6.4 Rough Idle when Cold

[Nrochwal]

Got an interesting issue, when starting the truck when not warmed up, if i let it idle for more than 20 seconds or so it "misses" and goes to a rough idle (noticeably different sound) If i keep driving after the rough idle starts it lopes and misses and runs real rough until it gets to the higher RPM's then flattens out. If i shut it off, open the door then restart problem goes away and runs normal. When its not warmed up I keep the RPMs at about 1000 at stop lights and it wont ever get to the rough idle.

Once the truck reaches normal operating temps it will idle fine for an extended period of time (20 mins or more) without missing. It almost feels like it could be a sensor or something because its like a light switch when it happens and wont go away until the truck is restarted. sometimes the check engine light comes on and the codes have been either P0302 misfire or P0266 misfire. if the check engine light does come on it clears itself after a few drive cycles because i do not let it idle below operating temperature at all anymore.

I've looked around everywhere and cant seem to find any information on this other than possible a fuel pressure rail sensor but those symptoms seemed to be an uneven idle all the time and mine is a significant noticeable switch from smooth to rough when the issue happens. I'm lost and want to avoid a trip to the dealer.

 

[DaveBen]

I don't know if this applies to your engine. My 6.0 would run rough when it was started cold. I did pretty much the same thing as yours without throwing any codes. I was running 15W-40 oil. I changed it to 5W-40 and it runs perfectly all the time. 15W-40 is too thick for Phoenix weather. I know about Aridzona as I went to the UofA in Tucson.

 

[Nrochwal]

appreciate the insight, i will check it out. Out of all the searching I did 1 guy had the same issue and said it went away when he changed the oil, so you may be on to something.