a code in the system that still will not require a CEL to come on. There are several options that could be pursued. One thing you did not mention replacing is the fuel filter. A problem with a filter can cause erratic fuel pressure which can induce a miss.
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Old 09-30-2012, 09:15 PM   #3
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Is there an ignition module in the distributor? J don't know what year they stopped doing that. We had issues exactly like that on a dirt track car one time.
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Old 10-01-2012, 12:10 AM   #4
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Tank Filter Inspection

Okay-- Thank you very much for the advice, I too do not really think the problem lyes within the fuel injectors- Reason: I ran the truck at 95 mph for approximately 5 miles last Monday night and the engine, which continued to miss- even at 3700rpm maintained the speed and that was with some pedel left too, so it does appear to be getting enough fuel under load. The mechanic at the Concord, N.H.Toyota dealer I brought the truck to two weeks ago mentioned that all systems were functioning correctly as that year truck only has an OBD1 diagnostic computer, which he explained does not have the capability of diagnosising individual systems compared to the newer OBD2 and he did mention that each injector has its own filter and that the tank fuel filter is considered a non servicable component. During the last year since I had the fuel tank replaced I have driven approximately 12,000 road miles as this truck does not go off road and I have not experienced any mechanical problems either, that is until this one began rather suddenly one month ago.

When I replaced the condenser I needed to loosen up the transducer in the distributer, it appeared to be set approximately at .001" away from the magnetic flag on the rotor shaft. I decreased the distance to .030 with a feeler guage, but doing that has had no effect on this problem.

Do you think a bad tank of gas or if somebody put suger or sand in the tank would result in this type of problem?
Again thank you for your advice and I will look at the tank filter this week- tank is full at the moment. I will need to remove the tank is that correct?
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Old 10-01-2012, 12:22 AM   #5
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Ignition Transducer Module

