2000 4x4 4 door LT with 155K miles, I've done a whole bunch of stuff to. New fuel pump, new MFI spider, Manifold/valve cover/plenum gaskets, plugs, wires, cap and rotor, as well as crazy cleanout of intake/ throttle body, etc.

Problem: When I start it cold, it never initially idles up around 1K. Almost Immediately after starting cold, it idles at around 5-600. If it is cold air temp, (mid 20's F) it will die right away unless I get on the gas to get it idled up for a little bit. Additionally, if I rev it high, like 4-5K, then drop off the gas, it will almost die before recovering to about 600 rpm. No codes. Runs great otherwise. Warm start is better, ramps up to 1K, then drop to 500 very quickly, then levels out at 650.

What do you think? IAC, EGR, MAF, vacuum leak, clogged cat? Not sure the best way to diagnose this one....

 

Process of elimination.
1) I would start with getting some MAF cleaner and clean the MAF real good.
2)With the engine running, spray some brake cleaner around the top of the plenum where the new fuel injection unit rises up out of the top of the manifold, just to make sure its seated in real good (if theres a vacuum leak, rpm's will increase right away).
3) double check all vacuum hoses

I wouldn't think a clogged cat on a cold start up, usually that tends to happen once engine is warmed up and higher rpm's. It's easy to take the EGR off and inspect the inside for carbon build up, clean if necessary.

I would think also maybe the throttle body sensor since its an idle issue, and a long shot would be the coolant temp sensor as it sends a signal to the engine computer as far as letting the computer know the engine is cold.

without a code its a hit and miss.

 

Not too much of a long shot. A symptom of a lack of fast idle when engine is cold can be a failed ECT or circuit issue with the ECT.

However, I'm also looking at the "crazy cleanout" of the throttle body part. Did you remove the IAC? If you did hopefully you didn't push too hard on the IAC pintle. The force required to move the pintle on an IAC that has been in operation can damage the internal threads. GM instructs not to try and move the pintle on an IAC that has been in use for that reason.

The IAC is a 4-wire stepper motor. You can check the resistance on the motor windings by unplugging the IAC (key off) and testing with an ohm meter across each set of pins on the IAC. Pin "A" and Pin "B". Then do the same on pins "C" & "D". You should read between 40-60 ohms. This won't tell you if the gears are stripped out, only if the windings have the correct resistance.

 

Thanks for the ideas. Here are some notes:
#1 MAF is in great shape, I also cleaned it with MAF cleaner.
#2 I did do this when I put it back together to make sure. Good call though.
#3 Thinking I should take my vac gauge and just start checking all of them. They all seem to be intact, I don't hear any hissing, and the standing idle is nice and stable, but you never know.
--Good to know on the CAT, I've never dealt with any before...
--Coolant sensor is good, watched that a bunch on the scan app after doing all the top end work. Could be the throttle position sensor, but it seems to run right through all RPM's other than when it drops down all the way.

Thanks Rockp2, One question, what is the ECT?
To answer yours, yes, I took the IAC out and deep cleaned the passageway and the IAC itself. I don't remember ever pushing on the Pintle at all. I may just put a new one in, they're only like 20 bucks or so for a Dormann. I also took the EGR out and cleaned it and the passage really well, but the gasket looked ok so I reused it. Do you think a stuck EGR valve would give me low idle symtoms?

 

The ECT is the Engine Coolant Temp sensor. Your symptoms could be caused by the IAC pintle not retracting as it should to raise the RPM. EGR should not effect that. It would be best if you had access to a scan tool that could read live data. It would be good to know what the commanded IAC Counts are.

There is an IAC reset that is supposed to be done anytime the IAC is disconnected.. But with your driving it, I would think by now it would have learned itself by now. But you could try it, it's simple. There's actually two different ones listed. Might as well try them both, one at a time:

First version
1. Start the engine for 20 seconds (use a watch).
2. Turn OFF the ignition for 10 seconds.
3. Start the engine
4. Check for proper idle speed. Cold engine it should be ~1,000 rpm, hot engine should be 650.

