5VZ-FE, 3.4L V6, Detailed Post on Performance Problem. Please, PLEASE Offer Informed Input

Hello All...

I am experiencing a performance problem with my '99 5VZ-FE 3.4L V6, Automatic Transmission, 4WD Taco with about 235k miles. I'm hoping to get some guidance/suggestions on rectifying my problem. I am currently at a loss. I suspect catalytic converter, but that's just a suspicion. Don't want to buy another, unless I have some level of confidence that is the problem.

Symptoms:

• Difficult starting. I have to go through about three cycles, before it will start, and then it stumble-starts
  
• Very low power until it warms up, especially at low RPM's. Acceleration improves after full warm up and as RPM's increase, but still not normal. When cold and accelerating from stopped, VERY Sluggish. Dangerously so, if entering traffic. Power during acceleration seems follows a "step pattern". Not smooth.

• Mileage has fallen to about 10 MPG, City. Seems to be stable at that level.

• Idles rough after warmup, stumbling (I've been shifting to "Neutral" at stop-lights, to avoid stalling)

OBD-II Readings:

• Previously had code P0136 (O2 Sensor, Bank #1, Sensor #2)
• Prior to code P0136 and performance issues, was getting flashing CEL and codes P0300 (random misfire) and codes P0302 and P0305 (misfire cyl's 2 & 5), above 70 MPH, but only above 70 MPH, but no noticeable missing or performance issues.
  
• No codes currently being thrown
• Catalyst Test is "Incomplete". Other tests are Green (see screenshot from Torque, below)
• Goes from Open Loop to Closed Loop at what appears to be normal operating

Actions taken to date:

• Swapped coil-pack feeding Cyl's 2 & 5 to another cylinder pair (no change. Misfire remained on Cyl's 2 & 5)
  
• Renewed downstream sensor with new two weeks ago with Denso part number
  $51 234-4162 Oxygen Sensor with 4-Wire 10.5” Harness for Non-California, Non-4WD Lexus/Toyota
  234-4162
• Changed spark plugs last week. (wires renewed with OEM, about four years, 50,000 miles ago) Old plugs looked good. No oil fouling or anything like that.
  
• Cleaned MAF Sensor with MAF sensor cleaner last week.
• Checked for vacuum leaks, using carburetor cleaner. Test was negative for vacuum leaks
  
• Ordered new upstream O2 sensor, Denso part number 234-4162. (not yet received, but OBD-II readings from this sensor appear to be nominal, fluctuating between .02 and .07 Volts) Changing this sensor, because it has over 80k miles on it now, and "just in case" it has some bearing on this issue.

Obviously, I have a blue-tooth enabled OBD-II tool and Torque Pro. Therefore, I can perform any tests/capture any readings possible with those tools. Please offer guidance and/or offer suggestions for the information necessary to resolve my problem.

Please ask any clarifying questions and/or offer input.

My truck was running well before all of this. I need it back again.

Thanks!

 

Can you get data at idle to include the following:
Long term fuel trim (banks 1 and 2)
Sort Term fuel trim
MAF sensor input
AIT sensor input (Also need actual outside air temperature when you take this reading)


Have you tried to disconnect the exhaust at the front of the catalytic converter to see if it will start/idle and better?

 

Thanks for your help, BamaToy! My Tacoma has only Bank 1. No Bank 2. All readings after warm-up.

LTFT = -8.6
STFT Bank 1 = Fluctuates between ~ -1.6 and + 2.3
MAF = 5.8 to 5.9 g/s
Intake Air Temp = 69.8 (climbing, as vehicle further warms up)
Ambient air temp ~ 63 degrees (taken from local weather. No thermometer for this)

I have not done anything with the cat yet, including disconnecting it to see if the truck runs better without it. The truck actually idles OK, unless I am stopped in gear, with the brake on, as at a red light.

 

Your MAF reading seems a little high at idle. Not out of range mind you, but a little on the high end. (It could be this way because of the idle speed for your truck being slightly higher than spec for the engine model) If it is high on the reading, but lower slightly on the actual airflow, then the computer would think you were getting say 5.9 g/s when you really were getting 5.2 g/s. The computer would be injecting fuel based on 5.9 g/s which would be more fuel than it SHOULD be getting at 5.2 g/s. (assuming again that the reading is incorrect, even if so slightly) The result of this would be an engine running slightly rich. And engine running slightly rich would result in a negative long term fuel trim number. (as the computer is trying to take away fuel) Your LTFT numbers are showing a negative fuel trim. (i.e. a rich condition)

Now this is a small amount, and SHOULDN'T result in an engine stalling at idle all by itself. If I saw LTFT numbers of -14 or more, then yes, but -8 should give you some stumble, but not a stall. (Again, this is according to numbers.... anything else that could be going on could cause the engine to stall as a compounded issue) You mentioned that you cleaned the MAF sensor before. I am concerned that perhaps cleaning is not going to help it, IF that is the root cause. So here is a test I want you to try. Find a vacuum line that is small, and attached directly to the intake. You need one that is ported, meaning that at idle there should be a small vacuum draw on the line going to the intake. Now start the engine. Get your gauge/scanner and keep an eye on your LTFT and STFT numbers. Disconnect the vacuum line and watch the numbers. They SHOULD go towards positive, with the LTFT coming closer to zero. If they do not, then your MAF sensor is probably bad.

 

Thanks for the comprehensive reply.

I have a multimeter. Is there a way to test the flow sensor at the pins?

