Hello all, my name is Dave and this is first post on this site. Seems like you are all pretty well informed on jeep issues and hope you can give me some advice.
I have a 94' jeep grand Cherokee with a 5.2 ltr engine, auto trans, awd with 230,000 miles on the clock.
Recently I've had some problems with rough running, huge amounts of black fuel smoke, and now blue oil smoke. Engine has had no issues until this happened. Was in the shop 1 1/2 weeks and they tell me they cannot figure it out.
Was told by mechanic that it could be, MAP sensor, O2 sensor or other sensor. No computer link up so I'm stumped. Changed the MAP and did not help at all.
New plugs, wires, rotor and cap did not solve the problem.
This problem is not steady, most of the time it will start clean then begin to run rough and smoke.
I would be grateful for any ideas or advice. Thank you, olddave.

 

Black fuel smoke seems to me would indicate too much fuel in the combustion chamber, or the fuel not being burned in the cylinder but rather in the exhaust. I don't know if the computer can put that much extra fuel in if there was a bad sensor causing it to think it needed more. So to get a lot of extra fuel would mean bad fuel injector(s). On a Chev truck I used to have, the fuel pressure regulator was inside the intake manifold, so when it went bad and started leaking, the engine ran real rich.

For the case of the fuel not burning in the cylinder, but then burning off in the exhaust, that would mean it is losing spark at some point. If it starts OK, then goes bad after running a few minutes, that sounds like something is happening after it warms up. Maybe the computer is making a change based on the engine temp, or maybe something is heating up and not working properly (coil, cam sensor, O2 sensor, etc.) If the computer is changing something too far, maybe the engine temp sensor is bad or maybe the computer is screwed up.

I don't know about yours, but the experts on here say that on my '97 with Straight 6, you can unplug the front O2 sensor and that will throw the computer into default basic engine tune, then if it runs right, you know that it was the computer's adjustments causing the issue. However if after unplugging O2 sensor it acts the same as before, that points more toward an actual physical problem such as a leaking injector or bad coil.

 

Check your plenum gasket. What you describe are the typical symtoms of a bad plenum gasket.

 

Thank you for the replies.
I had thought maybe something going bad in the ignition, time for more checking that department. Hoping it's not time for injectors.
Plenum gasket, would never have thought to look in that direction.
Thank you both, certainly gives me more to go on.

 

I second the plenum gasket. When it's cold, you're leaking vacuum so the computer starts dumping more fuel thinking the mixture is too lean. As the engine warms, you start sucking oil into the intake thru the bad gasket. This is common on the 5.2.