Periodic Rough Start Clears Up in Closed Loop

I have been having a rough start issue that only seems to happen when the truck sits for a few days. 1997 3.4 V6. Based on research I started to think I had a small head gasket leak issue, but I haven’t done much to confirm this yet. Coolant level seems relatively stable but I do notice it drops an inch or two in the reservoir after a while, but I also have a few known external coolant leaks I need to address. Anyways, I noticed the last few times this rough start happens, if I sit at idle and watch my scan gauge while it’s running rough (almost like it’s missing) it slowly goes away, but IMMEDIATELY clears up when the ECU switches to closed loop. I usually never did this and would give the throttle a blip or two to clear it. Does anyone one know what this key piece of information may mean? Maybe a leaking injector?? Never throws a CEL. Really hoping it’s not a head gasket thing. Planning on doing a compression test and a few other things this Fall. Thanks for any help.

 

Bump

 

Failing coolant temp sensor?

 

thought about this but unsure why it would only happen after truck sits for a long time.

 

I’m a bit perplexed as well. If I understand correctly closed loop (engine warm) allows O2 and MAF tighter control over afr. Could coolant temp sensor give a false warm reading, causing poor running until engine is actually warm? Seems low probability, but I don’t know the typical failure mode of coolant temp sensors.

Why several days? No idea, unless that’s the only time engine gets to ambient temp, like if you otherwise typically drive on many shortly spaced trips when the truck is used (you could rule this out).

Look at it this way, coolant temp sensor is easier and cheaper to replace than a head gasket. Any other signs of head gasket failure?

 

Have you looked at the iac valve to see if it may be sticking?

 

Update: Had the rough start occur today after it sat for about 24 hrs. I immediately got out to see if I could see any smoke in the exhaust indicating burning coolant. Nothing. I did however notice a few drops of liquid coming from the exhaust. I am in Louisiana though and it has been getting colder, so may be condensation. I caught some drips on my hand and on a piece a paper. Looks clear to me but kind of smelled like coolant I think? I let the truck idle and it switched to closed loop but still was slightly rough. I went and took off the radiator cap and fluid shot out so I hurried up and put it back on. Is this normal for there to be this much pressure after truck was running for 2 minutes or so? Made me think possibly combustion gases were building up pressure in the coolant system. Anyways I’m starting to really think head gasket.

Just strange how intermittent it is and how long this has been occurring without getting worse.

What would you guys recommend doing next? This is my DD and I have a pretty far drive to work.

I am moving into a new house Tuesday so I will be very busy moving etc. over the next week. I plan on doing a compression test once I am settled in my house and have all my tools in my garage. Just hope nothing bad happens until then. I do have a backup, but it’s not a vehicle I would like to drive to work every day for an extended period of time.