 

Found this

https://www.autozone.com/repairguid...tems/Mass-Air-Flow-Meter/_/P-0996b43f802c5570

and this

https://workshop-manuals.com/toyota...component_information/testing_and_inspection/


Going to change the upstream O2 sensor and do the vacuum test you suggested, as well as the ohm meter test, today. Will upfate with results.

 

Whats messed up, is that the two guides linked above show different locations for the indicated pins for the resistance test

 

Yesterday, I replaced the upstream O2 sensor with a new Denso and tested - sort of - the MAF sensor, as well as cleaning it again with MAF sensor cleaner. ( I hadn't realized before, that there is a sensor inside the tube, in addition to the one that's readily seen) No change in performance, after this change.

I say "sort of" tested the MAF sensor, because while I performed a "static" test with the sensor unplugged, I could not perform an active test with power being supplied to the sensor. (I do not have the pin probes necessary to penetrate the insulation on the wires leading to the sensor)

Given that the temperature of the sensor was unknown (though in retrospect, I may have been able to get this from the OBD-II), I'm not sure if the reading was correct. However, estimating the temp at the sensor as about 90 degrees, the reading of ~ 1.6 Kilo-Ohms seems about right.

As suggested, I warmed the engine and checked the LTFT, which was about -7%. Sadly, I didn't capture a screenshot of this. Upon opening a vacuum port, both the STFT and LTFT did rise, but the LTFT would no pass +1.6%. In the screenshots below, you can see the STFT & LTFT with the vacuum port sealed and then with the port partially open (you can see the changes to the STFT, as I modulate the size of the opening on the ported vacuum hose). Note that in the second screenshot, even though the STFT remained high while the port was open, the LTFT remains flat at +1.6%


Not sure how many data points the ECM needs to adjust the LTFT, but seeing how it quickly increased when I first opened the port, I feel it should have increased beyond +1.6%.

I'm considering renewing the MAF sensor, to eliminate that as a potential source of my problem. The Denso catalogue says the correct Denso part number is 197-6020. I found this one, but I'm a little concerned that it doesn't say "Denso" on the side:

https://www.ebay.com/i/152919560025?chn=ps&dispItem=1

Grateful for any additional input/guidance, BamaToy and / or others!

 

I've ordered the MAF sensor listed above. Should arrive Friday, the 23rd of March. Will share the impact on performance changing that has.

 

In general the system has a specific algo that it uses to decide LTFT adjustments. The STFT is just that, short term adjustments. This is use to make fast and quick changes in AF ratio. If a steady STFT doesn't correct an issue, then the system will make a LTFT adjustment, and continue to attempt balancing out STFT.

For example, in a nominally running engine, LTFT should be at or near 0. STFT will fluctuate between say -1.0 and +1.0. Fuel injectors are firing for say 0.100ms of fuel. Now let's say there is a vacuum leak letting in unmetered air. The first thing that happens is the upstream sensor (AKA AF sensor) will see a lean condition and move STFT to +2.0 (this is an arbitrary amount to help explain how it works.) Now typically the system want's to be able to make small STFT adjustments, keeping the numbers low, as high STFT adjustments cause erratic engine operation. So STFT is pushing +5, and injectors are fluctuating between standard 0.100ms and 0.150ms. Since this fluctuation is bad, LTFT gets adjusted and all injectors in that bank get a new baseline of 0.120ms. STFT will attempt to return to -1.0 and +1.0 to maintain the proper fuel to air ratio. The new standard for injector operation is now at a steady 0.120ms and minute adjustments of STFT can balance the system and the engine operation is steady, emissions are corrected, etc.
You can't have STFT corrections making large adjustments since it would cause engine operation to suffer, and emissions to increase.

So what does this all mean? It means that your system is reacting to the change (i.e. your removal of vacuum line to induce a lean condition) and is reacting to the lean condition. (The original number of LTFT means a rich condition. A negative number meaning that it is trying to remove fuel from the injector flow. Now a rich condition can be one of several things. Dirty injectors, inaccurate MAF sensor, inaccurate IAT sensor, high fuel pressure.... There are several possibilities here. The one thing that is clear, is that the system IS correctly responding to a change, so it is MOST likely that the AF sensor is working properly. I would seriously lean towards a bad MAF sensor at this point, but without knowing the actual fuel pressure, I can't rule that out.

 

Thank you Bama. I'm gonna read all this this evening, but having glanced at it, I do want to point out that my vehicle is non-californa, and I therefore understand this vehicle does not have an A/F sensor, but an upstream O2 sensor. (it's the same Denso part number as the downstream sensor, on my vehicle).

Not sure that changes anything.

 

Hello Bama and Others,

I received the new MAF sensor Friday and installed it Saturday. In addition, I removed and cleaned the throttle body and Idle Air Control port. Sadly, no change in performance: all of the symptoms listed at the beginning of this thread are still present. LTFT is still running negative, as illustrated in the this screenshot from Torque:



Can anyone offer any suggestions/guidance?

Thanks!

 

Have you checked cam timing and the timing belt?

 

Thanks for the suggestion. The symptoms don't suggest that, based on my experience. The truck runs better, than if it had skipped a cog or two.

 

Doesn't hurt to check and not hard to do.

 

Well, I have to eat my words. I took my truck to the shop, and they discovered that the timing was indeed off. The belt has moved by a lug or two. This explains why no codes are being thrown.

I changed the timing belt and water pump about a year ago, but did not change the tensioner, though I DID change it during the previous belt change about 5 years ago. I can only guess that the tensioner is bad. Not sure there are any other reasons the belt could move.

I've ordered a new belt and tensioner and will change those ASAP.