Low RPM miss (no codes)

R

rol1964

Member
Joined
Oct 10, 2020
Messages
58
Location
montreal, canada
2004 6 cyl 85000 miles
Initially diagnosed as rough idle. IAC valve was removed and cleaned.
Now, just after start up, when accelerating from stop, get slow engine acceleration as vehicle slowly stumbles up to speed. Eventually (10 - 15 secs) it will accelerate as normal. You can feel it suddenly firing on all cylinders as it stumbles in and out of normal function then after a 1/2 mile or so it’s good to go and no further problems.
Im leaning towards ignition coils….
Thoughts on where else to look?
Thx, Rol
 
There are multiple possibilities. First, you only cleaned the IAC itself, not the orifice it fits into. It's best to feed the cleaner in through the mounted IAC while the engine is running at IDLE (only) rpms so the orifice in the throttle body gets cleaned too.

Exactly what brand and model # spark plugs are installed? There are some supposedly correct spark plugs listed in some spark plug guides that will cause misfires and a bad idle.

Finally, does the engine run just as badly when cold and after it's warmed up?
 
IF this problem occurs after you have driven the Jeep and fully warmed up, then shut down for 15- 30 mins and you restart the engine. The problem could be the fuel injectors overheating from the exhaust manifold heat and dripping fuel into the cylinders (common problem with #3). There is a TSB addressing this problem where installation of insulating sleeves on each fuel injector and an insulation shield between the intake manifold runners will alleviate the problem.

https://www.amazon.com/dp/B072HGFZ6W/?tag=wranglerorg-20
https://www.amazon.com/dp/B015NOKWQY/?tag=wranglerorg-20
 
It had NGK plugs as this began to happen so I went ahead and changed them out. I replaced with Champions as they were readily available. The symptoms have mostly remained the same. A little worse maybe.
The vehicle runs like a champ after I get through this “cold” phase.
I will remove and clean the entire throttle body assembly.
 
Without codes, you'll need to do some systematic testing. Have you checked for vacuum leaks? Sometimes rubber parts can soften as they warm up and re-seal.
 
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