I have a 1999 Jeep Cherokee Sport 4WD with the 4.0L inline 6-cylender & automatic. It has 45K miles and all the recommended maintainace has been done onschedule.

Recently the check engine light has been comming on and the engine is idling rough. The problem is intermitent. There is plenty of power, and it runs smooth at anything above idle.

Any Jeep mkechanics want to take a stab at diagnosing the problem so i dont get screwed when i take it in. If it were a carburated vehicle i would say the air fuel mix or timing needs adjusted. But this being a fuel injectd vehicle, I ASSume those things are computer controlled.

ever changed the fuel filter?

Check the spark plugs/wires. My truck (97 F-150) was running kinda rough one day, then the chaeck engine light came on. The rough idle lasted only a few seconds, but the light stayed on for 2 or 3 days. Per the owners manual if the ck eng light comes on and stays on a malfunction has occured but corrected itself. If it's not the plugs/wires Check the O2 sensor. One of these are most likely what's wrong. If not...~$80 for a diagnostic.

PONY_DRIVER

P.S. When was your last oil change? (Change the fuel filter while your at it)

Hi. When I begin working on stuff on this nature I do several things FIRST.

1. Check and clean battery and cable connections and check ground connections to body etc. these can be the root cause of problems.

2. Alway give a GOOD VISUAL INSPECTION of components and connections, connectors to sensors, spark plug wires, wires rubbing metal, loose parts, etc.

3. Then begin with diagnostic tool (scanner)

4. Go from there once you have the DTC'c ( diagnostic trouble codes)

This procedure has never failed me, and I learned it the hard way. After chasing my tail for several hours looking for big problems and only finding small ones. Although you can run into difficulties that require a real technician.

Good Luck.
MM419

The Jeep 4.0L I-6 has a reputation for the crank angle sensor to slowly die. As it gets worse, you sometimes won't be able to start the engine.

Also, 45K is a little early for this, but a dirty EGR valve is a very common cause of drivability problems. You can clean it with an old toothbrush and a can of carb cleaner.

If you'll go to Autozone they'll hook up an analizer for free. Then you simply replace what it reads as bad.

An amber or yellow 'check engine' or 'service engine soon' light is related to emissions.  This can mean anything from a miss (i.e. bad spark plug wire) to any one of the sensors not working properly.  It's best NOT to go hog wild and replace everything though.  

Does the CHECK ENGINE light stay on even under heavy acceleration? If no then there is a part that gets carboned up over time and needs cleaning. I am under some badass medication right now and can't for the life of me remember the name of this part. On my Lincoln it is just before the intake manifold. One gasket and a wire brush and your problem is solved untill it funks itself up again. Good luck.

It could also be the oxygen sensor. When one goes bad the computer resets everything to a default setting. This is very easy to replace. Looks like a sparkplug screwed into a port in the exhaust pipe just after the header.

find a place to get the codes scanned.  99% of the time this will tell you what the problem is, or at least a very good place to start looking.


Untill you scan the codes you are wasting your time guessing.  there are a "pissload" of sensors that can cause this kind of problem.