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Hi Everyone,

Newbie to the forum here, I have a 2004 Impala LS with the 3800 Series II. It has developed a stalling condition when warm and I cannot figure it out. I need some help.

So, in a nutshell, it starts perfectly when cold, idles fine, no misses, etc. It will continue to run perfectly until it warms up, typically somewhere between the quarter to half way mark on the temperature guage and then it will quit and will not restart until left for a while to cool. The other weird thing that happens before it stalls is when reving the engine the tach will not follow correctly. It bounces up and down quite erratically.

So, here's what I have done so far in troubleshooting....replaced crankshaft sensor, replaced MAF sensor, removed ICM, cleaned all connections/grounds and re-installed. Scanned for codes (engine light has been on for a long time) and only got P0440 which I am positive is not related to this issue at all. I have seen this code on many GM vehicles before and had no issues with it, especially stalling.

So, in my mind, all that is left to cause this issue is ICM failure due to heat, cam position sensor, or damaged/melted wires to either the camshaft position sensor or crankshaft position sensor. I looked a bit at the wiring and didn't see any reason to dig into it deeper as there was no obvious signs of melting/damage. I have not ruled it out completely though.

A mechanic friend said it's most likely the ICM failing when warm. Could this be the case based on what is going on?

Also, I can't seem to find a definite answer to this, the tach is controlled by the PCM but is there one specific sensor that the PCM calculates the tach/engine speed from or is it a combination of things? I am very sure that the tach going erratic is the key to telling me what is wrong but I don't know how that signal is generated.
 

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warm stalling, hard starting when warm

I have seen this problem before. Believe it or not, usually when a series II acts like what you are describing, it is a dirty throttle body that is at fault. Without any scan readings, it is hard to say for sure that is the problem but it is easy to check. Just remove the air inlet from the t/body and look for carbon build up on the throttle plate and throttle body housing. It's easy to clean with a toothbrush and carburetor cleaner. If it was my car, I would clean the t/body first because it is highly likely to be the only problem, and it doesn't cost much to clean if you do it yourself. :)
 

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Wow, really? I'll give it a try. I am at the point I'll try anything cause I need to get it figured out and running sooner than later. It's been an ongoing problem I have been trying to figure out for quite a while. Luckily it's not the daily driver however it's about to become it as soon as I fix this. Our other vehicle is on it's last legs I think.

I never would have thought something like that would cause it to stall when warm. Anyways, I'll give it a shot.

Thanks for the reply.
 

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I am having a similar problem with my 2002 Buick Lesabre. It happened a few months ago, but is starting to do it almost daily now, in the extreme cold. I start the car and it idles fine. I let it warm up for a few minutes and it will slowly start to run rough until it is completely chugging and eventually shuts off and smells of raw gas. If I try to restart immediately it will NOT restart. If I hit the gas it will continue to run poorly at a higher rpm. If I let it sit 5-10 mins, it will restart and run like a top. It only seems to do this on a cold start up while letting it idle for awhile. So, the first few times I got no codes, but then it popped up a P0101 and P0108(MAF and MAP sensors). The MAP is pretty new and I removed it and it wasn't gunked up. The MAF is probably original (236,000 miles). I have a suspicion it is neither of these items and might be the idle control valve not opening and closing properly. Has anyone else had or heard of this problem? I might just buy a cheap idle control valve off ebay and swap it out. Does anyone know what size torx screws hold that on?
 

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The symptoms of the erratic rpm's when warm and revving prior to dying is
indicative of a possible breakdown of an electronic sensor or module. Definitely worth changing out the ICM before the cam sensor.
I have experienced this in the old days with the ignition module on GM's. Re-start would happen only when the module would cool down.

An experienced engineer on another site for 3800's strongly suggested to replace the ICM and the Crank sensor after 100K miles for reliability. His reasoning is that if they failed when you are on a desolate road....no fun

I am concerned that my Cranks Sensor is at 110K miles. My engine bay is cooler than stock, and my engine temps do not go much above 180...best of luck to you

this may help

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Discussion Starter #6
My thoughts exactly the more I think about it. It has to be something electrical/electronic just based on the fact that it's heat related. I think I'm convinced that changing the ICM is definitely the next move for me. It's very frustrating to have it start right up and purr like a kitten until it starts to warm up and then begin to spit and sputter and quit and won't restart until it cools down a bit.

