rhodejeep

2006 GC Laredo, 3.7L, 175k miles. was running fine until last week when this started

---ONLY WHEN COLD---
After sitting for a long time (e.g. overnight), I am getting very poor idling with misfires when placed into gear with a P0300 code. It seems like it is fine for the first 20-30 seconds or so, then when the idle settles down from ~900-1000rpm to its normal ~700 rpm, the missing starts. onced warmed up, this issue disappears.

----ONCED WARMED UP---
Really drives pretty well although I occasionally get a flat spot just off idle when starting from a full stop. Give it a bit more gas and everything is fine.

---WHAT I HAVE DONE----
-replaced crank sensor (albeit a non-MOPAR aftermarket part) (this seemed to help but not solve the issue), don't have the original part to put back in and confirm any improvement
-tested TPS (was fine) but replaced it anyway (no change)
-replaced cam position sensor (no change)
-changed ASD relay (no change)
-new plugs (no change)
-cleaned out IAC (no change)
-checked & cleaned PCV valve (no change)
-checked for vacuum leaks (none found)

I am at a loss for what to try next. as you can probably tell I am throwing parts at it at this point. A few other interesting things:
-when 'taking off' without letting the engine warm, the engine can bog and you get VERY poor accel from a stop. pulling over, turning off the jeep, restarting fixes this issue. Makes me think it's electrical.

I was thinking of swapping out O2 sensors next, but cannot reach the plugs on the upstream sensors...how the heck are you supposed to get your hand up there?? I would think that they would throw their own code if they were bad but??? Help please!

rhodejeep

well I guess I picked the wrong forum LOL.

anyway, I am posting this update just in case someone is searching someday with the same symptoms.

I ended up replacing the crank sensor with a genuine MOPAR part to the tune of $46.50....exact same part as what came out...a BWD 987...literally the same part that you can get at O'Reilly's, carquest, etc for $20 (didn't realize it was exactly the same until I got the old one out). Obviously no change. Another note about replacing the crank sensor is that is was REALLY stuck in there...I sprayed w/PB and twisted back and forth and pried with a long screwdriver to get it out. To install the new one, I put a gradual chamfer on the leading edge of the plastic shoulder so I could get it started, installed the bolt loosely so it didn't rotate, and then tapped it home.

THE FIX:
replaced the upstream O2 sensors, all is well. could not get to the plugs so the wires were cut at the sensors and new connectors installed.

LAST WORDS
the symptoms I describe (crappy idle when cold/multiple cylinder misfire P0300 code) seems to be quite common w/these vehicles. and the challenge is that there seems to be many many causes of these common symptoms making diagnosis difficult. I would say after wrestling w/this one for a week, just keep at it and unfortunately I don't see how you can find the proper fix without throwing parts at it to some extent. one other way to look at it is that if you just drive the jeep with the issue present, maybe whatever is causing the problem will eventually get worse and throw its own code...

93_xjcherokee

Glad to hear you got it fixed. Glad we could help

rhodejeep

oh I see, unlike most forums, you guys post after the issue is solved. novel!