Stumblin bumblin Jeep


#1

We have a '98 Cherokee (4.0, auto, 4WD, ~50K mi) with a peculiar tendency to run rough (misfire, hesitate, stumble) for about a minute or two at certain times. This almost always happens when the engine is warm and has been turned off for a short period of time and restarted. Approximately a minute after restarting the vehicle will buck & misfire as described when accelerating. After acting this way for a minute ot two the problem goes away for the remainder of that drive. This doesn’t always happen, but is becoming more frequent. We just installed a new complete fuel pump module, which fixed an erratic fuel gauge, but not the misfiring dilemma.

It seems there must be some system process going on about a minute after restarting a warm engine that is malfunctioning, but I’m at a loss as to what would cause the engine to act in this peculiarly specific way on such a regular basis.

No codes are being thrown by this malfunction. Cap & rotor look fine.

What do you think?



ThaNX


#2

[b]You might want to have a fuel pressure leak down test performed.

If there are any leaking fuel injectors or a leaking fuel pressure regulator, this extra fuel that enters the engine after it is turned off hot can cause the symtoms you describe. Because the engine is running too rich when it’s restarted.

Tester[/b]


#3

The fuel pump module we installed included a new pressure regulator (and pump, level sensor, strainer, etc.)
Why would it run run fine for the first minute or so if there’s a leak down problem at the injector(s)? Seems it would act up immediately upon restarting & not wait a minute or more…
You may be right, but I’m beginning to suspect something other than the fuel system at this point.
Any other ideas?


#4

Well, it hadn’t been throwing codes while this problem gradually happened more frequently over the past year or so. Finally two days ago it started tripping an O2 sensor related code. Replacing the forward O2 sensor appears to have fixed the issue.