The car has 164,000 KMS on it so it's just over the 100,000 mile mark. Guess the engineer you refer to is pretty much right on the money. I had never heard of that before as preventative maintenance. Thanks for the heads up.
 

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The first thought that I had when I read your post was the IAT sensor was not responding to your car warming up causing it to run rich as it warms.
Good Luck!
 

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Update on my issue:

I removed the idle air control valve last night and the thing was dirtier than hell. The o-ring was pretty much shot as I couldn't even get the IAC to go back into the slot because of the elongated o-ring. I cleaned the IAC thinking I would put it back in and test it but decided just to get a new one in the morning since I needed a new o-ring anyways and two new bolts as one broke off coming out and the other would likely break off going back in. I went to NAPA and got a new IAC for $30 and a few new bolts and washers. One thing I noticed right away was the pintle on the new IAC and old IAC were in much different positions. The new pintle was much closer to the IAC housing and the old one was much further out. Not sure if that makes a difference, but I am pretty sure the old pintle was getting stuck open at some point considering how dirty black it was and letting too much air in the engine at operating temperature. I started the car and immediately it chuggged and hiccuped for a bit, but them after about 30 seconds to a minute or so, it straightened out and idled perfect. I am guessing it needed to relearn the correct settings. Hopefully this bit of info can help someone in the future with similar problems.
 

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The key to remember is off idle. If you are revving the engine ato say 3K and it is erratic, then the ICM is more suspect.
Starting and checking for idle is quite different. Glad your resolved your issue
 

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Update on my issue:

I removed the idle air control valve last night and the thing was dirtier than hell. The o-ring was pretty much shot as I couldn't even get the IAC to go back into the slot because of the elongated o-ring. I cleaned the IAC thinking I would put it back in and test it but decided just to get a new one in the morning since I needed a new o-ring anyways and two new bolts as one broke off coming out and the other would likely break off going back in. I went to NAPA and got a new IAC for $30 and a few new bolts and washers. One thing I noticed right away was the pintle on the new IAC and old IAC were in much different positions. The new pintle was much closer to the IAC housing and the old one was much further out. Not sure if that makes a difference, but I am pretty sure the old pintle was getting stuck open at some point considering how dirty black it was and letting too much air in the engine at operating temperature. I started the car and immediately it chuggged and hiccuped for a bit, but them after about 30 seconds to a minute or so, it straightened out and idled perfect. I am guessing it needed to relearn the correct settings. Hopefully this bit of info can help someone in the future with similar problems.
Yes, The PCM does have to relearn the IAC position after it is replaced. It is normal to have the plunger closer to the housing when it is new, if it was extended like the one you took out, then it could become damaged during install. That is why we always push the plunger downward before installing the part. Let us know if this problem goes away or if you need more help. There are other things to do to the idle circuit if that doesn't fix it. Good luck with it! lol :)
 

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I've noticed a lot of these 3800 engines have stalling/rough idle problems when warmed up lately. WTH is going on, did GM shut them off via the OnStar system ? really that's what I'm starting to think. it's uncanny.

or they are all getting worn out and losing vacuum at idle, throwing off the injection system monitoring.

another thing I noticed is, if you do a google search for "3800 stalling when warmed up" you will get 100's of hits. but if you follow the threads, almost NONE of them have a solution posted at the end.

what's up with that ? almost nobody follows up on their trouble threads with a final solution post.

My 2001 Monte Carlo is starting to act up like this.
 

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PS- these engines have a large plastic fitting in the back bottom of the intake manifold, facing the firewall rearward, with a large rubber vacuum hose going to the power brake booster.

the plastic fitting is a SNAP IN deal, with 2 large clips to hold it in, sealed by a rubber O-ring on the fitting, where it goes into the manifold

they become loose over time, and can leak vacuum there. but it can be intermittent and hard to pin down. remove the plastic fitting from the manifold, coat the fitting with black permatex, put it back in, and let it dry.

another issue can be, the power brake booster can be leaking internally. I've seen it happen before. unplug the power brake hose and plug the hose, and let it idle and see if it stalls. if it no longer stalls at idle, that's the problem. change the power brake booster.
 

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I've had this problem multiple times.always been the crank sensor for me
 
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