3800 RPMs drop after cold start, sometimes dies

[TmanP]

I have a 3800 Series 3 that I have had for a few months. This problem surfaced about when the weather got cold. When cold/almost cold (usually) it starts up fine. Then a second later the RPMs dip (to about 500-600). Sometimes the car will save itself and they come right up to ~1000. But once in a while it will die and require a restart. It runs OK otherwise. Any ideas???? Oh, and it doesn't seem to matter if headlights/radio/climate control are on.

 

[Recalculating]

Throttle Position Sensor.

Any stored codes?

 

[TmanP]

I'll check. Would a Throttle position sensor cause a code???

 

[901Memphis]

Any mis fire codes? It's common for the ignition coils to go bad on those, sometimes even the ignition control module if you have random multi misfire.

Pull the codes first. If no codes, ask yourself if a tune-up is in order, plugs, wires mainly.

Could be a lot of things, need more information

 

[Fsharp]

You say it's not related to AC climate control, but that was my first thought that the AC compressor is kicking on and putting a load on the engine.

It could be the IAC (idle air control) actuator is faulty. The fact that it sometimes happens and sometimes the RPMs go right to 1000 makes me think it could be this.

It could also be that the engine doesn't realize it's cold and needs to enrich the air fuel mixture. A faulty ECT (coolant temperature sensor) could do this.

Meh

 

[TmanP]

It's in a 2006 Lacrosse with 140k miles. Wires are original. plugs may be. Not sure. It runs fine warm other than this cold start, which didn't seem to exist until recently. I can check for codes tomorrow.

 

[TmanP]

Oh also, the (automatic) headlights dim, then get brighter after the rpms come up. But it still has the issue with all electronics turned off.

 

[crazyoildude]

try cleaning the throttle body really good with throttle body cleaner and putting a good dose of techron in the gas tank..Also scan for codes on that vehicle sometimes if the air flow sensor is dirty or defective it can do all kinds of things to the rpm and sometimes show no code. If this all checks out a good tune up with new plugs and wires would be next. If the lights dim when the idle goes down i would check the battery if that is over 5 years old change that also just to save your alternator.

 

[Merkava_4]

Originally Posted By: TmanP
It runs fine warm other than this cold start, which didn't seem to exist until recently.


Water temp sensor. The computer doesn't know the engine's cold.

 

[Drew99GT]

It's having trouble "idling"; I'd look at the most likely culprit: IAC valve.

http://www.gmforum.com/normal-maintainence-138/how-clean-iac-intake-air-control-3800-v6-307092/

Coolant temp sensor would cause poor running conditions other than idle.

 

[Lubener]

If you have a VOM, you can check the sensors mentioned without spending money needlessly. A carboned up idle air control assy. can cause issues like yours. I have to do a cleaning every few years for idling issues.

 

[Hokiefyd]

At 140k miles, it probably needs a general tune-up. I'd give it new spark plugs and wires at the minimum -- and also clean the throttle body with some attention paid to the idle air control function. It's different on every vehicle -- sometimes it's a stepper motor that kicks the throttle up, sometimes it's integrated into a DBW throttle body, sometimes it's an extra air passage with a small pintle in it (Chrysler), etc.

You might also consider pulling the battery cable for 20 minutes and then letting the car re-learn how to idle and run when cold.

 

[Trav]

Does it have a CEL? IAC, TPS are probable suspects but a dirty or failing MAF can affect the idle as you describe.
I had one failing on a 3800 Gen III do the same thing and had no CEL or pending codes.

 

[gregk24]

Maybe the coolant temp sensor

 

[HerrStig]

Lazy manifold O2 sensor.

 

[demarpaint]

TPS
IAC
MAF
CTS
ATS

All come to mind, check for codes. No codes I'd probably clean the MAF first.

 

[Lubener]

Originally Posted By: HerrStig
Lazy manifold O2 sensor.


Not likely. He said it happens on a cold start, the engine is still in an open loop at that point. The O2 sensor has no effect until it is heated up and the CTS indicates to the processor that things are warming up. which would take several minutes.

 

[TFB1]

Originally Posted By: HerrStig
Lazy manifold O2 sensor.

New or lazy the O2 sensor does ZERO on cold start, only after 45-60 seconds or so of warm up does the PCM begin to receive data...

I'd be suspect of a dirty IAC, possibly a little gummy, so it's responding slower than PCM expects...

 

[TmanP]

Okay everybody....
Went to Oreillys and had him scan for codes. He saw nothing, also ran some diagnostics. He said my charging voltage was 14.7 Volts at idle. Oh and the battery is a newer AC Delco. I think under 3 years old. Did it once more today. No check engine light, obviously. He thought maybe a fuel filter.

 

[Nyogtha]

I've been helping a friend rehabilitate a poorly maintained 3800 Series III 2005 Buick LaCrosse he bought last year. It was giving these same symptoms. No codes thrown. It still had factory plugs & plug wires at 130K miles. We replaced the plugs & plug wires last month and his problem disappeared. We used Bosch OE Platinum Plugs which had $2 / plug rebate and Bosch plug wires. I figure that's good for the next 100K miles.