Rough idle once engine is warm

First post, so bear with me. I've been using this site for quite some time and it's been great for most my issues, but I've hit a wall. I have a 2003 2.7L Tacoma that has a pretty low idle (400-500 rpm) once my engine is warm. It slightly shakes the car when it's that low. Feels like it wants to die, but doesn't. Here is what I've done:

1. Ran a manifold vacuum test. Resulted in a vibrating needle at 17-19 in Hg vacuum. Repair manual said check ignition system.
2. Coil packs all perfect
3. Checked spark plugs. Misread proper gap and so I changed it from 0.041 to 0.031. However, this improved my gas mileage and acceleration response time. Why??? I later caught my error and changed the spark plugs back.
4. Cleaned my IAC valve to the best of my ability. Borderline stripped screws makes me not want to have to take that off again.
5. Cleaned MAF sensor.
6. Did a compression test. Results from 1 to 4: 155, 155, 140, 170

Not sure what else to do. Maybe it's a vacuum leak somewhere and I need to replace some hoses? Any suggestions are greatly appreciated.

 

Check your pcv system and clean the throttle body.

 

I agree with Clay, 4-500 is too low and check your PCV valve. If you can make it rattle, it's good. Do you have a check engine light, I'm assuming not. Do you have the ability to put a scanner on it that would show the long term fuel trim (LTFT). If it's reading high (like 15%+) you definitely have a vacuum leak, and likely from PCV valve. Usually a big vacuum leak will max out the LTFT and throw a code eventually though.

Also your MAF could be bad. See if you can swap it out. I don't suspect ignition system with a low idle like you've got.

The 2.7 has an EGR valve, yes? It could be slightly open due to carbon build up, or the vacuum line to it could be pulled off?

After cleaning or changing parts, you HAVE to pull your EFI fuse for a few seconds to reset your factory ECU so it can relearn the new "clean" parameters. If you havent' done that yet, it may solve your problem.

 

Update:

Replaced the PCV valve. When I shook mine, it still rattled, but it looked rough. Oh well, $7.00 part. Pulled the EFI fuse (didn't know that was what you are supposed to do). Idle was still pretty low (picture shows where it idles). Not as low as I put in my first post, but it's still not at 650rpm. Drove it around awhile and the CEL came on. P0420. Guessing that's going to need a cat replacement? I'm at 200,000 miles and I doubt it's ever been looked at. My Short Term Fuel Trim Bank 1 is 0.8%. My Long term is 18%. Does this all add up to a bad cat converter?

 

A high LTFT indicated a lean condition and the ECU is adding 18% fuel at that particular RPM. Rev the the engine to 3k and see what the LTFT does. Vacuum leaks are most aparent at idle. They will disappear at higher RPM (when there's less vacuum) and thus your LTFT will get closer to 0, if it's a vacuum leak.

So, could still be a vacuum leak somewhere, failing MAF, or failing bank 1 sensor 1 o2 sensor. If your truck has been running rich for a while, you would probably throw a po420 anyway, I honestly wouldn't worry about a bad cat yet. It's just telling you the catylist emissions are bad as read by the rear o2, you knew that already cuz your truck is running bad. The rear o2 does nothing (with a small exception which isn't relavent here) to the ECU "tune"

 

have you checked all your vacuum hoses to make sure they were not degraded and/or cracked?

 

Hmm. Check the hose to your fuel pressure regulator. Also, did you pop the TB off and give it a good cleaning? Particularly around where the blade meets the inner cylinder wall.

 

TB has been cleaned recently, same with the IAC valve. I sprayed starter fluid around the hoses. No changes in RPM. Only changes happened when A/C kicked on and off. I've recently replaced the upstream o2 sensor about a month or two ago. Have yet to test if the fuel trim changes when I increase RPM, but when I pulled the code, it said that it was 18% when RPM was at 1900 RPM. Close enough to 2500?

 

tag for now, I occasionally chase high and low RPM swings (3-400 up to 1200) at idle.

 

Quite the pickle. What are your fuel trims at idle, and at 750 rpm? Does the truck run fine otherwise?

 

Update:

Ran the scanner on the truck. I have 18.8% LTFT at idle, and when increased to 2500 rpm, LTFT increases to 24%. STFL at idle was 0.8%, then increased if I reved the engine, and once I let off, it would go to about -6% and within 3 seconds return back to 0.8%. Played around with the scanner for a bit and it said my idle rpm bounced between 680 and 720 rpm pretty rapidly. So it's not a low idle, i guess? Also checked the MAF sensor. There was a direct increase when I reved the engine, which makes sense. Think it could possibly be the TPS? Or fuel injectors? I plan on running fuel injector cleaner through it on the next fill up.

 

Based on your trims it looks like you're getting less air at idle than the truck is used to. Might help to reset the ecu, disconnect the battery for 30 minutes.

Idle should be at 750. Have you checked your air box and intake tract?

And let's not rule out o2 sensor. It's hard to check without an afr gauge. Check your o2 sensor outputs. I'm not sure where they should be at idle but they should smoothly increase with rpms.

 

While checking engine related stuff for my Honda prelude, I saw a video where a technician stated that it was common to have idle issues when there was a air bubble/pocket with the cooling system. It may be just honda related...? There is a air bleed valve on typical honda 4 cylinder engines.

 

Update:

Unplugged the battery and instantly I got a P0120 code. TPS problem. I ordered a new one, going to replace it. However, I erased that code and for whatever reason, it hasn't come back yet. The truck seems to have a mind of its own. Once the code went away, it seemed to drive extremely smooth and acceleration was all there. Going to give it a couple days to see how it all reacts

 

Any update on this one?

 

Why don't people replace ''almost'' stripped screws when they take stuff apart?

I've caught myself doing the same thing so I don't let myself off the hook but I've gotten pretty good over the years FINALLY and changing them out at the first sign of head damage.