This would be another good thing to have a scan tool w/live data. So you could compare VCM "Desired RPM" to actual RPM. If you don't have access to a scan tool, there are DLC dongles you could get that would display data on your phone via bluetooth. Search around on the forum and you will see some threads discussing it.

Second version:
1. Turn the ignition switch on for 10 seconds.
2. Turn ignition OFF for 5 seconds.
3. Start the truck and let idle.

 

Very cool. I do have a bluetooth scan tool. I can't remember the app I'm using though. I know I can see live data, but can't record or anything like that. It was just a cheap thing. I will have to look and see if there is a readout for that.

Yeah, it has been quite awhile since I did it, so if it is working properly, I'm guessing it would have relearned by now but I will try this reset tonight anyway since it's quick and easy. Thanks for the tips!

 

Good deal, the PIDS we want to see at idle are IAC Counts, Throttle Position, Desired Throttle Position, RPM, Desired RPM. Record those on a piece of paper both with all accessories of and with accessory loads on (hi-beams, a/c, rear window defroster).

One of the dowsides of just buying a new one is your supposed to adjust the pintle on the new one, based off the old one. If the old pintle is out of correct position, then the new one would be also. Did you turn the pintle or anything like that when you were cleaning it? Another downside is if the IAC isn't the problem, you spent money you didn't need to. Honestly, I wouldn't go with a Dorman on this if you did end up needing one. I'd get the AC Delco which is much more. But that's my personal opinion.

 

I just read back to your first post and noticed you stated there is no momentary flare up when you first start the engine (to paraphrase). So, if when the engine is hot and you start the truck it should just for a moment flare up and then settle down to correct idle. If it is not doing that, I am very much leaning towards an IAC issue. Check your throttle plate also and make sure it is not sticking when you push on it.

It is possible that you damaged it internally when you were cleaning it. Did you do the ohm test yet? Also ohm check pins B & C as well as pins A & D. You should get infinite resistance on those (probably shows as "OL" on your meter). Ohm checks are somewhat limited in diagnosing a bad motor, but it is a quick and simple test.

When you unplug the IAC the tab you push on the harness connector is considered the top. So looking at the plug on the motor visualizing the locking tab as the top, pins are from left to right ABCD.

EDIT: Here is an image of the harness side connector. So it is a MIRROR image of what you will see on the IAC motor itself ("A" would be on the left side on the motor, as opposed to on the right on the harness connector side). I'm pretty sure that as you look on the motor connector it will be kinda upside down. So just insure you note that the locking tab is top and "A" is to the left if you viewed it that way.

 

Yes, there's not really any "flare up" when you start it, maybe 700 at most for a split second, then immediately down to a little above 500. Cold or Hot, doesn't matter.
I didn't get a chance to look at it yesterday, my son didn't get home until later. I should be able to check things tonight. I will have to update you with my findings. I appreciate the help! Throttle plate is not sticking, I cleaned it all out really well and made sure it wasn't because it was before... Had to use some brake cleaner to get a lot of the caked on stuff off, but then I cleaned it with a toothbrush and carb cleaner, then a finish clean with MAF cleaner to make sure I didn't have any deposits. I was careful about cleaning everything, but that doesn't mean the IAC isn't messed up. I'll check it with the meter, then check the scanner for RPMs, then if it checks out, I'll pull it and see what's going on.

 

The PIDS you want to record are; 1)IAC Counts, 2)Desired RPM, 3) RPM, 4)Desired Throttle Position, 5)Throttle Position.

With the ohm meter your looking for 40-60 ohms across pins A&B and across C&D. Then infinite ohms across pins A&D and B&C.

Might as well try those IAC relearns (one at a time) before you do any testing. Be nice if that solves the problem right there...but I'm not betting on it.