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99 XJ Stumble/Hesistation on Initial Acceleration

T

thejeepgod

NAXJA Forum User
Location
CO
Backstory: 99 XJ has a stumble on acceleration, usually at lower speeds. It only happens when it is cold, i.e. first pulling out of the driveway in the morning, or when the jeep has sat for a few hours. This usually happens in the 1500-3000 rpm range, it does not last very long (few seconds) then it will recover and return to a normal power level. It never feels like its about to stall, its just a hesitation. It will usually only do this once during the course of driving, and runs normally otherwise. The check engine light is not on.

I have access to many parts jeeps, so I have been swapping things over in an attempt to fix this, so far I have changed out the following with new or known to be good units and it still does the same thing:

fuel pump
fuel rail w/ injectors
throttle body w/ all attached sensors (MAP, TPS etc)
Distributor Cap/Rotor/Plug Wires
Hall Effect Sensor aka camshaft sensor
Crank Position Sensor
Pre-Cat Oxygen Sensor
Spark Plugs
ECU - this did help for a while, but then it seemed to re-learn things and it came back.

I have also done a visual inspection for vacuum leaks and broken grounds, did not find anything.

Now I'm stumped, I've changed out nearly every sensor on the engine...anyone else have any ideas on what this could be??
 
I have the same thing on my 91 and haven't solved it yet. Mine also occurs at mid throttle range randomly, but less or more throttle will make it go away.

I've tried replacing the CPS and TPS already, just as you have. Got somewhat better, but still an issue.

I had this issue on my 96 XJ as well at one point, it went away after I pulled the drivetrain, put a new transmission in the middle, and reinstalled it. No idea what part of that process actually fixed it unfortunately.
 
I really, really doubt it's the transmission as the symptoms described are an engine problem, and the engine and transmission control systems are almost completely disconnected except for diagnosis / DTCs.

Also, I have an AX15 in my jeep and I have exactly the same problem.
 
Dunno. I had EVERYONE i know stumped, until i had a buddy at a chevy dealer come in with the same issues then finally the guy burned up the tranny. They swapped tranny and it went away. I "lightly" burnt mine up, swapped the aw4 and problem went away. might have been luck of the draw.

did you think about possible vaporlock?
 
Also, i had absolutely NO issues while in reverse. i even ran trails on my clubs land in reverse cause i couldn't figure out the problem. That was a sight to see to say the least! Anyone ever flop over running trails in reverse on purpose? ya i did. haha
 
Mine's a 91 with the supply/return fuel system, so I'd probably have to have the engine block glowing to get vapor lock. I did consider it though.
 
it is an aw4, but i doubt its that. I have tried keeping it in first gear and it does it just the same so its not trying to shift and failing or anything. It also does it in reverse. Also it doesn't feel like a slipping transmission, just a lack of power like something electrical failing. But I guess its still a possibility, just dont want to swap out the tranny on a hunch:dunno:

I do not suspect vapor lock as it always starts right up normally.
 
Sounds like it could be exhaust/vacuum related.... if you have a bad enough leak in either it would produce these sort of symptoms. Check over all your vacuum lines, not just a visual inspection but really go over them all, looking closely over each inch, and especially the terminations/junctions. Look over your exhaust system, headers/exhaust manifolds are prone to crackage, and the spot may be out of sight/hard to see. (see my build thread page 5 (i think) for where to look for the invisible crack.) Look over the rest of the exhaust for soot marks around joints - this is a sign of a leak.
Another issue could be intake manifold leak. This is harder to track down, especially if you cant re-create the problem at will. Also, if you haven't replaced your catalytic converter or if it hasn't been swapped out in 100k it may be time. You're o2 sensors wont always tell you when its time.
 
Having this same issue on my 95. From 1500-1750 RPM, any lower or higher and it's good, or if I run in third, no overdrive. My guess is tranny since I changed all vacuum lines and the exhaust manifold. Everything behind the manifold was good. hasta
 
TPS, O2 sensor, or MAP sensor vacuum leak are common causes.
 
Just a thought....I have a 98 XJ but I also have an 89 Celica GTS. When I bought it 10 years ago it had a stumble on initial acceleration when cold. It ended up being a crack in the air intake boot. When the engine was cold, the crack in the rubber would expand and as the engine moved slightly upon acceleration the crack would open and allow unmetered air past the MAF. A replacement rubber intake boot from the junkyard solved this problem.
 
I had a nearly identical situation when I first picked up my XJ, although it was throwing a few codes along with the cold stumble. It turned out that the oxygen sensor heater fuse was blown.

3427d1290480984-o2-sensor-high-voltage-heater-circuit-malfunction-wtf-pre-cat-o2-fuse.jpg
 
Subscribed.

My 98 sport, 4.0L w/ AW4 has had a stumble. I notice it off the line often, and lately at slow speed. This was after the tune-up. I replaced cap, rotor (since I knew they were old) just to see if it would help.

I have a very cracked exhaust manifold. I'm running an injector cleaner (BG44K) through it on this tank, and I'm getting a lot of smoke from underneath the intake manifold.

I have an APN header sitting in the garage, just need to get a weekend to put it in. I just got around to replacing the motor mounts with Brown Dogs, so I can finally replace the cracked header. Since the rig has 204K, I bought a new OTK O2 sensor, Magniflow direct fit cat, 10" flexpipe with 6" braided section, and will get everything fixed at the same time.

Keep this thread alive and I'll check back in and let you know if it fixed it.
 
I also had a horrible stumble upon start up. Once the engine warmed up it would go away. I replaced the usual TPS, IAC and CPS nothing changed. Finally changed the O2 sensor on the down pipe and fixed the problem. I will occasionally get a slight stumble but I figure its my other o2 sensor. Haven't had time to replace it and it has 160k on it.
 
Out of everything I checked and replaced on my jeep, turned out to be old spark plugs. Go figure. At least I have some new parts on there and the wife wasn't complaining about me spending money on the jeep. :conceited
 
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