Strange Cold-Start Problem
 

So I'm not sure exactly what to call what is happening. I'm not sure if it's a stumble, misfire, or what, so pardon my lack of proper terminology.

Whenever I start my car (2008 Cobalt SS TC, stock tune), and only when it's cold (when it does the cat warmup cycle) it starts perfectly every time, but after the rpms hit ~1300, the car starts to stumble/misfire for about 2-3 seconds and then goes back to normal from there on out. The RPMs drop to about 900-1000 during this time. The car is always switched to on to let the fuel pump prime before starting (I've tried not letting it prime and does the same thing).

This has also happened to me once when I cleared my DTC codes with the engine running. It did it until the scan tool was finished clearing the codes (it was just a no rear-O2 code that I'm waiting to get disabled). I've never had a misfire code or any drivability issues (except shitty gas mileage)

I've tried: Cleaning MAF, Replacing air filter, new spark plugs gapped to proper spec (it was happening before all of this too) My next options are a thing of Techron (some forum said it fixed their similar issue) or a seamfoam. The engine has ~125k on it.

Any ideas on how to fix this? I can provide a video if absolutely necessary. Thanks!

you have a catless downpipe?

Yeah, catless downpipe without a rear O2. Could that have an effect on it?

get a tune

I do have a tune. This problem happens no matter if it's enabled or on stock settings.

It always does this on stock tune, sometimes with an hptune.

So it's normal even on Trifecta and stock? There's no way to stop it from happening?

Don't listen to him. He's constantly giving his two cents and it's usually wrong.

My car has been doing it for years on stock tune with a catless pipe.

Bumping this.
As it gets colder, this is happening worse and worse. I've also been getting pretty bad gas mileage especially when the car isn't 100% up to operating temp (around 205 degrees F)

I always let my coolant temp get up to at least 100 F before going anywhere. Someone please help me out. I think it may have something to do with my fuel injectors as I read another thread about it being the problem and have no idea how to diagnose that in a Direct Injection motor.

I would check your o2 sensor, especially since you say gas milage is getting worse. I'm wondering if it's running too rich. Also I'm sure cleaning your valves will do wonders for idle smoothness if you are at 125,000 and they haven't been touched.

Evap purge solenoid. Mine did it for years. Replaced it a month or so ago and it's been fine since.

It's only $20.

Once I put my catless DP on the car it did the same thing. It is annoying but i just let the car finish the cat warmup cycle then it is fine. One of these days I fix the tune to get rid of it I just haven't had time to mess with it.

I'll definitely be doing a complete valve cleaning next oil-change. Also, O2 is fine after checking it, and my LTFT's are only -2% so it's not that rich.

Changed it about 2 months ago, didn't do anything.

Mine is fine literally 3-5 seconds after the start. If I blip the throttle slightly, it goes away. I have a trifecta switchable tune and the guys from Trifecta said there's nothing they can do about it.


Any other ideas?

Update: I reached out to the previous owner who said he was having issues with it as well. He said he disabled some rear O2 codes because he thought it was the catless downpipe causing them.

I reflashed the trifecta tune and sure enough it has Rear O2 running lean, and rear O2 no input (no idea how both of them are there)

I removed the rear O2 and found that it was absolutely disgusting. It almost looked like it was covered in oily water. I cleaned it off, reset codes, and put it back in and hopefully that fixes it. If not I'll replace that O2 and see if it fixes it.

I also learned my catless downpipe has an angled bung, when I originally was told it had no O2 bung with the O2 disconnected.

EDIT: Just got back from a drive, issue is a lot less violent now. My Long Term Fuel Trims are also up to 0.8% instead of 4.3% which is good.