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Originally Posted by PAlittlematty View Post
Is there an ignition module in the distributor? J don't know what year they stopped doing that. We had issues exactly like that on a dirt track car one time.
Hi Matt, There only appears to be a transducer of some sort that responds to a magnetic flag on the rotor shaft, as you may read in the post below and I did reduce the distance inwhich the flag bypasses the transducer. interesting you mention that because the original ignition coil saw a high voltage burn hole in the insulation on the side facing the distributor and I am thinking what if radom arcing was occuring at some point damaging the transducer- I did inspect it when I replaced the condensers. what seems confusing is that when the engine is completly cold until maybe two to three minutes or running does the problem begin to occur, then becomes progressively worse. Thank you very much for your interest and reply.
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Old 10-01-2012, 12:32 AM   #6
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Quote:
Originally Posted by BamaToy1997 View Post
The fact that your injectors can rotate in not an issue. It is normal for that. The fact that it runs fine cold, but misfires as it warms up suggests that as a most likely cause, an ignition component is failing slightly after it gets hot. However that is not exclusive. Considering all the components you have replaced, you have pretty much covered the secondary ignition system. I would first suggest getting the system checked for codes. As I have mentioned in several other threads, you can have a code in the system that still will not require a CEL to come on. There are several options that could be pursued. One thing you did not mention replacing is the fuel filter. A problem with a filter can cause erratic fuel pressure which can induce a miss.
Hi Bill, Thanks for you post. I created a separate response to your response below- I am new here and this is my first posting!! I do agree that a fuel filter could potentially be dirty being that I have tried three fuel injector cleaners and even added a half quart of ATF to the tank two weeks ago if the problem was a sticky fuel injector. What do think about the small filters inside the fuel injectors, would the main filter strain out contaminates small enough that would cause a plugged injector filter?
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Old 10-01-2012, 01:31 AM   #7
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Coolant temp sensor?
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Old 10-01-2012, 08:04 AM   #8
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Quote:
Originally Posted by josh. View Post
Hi Bill, Thanks for you post. I created a separate response to your response below- I am new here and this is my first posting!! I do agree that a fuel filter could potentially be dirty being that I have tried three fuel injector cleaners and even added a half quart of ATF to the tank two weeks ago if the problem was a sticky fuel injector. What do think about the small filters inside the fuel injectors, would the main filter strain out contaminates small enough that would cause a plugged injector filter?
To be honest, I don't think you have a clogged filter internal to the injector. Now rather thank just saying that, I will explain why. If the filter were clogged, it is more likely that your problem would be consistent from engine start up, and not just after it has warmed up. A dirty filter is dirty all the time, not just when it gets warm.
Personally I am more concerned with the fact that you put ATF in your fuel system. That is the LAST thing I would ever suggest one of my customers to do. ATF has it's own properties that are not really conducive to a fuel system. Too many people have these back-home remedies that get passed down over the grapevine, and most of them are just plain wrong, or even damaging. If you want to properly attempt to clean your injectors, use a quality fuel injector cleaner that is designed to lubricate and clean your fuel injectors, and try to break up the deposits that may have accumulated on the tips. I recommend STP, but again, that is a personal opinion. Sometimes the simple method is the best.
Again we are still making the assumption that it IS an injector at fault. There are other issues such as battery grounds, poor terminal tension (Both at the battery, and at the individual connectors for the coil, modules, etc) that could be at fault. The best way to eliminate the system that is failed is to do a waveform check of your ignition system. Many shops have the equipment to test this, and usually will not charge an arm and a leg to check it for you.
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Old 10-01-2012, 09:45 AM   #9
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Quote:
Originally Posted by BamaToy1997 View Post
To be honest, I don't think you have a clogged filter internal to the injector. Now rather thank just saying that, I will explain why. If the filter were clogged, it is more likely that your problem would be consistent from engine start up, and not just after it has warmed up. A dirty filter is dirty all the time, not just when it gets warm.
Personally I am more concerned with the fact that you put ATF in your fuel system. That is the LAST thing I would ever suggest one of my customers to do. ATF has it's own properties that are not really conducive to a fuel system. Too many people have these back-home remedies that get passed down over the grapevine, and most of them are just plain wrong, or even damaging. If you want to properly attempt to clean your injectors, use a quality fuel injector cleaner that is designed to lubricate and clean your fuel injectors, and try to break up the deposits that may have accumulated on the tips. I recommend STP, but again, that is a personal opinion. Sometimes the simple method is the best.
Again we are still making the assumption that it IS an injector at fault. There are other issues such as battery grounds, poor terminal tension (Both at the battery, and at the individual connectors for the coil, modules, etc) that could be at fault. The best way to eliminate the system that is failed is to do a waveform check of your ignition system. Many shops have the equipment to test this, and usually will not charge an arm and a leg to check it for you.
Hi matt, Acually it was the tech from Toyota that suggested the ATF, his thinking was that if an injector were sticking and there was carbon build-up within the cylinders (although I never hear a knock on acceleration!) the ATF would help lubricate the injector, and have tried three different fuel system/ fuel injector cleaners; as for the electrical test one of the mechanics I brought the truck to did check the electrical system with an oscillscope- is that same as a wave lenght tester? I did check the body ground, and installed a new battery and alternator within the last 6 months - all connections appear to be solid and tight with no corrosion, also the internal distributor connections/ external connectors I thoughly checked when I replaced the second condensor - all were /are tight; also checked all vacuum lines for breaks and cracks, again all appear to be okay. The way in which the engine misses o rapidly and then recovers its almost like turning the key on and off- I can even hear the gears in the transmission and drive train make a clunking sound as it happens- it losses power quick and then recovers just as quickly- but again the electrical system was "scoped" and everything checked out okay. What do think about a warn valve guide? in that when the temp/ rpms reach a certain level a leak in one cylinder occurs.
What do think about me buying two injectors and replacing two at a time and see what happens each time?
Thank you sticking with me on this, I have a lot of money in my truck- replaced all light bulbs, all fluids, wtaer pump, change the oil every 2000 miles, alternator, starter, battery, new Toyo tires, brakes- rotors/calipers/ wheel bearings/ rear drums and spring kit, fuel tank and now the ignition system!!
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Old 10-01-2012, 01:29 PM   #10
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If the injectors are original, replacing all of them at one time wouldn't be considered wasteful. 17 year old injectors should be suspect until proven to be in good shape. You coud just replace one, and use it as a diagnostic tool, but you have a one-in-four shot at getting it right. Plus, the time to change positions of the injector would be long.
In short, change all of the injectors at one time. You already replaced the ignition components. Best to fix the other half of the equation.
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Old 10-01-2012, 08:43 PM   #11
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Noticed the age of your truck. Might be rust in the fuel tank. As truck is driven, flakes can start to accumulate onto the fuel inlet filter. Once the engine has stopped so too the fuel flow holding the flakes against the filter. If you do pull the tank for a look-see, you might expect to replace the internal fuel pump as it has a rust issue where it connects to the fuel line. My tank was pulled for a weld fix on the chassis. Good luck.
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Old 10-02-2012, 08:17 AM   #12
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Quote:
Originally Posted by josh. View Post
Hi matt, Acually it was the tech from Toyota that suggested the ATF, his thinking was that if an injector were sticking and there was carbon build-up within the cylinders (although I never hear a knock on acceleration!) the ATF would help lubricate the injector, and have tried three different fuel system/ fuel injector cleaners; as for the electrical test one of the mechanics I brought the truck to did check the electrical system with an oscillscope- is that same as a wave lenght tester? I did check the body ground, and installed a new battery and alternator within the last 6 months - all connections appear to be solid and tight with no corrosion, also the internal distributor connections/ external connectors I thoughly checked when I replaced the second condensor - all were /are tight; also checked all vacuum lines for breaks and cracks, again all appear to be okay. The way in which the engine misses o rapidly and then recovers its almost like turning the key on and off- I can even hear the gears in the transmission and drive train make a clunking sound as it happens- it losses power quick and then recovers just as quickly- but again the electrical system was "scoped" and everything checked out okay. What do think about a warn valve guide? in that when the temp/ rpms reach a certain level a leak in one cylinder occurs.
What do think about me buying two injectors and replacing two at a time and see what happens each time?
Thank you sticking with me on this, I have a lot of money in my truck- replaced all light bulbs, all fluids, wtaer pump, change the oil every 2000 miles, alternator, starter, battery, new Toyo tires, brakes- rotors/calipers/ wheel bearings/ rear drums and spring kit, fuel tank and now the ignition system!!
Name is Bill, but hey, it's all good. I think the Toyota tech may have been sniffing too much gasoline fumes. Haha. I won't bash him though. I have worked in this industry for over 20 years, and have never put anything that did not belong in the system. ATF is transmission fluid and belongs in the transmission. While it does burn, it will put unnecessary byproducts into your exhaust and catalytic converter. while the risk is low, there IS a chance that even a small amount of a chemical not designed to be in there could damage a very expensive catalytic converter. Just my professional opinion on this.

