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4.0L Runs Rough

bmar99

NAXJA Forum User
Location
Ma
I just purchased a 1996 Sport with the 4.0L with 120,000 miles. It runs pretty rough at idle, and the check engine light is on. I did a complete tuneup and cleaned the throttle body. Didn't make a difference. Any ideas???
 
did you clean/replace the IACV on the throttle body?

did you disconnect the neg battery cable for 10 minuts or so to reset the ECU?
 
any chance it got wet lately?
Quite a few times I have seen a Jeep run rough, it has gotten wet recently and the Throttle Position Sensor got wet, and caused problems. That said its about $70, so hopefully someone chimes in with another answer that may cost less. Rough just at idle tho may not be the TPS.
Does the key on-off-on-off-leave on trick work on a 96, my 97 has that featuer but I am not sure the years it is specific too.
 
Bmar99,

A couple suggestions. Some injectors may be dribbling and not shutting off completely. Run some good injector cleaner through the engine. Also check that all your injectors have the correct colored electrical wire running to each injector. I assume you pulled the IAC and cleaned the passageway it controls of any carbon build-up. If this happened when it was warm, maybe you're heat-soaking the fuel rail or injectors and causing vapor lock that comes and goes. A sluggish O2 sensor (i.e. coated) can cause a rich condition and a rough idle. Finally, spray a mist of water on the ignition wires while the engine is running and see if there's any spark arcing from the wires.

Best regards,

CJR
 
I noticed on the fuel rail that there is a cap that seems to some kind of a valve, is this for a cleaning kit on some kind? I did put a bottle of fuel injector cleaner in the gas tank. Yes I did pull the IAC and clean it, should i pull the O2 sensors and clean them? The wires are brand new.
 
the schrader valve on the fuel rail is for testing fuel pressure and pressure leakdown...If your check engine light is on, can you get it scanned at a nearby Autozone?...the check engine DTC code can be very helpful in troubleshooting the problem...
 
What exactly did you replace in the tune up?

Many things can cause a rough idle, the worst being a bad head gasket (my current problem with a recent purchase that still has great power).

I would start by going through everthing in the fuel and igntion system starting with checking the spark plug wires for any that are criss crossed. Next pull one plug wire at a time to test for any cylinders that do not cause more of a miss when disconnected to check for week or bad cylinders. You can do the same test by disconnecting a fuel injectors control wires one at a time. If you find one or more that do affect the idle when disconnected and reconnected then you have isolated the problem to those cylinders and their fule injectors, plugs and wires.

A vacuum test will tell you a lot as well.

http://www.ncf.ca/~ag384/vacuum.txt

Vacuum leaks can cause idle problems too.
Loose grounds and loose connections can cause various idle and engine miss problems too. They can be difficult to locate. The best method is the wiggle the wire test as you go through the wiring harness while the engine is running looking for loose connections.

I take it the check engine light is not on? Have you had it scanned with the OBD-II scanner to check for error codes?

Oh, and leave the O2 sensor alone for now. If it dies it should throw an error code and the check engine light will come on.
 
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