Car Won't Idle Sometimes

My car just started doing this a few days ago.

When I let it sit for a while, like 12+ hours, when I crank it again, it will not idle on its own. It's almost as if the fast idle isn't activating (not sure on the technical term there). It runs fine as long as I keep my foot on the gas pedal. After about a minute or so of warming up, it runs perfectly fine, like absolutely nothing is wrong. I can cut it off, and come back to it within several hours, and it cranks up and idles fine.

I checked all the stuff under the hood, and everything looks fine. It is actually pulling a little better vacuum than usual, at about 21-22 inches versus 20-21. It still runs great at WOT.

I thought maybe it was because I have the idle set at 700 in my programmer (exhaust sounds better to me), so I moved it up to 900, but that didn't change anything other than the speed it idles at after it gets through the little hiccup.

Could it be the cold weather? It's been in the 20s here the last few nights. I wouldn't think it would be because I've never had it do this before, and it's definitely been in colder weather with no problem. Plus I didn't think fuel injected motors had problems in the cold like old carb motors do (that's exactly what it's acting like, an old carb motor on a cold morning).

Any ideas? It's not causing any problems, but is kind of annoying and I'm afraid it could be a sign of something else wrong.

wmburns said:

IMO, the fact that it activates the fast idle is an important clue.

Assuming there are NO VACUUM leaks, my vote is a bad TPS, or missing TB return spring, or loose TB linkage.

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But it isn't activating the fast idle when it's giving these problems. It will only activate it after the car has been warmed up.

I looked over everything vacuum related, and the gauge is showing a very good vacuum, so I very seriously doubt it has a vacuum leak.

I didn't look super closely at all the TB stuff, but I don't think anything is wrong with any of it. The TPS was replaced when I put on the supercharger (~8 months ago), so I don't think it is bad either. Why would any of that stuff only malfunction when the car has been sitting for a long time? Why wouldn't it malfunction all the time?

trinity_gt said:

Assuming your programmer also functions as a scanner, what do the coolant temperature and intake air temperature sensors read before your actually start it cold? Turn the ignition on and take a reading before starting the engine...

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I can and will do that tomorrow morning after my exam. Where you going with this?

2002BLGT said:

Maybe the IAC is sticking from the cold weather , do you have a wideband on it ? whats it reading when you do get it cranked when its dead cold ? maybe the MAF transfer needs a little tweak in the lower extremes

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I don't have a wideband, but I'm thinking about getting one. I thought about the tune, but can't really imagine it being it considered it hasn't done this until now.

The IAC is a possibility. It isn't the one that was on the car, but rather the one on the supercharger when I bought it.

I'm probably not going to change anything until I get to try cranking it after it's warmed up some outside. IMO, we need to decide if it's cold weather related, and that'll narrow the possibilities down quite a bit (I assume). It's supposed to be back in the 50s this weekend, and it hasn't given me trouble in that temperature range so far, so I'll hopefully get that tested out soon.

trinity_gt said:

If the PCM is not getting the correct info on the air or engine (coolant) temperature, it may not be fueling correctly when cold, before the O2 sensors start providing good feedback. A rough analogy: an old carbureted car with a choke that doesn't close as required on a cold day.

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I gotcha. Makes sense.

I datalogged before and during a cold start this morning; the IAT before start was 28 (about how cold it is outside) and the coolant temp was also 28. After I cranked it (had to use the accelerator to make it idle), the IAC warmed up to 30, and the coolant temp slowly rose up to around 70 before I cut it off. So to me, it seems that those things are fine.

wmburns said:

If the TB idle set screw is set too high, the IAC will not be able to slow the RPM's down. It may have become a problem when the air temperature dropped (cold air more dense).

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Just to be clear, when I said I adjusted the idle speed, it was with my SCT X3 programmer, not the screw on the throttle body.

wmburns said:

If you disconnect the IAC electrical connector what does the RPM's do?

The motor must idle very slowly or die outright when the IAC electrical connector is disconnected. Otherwise the IAC will not be able to correctly control RPM's. Remember, the IAC is a bypass device. It works be ADDing/bypassing air around the TB. Once the IAC's duty cycle falls to zero, it can't subtract air and the adjustment fails.

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I am now almost positive that the IAC is the culprit. I'll explain why.

When I got the car warm enough to where it would idle on its own, it was idling at about 650 rpm. As about a minute of time passed, the idle speed slowly increased up to about 900 rpm, which is where I have it set in the programmer. Now I don't fully understand how the IAC works, but that seems like it was stuck closed (as someone suggested) and slowly worked itself open as the engine warmed up. Seems like a logical explanation to me. But it gets better.

When I unplugged the connector from the IAC, the car shut right off, just as you said it should. But also, the IAC has about a quarter of an inch of rotational lost motion. I don't know if it's supposed to be that way or not, but if I wiggle the IAC a bit, the car shuts off immediately.

This sure seems like a bad IAC to me. What do you guys think?

I still have my old IAC (from before the blower) at home. I'll swap it on when I get home next week and see what happens.

wmburns said:

The loose IAC body is my vote. Think about it. The PCM can re-learn virtually any idle trim value within the IAC's duty cycle (0-100%). However, if the IAC is loose, this means that when the PCM calls for a given duty cycle, this could mean different amount of air. In a sense, a moving target.

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trinity_gt said:

Mine too, but for a different reason: This latest news makes it sound as if there's an intermittent electrical connection within the IAC itself. The OPer said "but if I wiggle the IAC a bit, the car shuts off immediately"; either there's a short internal to the IAC or the connection is intermittent: When the connection is lost, the IAC simply snaps shut and the engine stalls.

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Could be both. Either way, I'll know if the IAC is the problem by this time next week. In the meantime, I'll deal with it.

The IAC valve was the culprit. Went to the car this morning, tried cranking it; it would crank and cut right off; give it a little gas and it ran fine, but let off the gas and it cut off immediately. Swapped on my old IAC, turned the key and it cranked up and went into the fast idle just like it's supposed to. Runs like a charm.