As for your truck issues, it sounds like you have done a lot of shotgunning. While that approach does work sometimes, it does get expensive as you have learned. The oscilloscope test will check for waveform. The question is what part of the electrical system did he check? Your problem does look like on that will not be easy to find. I would scope the primary ignition, the secondary ignition, the charging system, and then proceed to check all of your grounds, and look for possible locations of EMI. Do you have any spark plug wires routed near (within 1 inch) of any harness or sensor? EMI can be difficult at times to find, but causes all sorts of possible havoc in an automobile.

I doubt a worn valve guide. A worn valve guide would tend to actually show up more cold than warm because as the parts warm up they microscopically expand, thus reducing the free play.

The way you are describing your condition is not uncommon in many of the vehicles I have repaired. The unfortunate part is that of all of the different failures, many had a different problem. The MOST common problem though has been ground issues. Is your engine ground strap to the body secure, and rust free? I know you have mentioned checking tightness of some of your grounds, but have you actually removed the cable and cleaned the surface? The older vehicles are less prone to small issues like this, but not impossible. As you have shown, you have been chasing this problem for a while now. I will do what I can to help, but obviously without your vehicle in my shop for me to lay hands and eyes on, I can only do so much. Have you personally done a voltage drop check on your battery terminals?
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Old 10-03-2012, 11:04 AM   #13
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Temperature Sensor

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Originally Posted by Quaker View Post
Coolant temp sensor?
Okay I have considered the coolant temp sensor as being the problem as well, but if that was case I think the check engine light would come on and fault code would be generated due to at least one of the two O2 sensors responding to a "rich" exhaust gas, because when the engine is cold at start-up the fuel to air mixture is more concentrated, but as the engine warms up (I think based on a coolant sensor) the mixture leans out as the temperature increases- probably based on electrical resistance, also I am on my third set of plugs over the last 5 weeks; the last set (NGK) I drove approximately 400 miles on- and all four plugs looked really clean with no carbon build-up- and the gas milage is still at about 23 miles to the gallon, so I don't think the air fuel mixture is to rich. Thanks for the suggestion, I guess I could try replacing that as well.
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Old 10-03-2012, 11:34 AM   #14
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Quote:
Originally Posted by BamaToy1997 View Post
Name is Bill, but hey, it's all good. I think the Toyota tech may have been sniffing too much gasoline fumes. Haha. I won't bash him though. I have worked in this industry for over 20 years, and have never put anything that did not belong in the system. ATF is transmission fluid and belongs in the transmission. While it does burn, it will put unnecessary byproducts into your exhaust and catalytic converter. while the risk is low, there IS a chance that even a small amount of a chemical not designed to be in there could damage a very expensive catalytic converter. Just my professional opinion on this.

