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[edm84]

So here goes..

About two months ago my 3.2 started to have problems with cold start, at first it was just a bit lazy, but would start on the first go, no EML. Had my scheduled maintenance - among everything replaced all plugs. It got worse and worse - started stalling in idle and would not start at all hot & cold. Read error codes - failed crank sensor = replaced.

Cold start problems still persist and get worse - could not start on the first go, when started it would idle really rough, like misfiring. Read error codes again - 1) cyl-1 misfire, 2) random misfire (not 100% sure if these are not old codes from when crank sensor was dying - they did not show up on another code read), 3) catalyst dying (both banks). Nothing unusual in the live data when warm. We had a theory and replaced 1-3-5 coil pack. No effect.

As it gets colder it's even harder to start, will not start on the first go, on the second will start, when rpm's drop to idle - stalls, on the third it won't stall in idle. Idle VERY rough (500-1000 rpm). For about 10-15 minutes, then everything gradually goes to normal. Drives super smooth. Starts on 1/2 turn when warm. No error codes (except for catalyst), no EML.

Would love to hear some ideas before driving off and spending a ton of $$$ in dealership (due to local specifics they have the equipment to test everything, but lack thinking out of box for such situations).

[Kevin Wood]

Coolant temperature sensor is one possibility BUT when the crank sensor went was it replaced with a genuine one?

[twiglet]

Are the codes you refer to as 'Catalysts dying' 0420 and 0430.  If so, don't worry about them.  It's just the pre-cats starting to fail, but will have no effect on engine performance at all. 

Did you replace the crank sensor with a genuine GM one?

[albitz]

Even genuine sensors have been known to fail quickly.I had one that was faulty when new,and Im pretty sure others have had the same,so not always easy to tell ime.

[symes]

when changing crank sensor you might have pulled plug lead or connector on dispack?

[edm84]

Are the codes you refer to as 'Catalysts dying' 0420 and 0430.  If so, don't worry about them.  It's just the pre-cats starting to fail, but will have no effect on engine performance at all. 

Did you replace the crank sensor with a genuine GM one?

Yes, those are the codes I have now. Not worried about that much, though lit EML is annoying.

So you're saying that low quality crank sensor could behave that way - only when cold and no errors? The replacement crank sensor was definitely not bought at a dealer. But then again is there such thing as a genuine GM sensor, aren't they made by Bosch? My mechanics know very well that only premium aftermarket parts are accepted, so it might easily be that same Bosch sensor. Anyhow I will check that.

[edm84]

when changing crank sensor you might have pulled plug lead or connector on dispack?

If you mean that if coil packs are properly connected, then yes they are.

[Kevin Wood]

It would be worth checking the live data to ensure that the coolant temperature is being reported accurately, IMHO.

I'd also check the MAF sensor output while I was at it. It's possible that the MAF is misreading, and when the engine is cold, there's no feedback from the lambda sensors to correct the fuelling.

You could try starting it cold with the MAF unplugged, actually. If it starts better, that might point to a MAF problem.