ROUGH starts on cold starts?

[barrak]

I have an old Cherokee that has a failed check valve (notorious on them). I replaced it once and it lasted 6 months before it went out again. The symptoms are long crank times, and often stumbling for a few seconds once it does fire until pressure builds up completely. After 18 years, I'm used to cycling the ignition 2-3 times on it to get around the rough starts.

My 1985 Bronco II needed a permanent diet of STP fuel additive to prevent rough starting/stalling in the early nineties. Bought used with 80,000 miles, then. Good times... grad student on a tight budget.

 

[A-TEAM]

So a little update, I bought a new mopar IAT sensor to try but my rough starts cleared up again. My educated guess here is a bad tank and/or too “hot” of gas. I usually always run 89 octane gas with 10% ethanol but my station was out so I filled up with 90 octane rec fuel (marine/ethanol free) when this started happening. After I burned off that tank with a bunch of rough starts and started running regular 89 again it seems to start up fine again. Glad it worked itself out, will update again if it happens or if I do anything with the new IAT sensor.

 

[HSKR R/T]

So a little update, I bought a new mopar IAT sensor to try but my rough starts cleared up again. My educated guess here is a bad tank and/or too “hot” of gas. I usually always run 89 octane gas with 10% ethanol but my station was out so I filled up with 90 octane rec fuel (marine/ethanol free) when this started happening. After I burned off that tank with a bunch of rough starts and started running regular 89 again it seems to start up fine again. Glad it worked itself out, will update again if it happens or if I do anything with the new IAT sensor.

Probably something wrong with the fuel itself, as in water in fuel or something, and not the octane rating.

 

[barrak]

Interesting. Maybe if you give it a couple seconds before cranking, it gives the fuel pump some time to prime up the fuel rails. The fuel pressure may be bleeding back into the tank when the truck sits for an extended period of time, hence leaning mixture on cold start. Not sure what mechanism keep the pressure while the trunk is off tho. (check valve?)

Curious to see your findings with this.

I think you're on to something. I do get the occasional rough cold start after sitting overnight, even stalling at times after the first crank. Nothing to do with ambient temperature. Now I'm convinced it only happens when the tank is near empty. 33-gallon tank, short commutes, high-octane diet.