As for your truck issues, it sounds like you have done a lot of shotgunning. While that approach does work sometimes, it does get expensive as you have learned. The oscilloscope test will check for waveform. The question is what part of the electrical system did he check? Your problem does look like on that will not be easy to find. I would scope the primary ignition, the secondary ignition, the charging system, and then proceed to check all of your grounds, and look for possible locations of EMI. Do you have any spark plug wires routed near (within 1 inch) of any harness or sensor? EMI can be difficult at times to find, but causes all sorts of possible havoc in an automobile.

I doubt a worn valve guide. A worn valve guide would tend to actually show up more cold than warm because as the parts warm up they microscopically expand, thus reducing the free play.

The way you are describing your condition is not uncommon in many of the vehicles I have repaired. The unfortunate part is that of all of the different failures, many had a different problem. The MOST common problem though has been ground issues. Is your engine ground strap to the body secure, and rust free? I know you have mentioned checking tightness of some of your grounds, but have you actually removed the cable and cleaned the surface? The older vehicles are less prone to small issues like this, but not impossible. As you have shown, you have been chasing this problem for a while now. I will do what I can to help, but obviously without your vehicle in my shop for me to lay hands and eyes on, I can only do so much. Have you personally done a voltage drop check on your battery terminals?
Hi Bill, There are two people here I am responding with that is you and and another guy named Matt- must have got my posts mixed up?? Okay- The mechanic I brought my truck to is guy who works on problems other shops in the area are unable to fix- I believe he is a retired electrical engineer. Anyway, he scoped the cam and crank sensors, ignition system and checked the alternator sine wave- I called him yesterday and he told me that all systems he checked looked fine with nice even pattern, even when the engine was missing at the higher RPM.
I just completed a check of the body ground and positive cable voltage drops with as much load on the electrcal system as I can create- headlighs/blower motor on high;the results are as follows:
Varified all connections are tight and clean.
Battery negative terminal to body---------------------- 12.4mv
Battery negative terminal to engine block--------------- 38.4mv
Battery positive terminal to alternator output stud------ 121.6mv

I am not sure if these readings are within and acceptable range though.

Also, Just to help eliminate the possiblity of a faulty fuel injector I purchased two injectors from Injector Warehouse out of S.C. and will replace two at time and see what happens- I will get back to you guys when I put them in this Saturday and let you know the results. Thanks for your help!!
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Old 10-09-2012, 07:25 AM   #15
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Update

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Originally Posted by BamaToy1997 View Post
Name is Bill, but hey, it's all good. I think the Toyota tech may have been sniffing too much gasoline fumes. Haha. I won't bash him though. I have worked in this industry for over 20 years, and have never put anything that did not belong in the system. ATF is transmission fluid and belongs in the transmission. While it does burn, it will put unnecessary byproducts into your exhaust and catalytic converter. while the risk is low, there IS a chance that even a small amount of a chemical not designed to be in there could damage a very expensive catalytic converter. Just my professional opinion on this.

As for your truck issues, it sounds like you have done a lot of shotgunning. While that approach does work sometimes, it does get expensive as you have learned. The oscilloscope test will check for waveform. The question is what part of the electrical system did he check? Your problem does look like on that will not be easy to find. I would scope the primary ignition, the secondary ignition, the charging system, and then proceed to check all of your grounds, and look for possible locations of EMI. Do you have any spark plug wires routed near (within 1 inch) of any harness or sensor? EMI can be difficult at times to find, but causes all sorts of possible havoc in an automobile.

I doubt a worn valve guide. A worn valve guide would tend to actually show up more cold than warm because as the parts warm up they microscopically expand, thus reducing the free play.

