Stalls at idle & low RPMS until engine warms up

[Babbles]

I have a 2002 Pontiac Grand Prix GT (3.8L naturally aspirated engine).

When the engine is cold the engine will stall out unless I give it a bit of gas. Also at while driving at low RPMs it will start to stall. After the engine warms up then it runs fine.

I check the MAF sensor and it seems fine; I blew some canned air over the resistor but it doesn't look like there was a lot of build-up on it.
I took off the EGR valve and shot some CRC Throttle Body cleaner into the opening. There really isn't much in there for me to scrub, and it did not appear to have any physical buildup of junk in there. Also the little spring piston thing (not sure what it is called) seems to move relatively easily.
I took off the Idle Air Control valve and I think there could be a problem with this. When I first took off the IAC the little plunger looking guy shot off from the spring, which didn't seem normal. I sprayed some throttle body cleaner in and around that part (the plunger was covered in carbon). I screwed this plunger part back into the IAC valve body and it really doesn't move up or down. Should this part freely move up and down by my finger, or should it have a lot of resistance?

If anybody has any advice on something else I can check please let me know.

 

[exdeath]

ECT sensor.

As for IAC valve, it's hard to say as there are numerous designs, anything from a simple electromagnetic shutter to worm screw looking things. Consult a service manual for your engine and follow the diagnostic procedures to confirm proper operation. Usually you just apply 12 volts in both directions and see if it opens or closes appropriately.

 

[boomerang]

It's been some time since I've had an IAC out and in my hand, but the plunger not being physically connected to the shaft does not sound right. I don't see how it can retract if it's not physically connected, as the spring would keep it always extended. In other words, how can it control the idle speed if it's in the out position all the time?

As far as screwing the plunger back in goes, that sounds correct. I believe that's the proper procedure for the situation you're in. You don't want the shaft and plunger bottoming out when you reattach the IAC itself. Yes, it should have resistance. It should move through the action of the stepper motor, not of it's own accord.

After starting the IAC will cycle and find it's happy spot based on the RPM and the signal it receives from the computer.

I'd go down to the local auto parts store and look at one there. See if the plunger is in fact connected to the shaft.

I'm sorry, I wish I could remember all this with exact clarity. It's been too long.

I agree with exdeath in that the first thing that came to my mind is the coolant temp sensor. I'm just elaborating on the IAC situation.

Looks like this? The plunger appears to be attached.

 

[Babbles]

boomerang - that is the exact same IAC that I have. The plunger part has a shaft of a couple of inches with the bottom inch or so being threaded. When I removed the part I assume it became unscrewed but was still set into the body of the unit. When I removed it, the plunger just shot out due to the spring - which by the way was a crazy pain in the ass to find that little freaking part in the engine bay. Anyhow, I screwed it back into the IAC body but I noticed it did easily move up or down, but as noted that could be resistance of the stepper motor. I was thinking about going ahead and replacing the IAC - I can get it for ~$30 - because there could be something physically broke with it which caused it to become unscrewed.

exdeath - I read a tip online about unscrewing the IAC and removing it from the throttle body but keeping it wired-up. Then turn the ignition to on to see if the IAC actuates with power. I haven't done that, but I may give it a try.

I'll look into the ECT sensor, but I'm not exactly sure where it is in my car and naturally I don't own a Chilton's manual.

 

[SparkyJJO]

The broken IAC could do that to you. Mine's the same design (I think all the GM 3.8L series II/III engines use the same IAC style).

Google is your friend on finding stuff like the ECT sensor... to be honest I've never heard of it

 

[boomerang]

Originally posted by: SparkyJJO
The broken IAC could do that to you. Mine's the same design (I think all the GM 3.8L series II/III engines use the same IAC style).

Google is your friend on finding stuff like the ECT sensor... to be honest I've never heard of it

ECT is a Ford term I believe. Engine Coolant Temperature (Sensor).

 

[boomerang]

Originally posted by: Babbles
boomerang - that is the exact same IAC that I have. The plunger part has a shaft of a couple of inches with the bottom inch or so being threaded. When I removed the part I assume it became unscrewed but was still set into the body of the unit. When I removed it, the plunger just shot out due to the spring - which by the way was a crazy pain in the ass to find that little freaking part in the engine bay. Anyhow, I screwed it back into the IAC body but I noticed it did easily move up or down, but as noted that could be resistance of the stepper motor. I was thinking about going ahead and replacing the IAC - I can get it for ~$30 - because there could be something physically broke with it which caused it to become unscrewed.

exdeath - I read a tip online about unscrewing the IAC and removing it from the throttle body but keeping it wired-up. Then turn the ignition to on to see if the IAC actuates with power. I haven't done that, but I may give it a try.

I'll look into the ECT sensor, but I'm not exactly sure where it is in my car and naturally I don't own a Chilton's manual.

I know my post was a little wishy-washy earlier, but I think the IAC is the problem, although the idle stabilizing after warm up has me a little confused. I hate to just throw parts at a problem, but based on what you've said so far, I'd replace the IAC first. The pintle should be an integral part of the IAC. It should be designed to not be removable.

With an OBD1 car, you could jumper the connector under the dash and with the key on, the IAC would cycle. You're going to need a scan tool or you're going to have to rig something up to check out yours. For ~$30, I'd just buy one.

Be sure to post back your results. I'm always interested to hear how someone makes out.

 

[Babbles]

My car has a OBD II computer, and I did get the codes read with a loaner from Advance Auto Parts:
P0102: MAF, or VAF circuit low input
P0171: System too lean
P0440: Evaporative Emission Control System Malfunction

boomerang - about the idle stabilizing after the engine is warm, I *think* what could be going on is there is grease, oil, or other random gunk that is in a valve or otherwise in the system somewhere. When the engine is cold it is somewhat solid, but after the engine warms up it liquefies and allows the parts or air to move freely. I figured it had to be the IAC or EGR valve since it only happens at idle and not when the engine is throttled.

Anyhow . . .
This morning the engine started up and it was able to idle normally without me giving it some gas to prevent it from stalling. So I may have fixed it - what the exact problem was, I don't know in that I yet again took off the IAC, EGR, and MAF sensor and gave each another quick clean. So I may have fixed it. However with all of that being said I think I may need to still replace the IAC, it could just be a temporary fix with the pintle working for now and may screw up on me at some point.

The bitch about troubleshooting this is that it only happens when the engine has had a chance to cool down and not run for a while, basically over-night. So I could not, and really still can not, tell if I fixed it until I wait a day and see how it starts up after sitting for a long while.

 

[exdeath]

Just for kicks check the rubber fuel line running from the charcoal canister/purge valve to the intake manifold and make sure you don't have a vacuum leak there. In fact, check everything related to the canister (the evap system, including your gas cap).

A simple thing like the evap hose going into the manifold being disconnected would cause all three codes simultaneously as well as cause your cold start problem (vacuum leak cancels out cold start enrichment).

A quick fix would be to disconnect the evap hose from the intake and plug the port on the manifold and clear your codes. If the MAF and lean codes stay away but the EVAP comes back (which it will, being disconnected) you know you have a leak somewhere in your evap system.

 

[Babbles]

Yeah, that's a good idea. I'll check out the hoses and see if that could be an issue.

Overall the car has been very good to me, but it's now pushing 8 years old and I hve ~120,000 miles on it so there have been a few odds and ends popping up here and there that need to be taken care of.