FIXED!.. I think ;) Need help chasing down sudden rough idle!

Subd. Got some pretty wicked idle behavior this morning, kind of shook the truck at a red light.

 

Just throwing it out there , are all connections (to the igniter specifically) solid ?

 

I checked by hand all the connections I could reach, igniter being one of them, to check for solid connection. It checked out.

 

Any chance you checked the IAC when you had it off? You do that by applying 12 volts to a pair of the terminals and watch it move, smoothly that is. When mine went I had cleaned it and the problem remained, in that case though it was a high idle vs. a rough idle.
Do you have an OBDII scanner to view live data? I'd look at the IAT and see what that's reading because you mention it runs better in cooler temps, cold air being denser of course means at any given throttle position there is more air being fed at lower temps. The computer measures this and adjusts as necessary but if the reading is off or low, you could be getting choked air flow. The scanner will allow you to view throttle position(% open) also and some other useful data. The compression test sounds worthwhile also, given the spark plug issue.

 

What are your LTFT and STFT %s?

 

I actually didn't electronically test the IAC at all other than manually checking the valve for freedom of motion. Right off the TB, the valve moved sluggishly because of all the carbon/gunk build up. After the cleaning, it was 100% free and difference in the lack of resistance to movement vs. how it was before the cleaning was totally obvious.

I may try your 12v suggestion. Do I ground the IAC for this test? I do not have a OBDII scanner. I can get the loaner from the parts store I think. Thanks for the help.

 

I am not even going to lie, I have zero knowledge of what the LTFT and STFT percentages are regarding or how to obtain them. I assume with a proper scanner? (which i do not own - only have a cheapy ohm meter). I'm a amateur mechanic for my own needs, but I learn as I go! Thanks for contributing though.

 

i think everyone should have one of these. an elm327 and the torque app for androids (there are similar apps for apple). you can load up quite some relevant OBD2 displays. I have 3 pages worth. I'll try and upload screenshots tomorrow while I get live data from my truck.

essentially your Long Term Fuel Trim and Short Term Fuel Trims tell you how much more or less fuel is being added based on what the O2 sensors report is being burned in the chambers. LTFT displays learned air-to-fuel ratios from months worth of data collection. while STFT displays i believe the live fuel trims.

Stoichiometry, being at a perfect 14.7:1 ratio all the time, is 0% fuel trim. the computer uses +- 3% to that 0% for error control.

vacuum leaks force the o2 sensors to read too lean (too much oxygen/not enough fuel because more air is getting in than is anticipated by the MAF sensor), and thereby tells the computer to add more fuel. which causes a positively displayed fuel trim (instead of -3% <-> +3%, it's more like between +9%<->+15%)

bad MAF can cause a too rich situation by not properly sensing how much air is coming through. making the computer think it's taking more in than it really is. causing it to idle very low and sputter a lot. this is a very lean situation. -15% <-> -9% fuel trims.

the 15 and 9 %s are just rough ideas. it could easily be lower or higher. but the computer will tolerate up to 25% fuel trims before kicking on a CEL.

 

Well that is a very precise and detailed explanation for my question. It definitely interests me to be able to dive deeper into vehicles then I currently do. I will refer back to this post if I get a proper device to get readings. Thank you.

 

I bought an OBD II setup like this when I threw a code. it is a great system to use with your phone and quickly diagnose a host of issues.

It's a good no-brainer investment for any wrench turner.

EDIT: I got a BAFX, heard they were more robust and consistent quality than ELMs.

for Android Phones ONLY - Wireless Bluetooth Diagnostic OBD2 Scanner Car Code Reader and Scan Tool for All 1996 & Newer Vehicles ELM327 Compatible OBDII

http://www.amazon.com/gp/product/B005NLQAHS?psc=1&redirect=true&ref_=od_aui_detailpages02

 

they probably are. but im too cheap to go that route lol.

 

BAFX is a brand, its all the ELM327 interface. I still recommend getting a handheld OBD unit like an Altel or a used Snapon. The bluetooth and Torque apps are perfect for what my friend @undyingvisage58 listed, fuel trims, codes, recorded values in the ECU. The PID refresh rate (the live readings, aka mode $06), will never update as fast as a dedicated unit.

 

That being said, ya'll rely way too much on scanners and OBD codes.

One fault with trying to rely on LT and ST fuel trim. IF this problem just started, it may take the LTFT a few cycles to show a lean condition (with it would show as enriched fuel trim). What you are trying to do is prove the existence of a vacuum leak via fuel trim reporting. That like trying to treat a disease by seeing how medication changes the condition instead of checking for the underlying problem. Not wrong per se, but not the most efficient or accurate way to diagnose.

Why has no one so far instructed the OP to get a good vacuum gauge? Pull up a pictogram of different conditions and related vacuum readings, this will tell you so much more then the LTFT readings on the OBD2 scanner. I guarantee you this can be solved with a vacuum gauge and a multimeter.

Manifold and ported idle vacuum readings can rule out or confirm a working IAC. This is another example of the FSM just complicating things instead of deferring to old school methods.

Also just having the IAC switching with 12V is not an indicator that it is working properly. All the crap getting into the solenoid will eventually cause it to fail to actuate accurately. I described a comparison in ohm values in my no start thread.

https://www.tacomaworld.com/threads...park-and-fuel-fried-ecu.387148/#post-10649301

These are for the 2.7L version, so results may vary. Not having $200 to splurge on a OEM one, i bought a Chinese knockoff from a local importer in Alhambra (East LA) for $50. Its been working like a champ for 2000 miles now.

 

Knowledge is power. I agree with the importance of having the best tools and knowledge to use these tools, and of course I'm very appreciative for everyone's input so far. I do however think the solution is swimming near the surface, I just haven't caught it yet. The trigger for the rough idle seems to have been kinda simple - replaced plug & wires. Caligula you have a valid point. I'm going to study up on performing the vacuum tests and go from there with the other suggestions (at least those that I can actually perform). By the way, I'll have to take a closer look at your garage sale. I'm in Chino and you might have stuff I need!

p.s. one thing that popped into my mind about this, I notice I don't feel the rough idle when the a/c is on, but I can feel it when the a/c is off.

 

That is fairly thorough info there. It's all new to me but I'm sure I can easily get a grip on these readings and more. I didn't know these obdII were so affordable so I for sure will add one to my tool box.

 

Well that's interesting, because AC always kick up idle a bit to deal with the addiitonal load. Good clue.

It sounded like you already checked for vaccuum leaks. But "checking" can be very different from systematically testing for correct pressure.

Old story, years ago in college, next door neighbor girl had a little red Toyota single cab with the 22R. She came over and said "it's running rough, I don't what to do." Popped the hood, looked around, found a vaccuum line that had come off, reattached it, smoothed right out. I was still pretty inexperienced (mechanically) at the time, and that was very informative and rewarding. The line was actually exceptionally short and may have vibrated off, so I told her to got to a part store and ask for a longer piece to avoid this in the future.

 

I agree. A mechanical test is in order.

Main suspects at this time from the info I've gathered:
1. Vacuum leak
2. Something with the TPS
3. Something with the IAC Solenoid/IAC
3. Something associated with the Elec Cnrtl Mod - A/C system actually not causing more strain (idle not rough with A/C on)
4. Something temperature related.

 

bump. So what is the rest of the story? Enquiring minds want to know

 

Don't know Christi, the jury is still out...