The way you are describing your condition is not uncommon in many of the vehicles I have repaired. The unfortunate part is that of all of the different failures, many had a different problem. The MOST common problem though has been ground issues. Is your engine ground strap to the body secure, and rust free? I know you have mentioned checking tightness of some of your grounds, but have you actually removed the cable and cleaned the surface? The older vehicles are less prone to small issues like this, but not impossible. As you have shown, you have been chasing this problem for a while now. I will do what I can to help, but obviously without your vehicle in my shop for me to lay hands and eyes on, I can only do so much. Have you personally done a voltage drop check on your battery terminals?
Hi Bill, Okay so I replaced the fuel injectors two at a time, and yes the problem still occured- I decided to unplug the mass air flow sensor just to see what would happen and the result is that the engine now runs smooth with no miss!! I don't know for sure if the sensor is really defective or by unplugging it just masks another problem, but my question is will it damage the engine if I temporaraly drive the truck with the mass air flow sensor? - the engine starts a little hard but is clearly running rich at lower rpm, but runs perfect at speeds above idle and appeares to a bit sluggish during acceleration.
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Old 10-13-2012, 07:46 AM   #16
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By disconnecting your MAF you have forced your computer to change the method in which it calculates fuel trim. Simple enough. Your computer is now using what is called "Speed density" rather than "Mass air" calculations. It takes input from other sensors, like the O2 sensors, ECT sensor, crank sensor, etc. and calculates what SHOULD be the amount of air going into your engine. This system has been used in several different manufacturers, and there are even a few still put in use today.

That being said, you mentioned that it is "Clearly running rich" at lower engine speeds. you know this from what? Smell? Engine data? I ask because if you richen up a fuel mixture, you can mask a smaller issue. If fuel IS the underlying cause, then a rich mixture would override the "failure" and the engine would run smoother. As mentioned by Quaker in an earlier post, even an off-kilt signal from an ECT can cause issues at idle. Have you been able to look at scan tool data? That would be the biggest help. Look at your fuel trim at idle. It should be pretty dang close to the same amount with the MAF connected, as disconnected (if the computer is getting the right calculations)
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Old 10-14-2012, 08:35 PM   #17
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Quote:
Originally Posted by BamaToy1997 View Post
By disconnecting your MAF you have forced your computer to change the method in which it calculates fuel trim. Simple enough. Your computer is now using what is called "Speed density" rather than "Mass air" calculations. It takes input from other sensors, like the O2 sensors, ECT sensor, crank sensor, etc. and calculates what SHOULD be the amount of air going into your engine. This system has been used in several different manufacturers, and there are even a few still put in use today.

That being said, you mentioned that it is "Clearly running rich" at lower engine speeds. you know this from what? Smell? Engine data? I ask because if you richen up a fuel mixture, you can mask a smaller issue. If fuel IS the underlying cause, then a rich mixture would override the "failure" and the engine would run smoother. As mentioned by Quaker in an earlier post, even an off-kilt signal from an ECT can cause issues at idle. Have you been able to look at scan tool data? That would be the biggest help. Look at your fuel trim at idle. It should be pretty dang close to the same amount with the MAF connected, as disconnected (if the computer is getting the right calculations)
Okay. I agree that by taking the MAF out of the system if the engine is really running too lean, that it would appear to make the engine run smoother; when I describe the engine as running "rich", what it is the smell of the exhaust as I do not see any black smoke, but rather a smell of mixture of burnt and unburnt fuel occuring at idle especially just after start up. I have not looked at the scan data personally, but the tech at the Toyota dealer mentioned that No codes are registering. What is fuel trim at idle?- is there an adjustment that can be made to the fuel pressure or flow? The truck idles really good with no misses.
I ordered a MAF sensor and will be recieving it by this coming Thursday and I will let you know the result- I would not be a bit surprised if that was not the problem, but at least it may eliminate one sensor as being the problem. Thanks for your help!!
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Old 10-15-2012, 06:45 AM   #18
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Fuel trim has two different groups. Long term and Short term. Short term is immediate adjustments to the amount of fuel that is sprayed from the injector to correctly adjust for the response of your air/fuel ratio (engine monitoring O2) sensor. As the computer makes these adjustments it decides if this adjustment is continuous, or temporary. If it is continuous, then it increases the Long term fuel trim until the Short term is again back at 0% (or within 2-3% of 0%). There are two directions fuel trim goes in. + and -. A positive fuel trim number means that the computer is having to add a small amount of extra fuel. (This is done by microscopically increasing the amount of time the fuel injector is held open) A negative fuel trim is just the opposite: The system is having to decrease the amount of fuel the injector sprays.

In a PERFECTLY running engine, you will find your LT and ST fuel trims at or near 0%. The system is within acceptable levels as long as the LT fuel trim is not over 10% in either direction. (Some manufacturers have a lower tolerance of 7% either direction) So you may not have a code, even if your LT fuel trim is +9% due to running lean depending on the tolerance set by the manufacturer. A quick check shows that Toyota, for a 1995 Tacoma, they allow up to +/- 20%, which is pretty big. So this means you could be running as lean as -20% and still not set a code. -20% is actually pretty dang lean, and could cause a lean miss-fire. This is why I suggest you find someone with a scan tool that can read your data stream and get you a print out or something. Without this information you are trying to solve a problem you may never find without replacing every component. Imagine a doctor trying to diagnose a person's stomach ache without actually having a stethescope or a lab for blood-work, or any way of giving you an exam. He's gonna only make a guess from experience. Good luck with the new MAF sensor. Hopefully that is the problem. (It is a common failure part on many vehicles)

As far as your question, on a fuel injected vehicle, fuel trim is computer controlled. You really can't adjust anything. (Some things can be tweaked, but not a recommendation)
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Old 10-17-2012, 10:34 PM   #19
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Latest Update- 18 Oct

Quote:
Originally Posted by BamaToy1997 View Post
Fuel trim has two different groups. Long term and Short term. Short term is immediate adjustments to the amount of fuel that is sprayed from the injector to correctly adjust for the response of your air/fuel ratio (engine monitoring O2) sensor. As the computer makes these adjustments it decides if this adjustment is continuous, or temporary. If it is continuous, then it increases the Long term fuel trim until the Short term is again back at 0% (or within 2-3% of 0%). There are two directions fuel trim goes in. + and -. A positive fuel trim number means that the computer is having to add a small amount of extra fuel. (This is done by microscopically increasing the amount of time the fuel injector is held open) A negative fuel trim is just the opposite: The system is having to decrease the amount of fuel the injector sprays.

In a PERFECTLY running engine, you will find your LT and ST fuel trims at or near 0%. The system is within acceptable levels as long as the LT fuel trim is not over 10% in either direction. (Some manufacturers have a lower tolerance of 7% either direction) So you may not have a code, even if your LT fuel trim is +9% due to running lean depending on the tolerance set by the manufacturer. A quick check shows that Toyota, for a 1995 Tacoma, they allow up to +/- 20%, which is pretty big. So this means you could be running as lean as -20% and still not set a code. -20% is actually pretty dang lean, and could cause a lean miss-fire. This is why I suggest you find someone with a scan tool that can read your data stream and get you a print out or something. Without this information you are trying to solve a problem you may never find without replacing every component. Imagine a doctor trying to diagnose a person's stomach ache without actually having a stethescope or a lab for blood-work, or any way of giving you an exam. He's gonna only make a guess from experience. Good luck with the new MAF sensor. Hopefully that is the problem. (It is a common failure part on many vehicles)

As far as your question, on a fuel injected vehicle, fuel trim is computer controlled. You really can't adjust anything. (Some things can be tweaked, but not a recommendation)
Okay- I received the MAF sensor yesterday- installed it and YES you are correct!! the problem still exists-- dammit!! Soo... I unplugged the upstream and downstream O2 sensors- one at a time- the problem still existed, next I unplugged the coolent temperature sensor (I did this because when the engine is cold it runs perfect- it's just as the engine temperature increases the miss progressively becomes more rapid until it occurs every 8-10 seconds- reguardless of any speed above 1800 RPM), but I did notice that the check engine light did not come on when unplugging these three sensors and I did disconnect the battery for 10 seconds between each test of unplugging sensors.
Thank you for explaining fuel trim; now I better understand how the fuel system functions as a relationship to time and fuel metering as engine demand varies based on ambient temperature and sensor feedback.
I have scheduled and appointment with yet another technician- one which owns a small garage in town who I contacted for the first time last week; this was prior to ordering the MAF sensor and explained to him the events involving this problem- he did state that the computer should go into a default mode if a sensor malfunctions.The Toyota dealer technician did perform a check (OBD 1 analysis) and I remember him mentioning that this is one of the earlier diganositic programs and uses a limited number codes, so he unable to look at each system in detail.
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Old 10-18-2012, 07:05 AM   #20
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Yes, the OBD1 systems have less codes, and less data to look at. I had hoped you would have been lucky, but as suspected, the MAF did not fix the issue. As for the check engine light not coming on when you disconnected different sensors: The older systems had less strictness to the requirements to set a code. Some codes will not set unless the failure is present for 2 consecutive engine starts, and some codes do not test for actual connection of the sensor. Hopefully the new tech you are taking it to will be able to find the problem quickly.
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