My 94 YJ 4.2L MPFI, 104000 miles started acting up a few weeks ago. It has since been to the my local mechanic and the dealer. The problem starts after 14 to 20 miles of driving. The engine starts running a little rough just barely noticeable until you push in the clutch. Then the idle drops off very quickly to about 200 to 300 rpm. When you let out the clutch and give it the gas its like driving into a very strong head wind. If you push on most of the time the engine will surge a couple of times and start running ok again. I have had the problem show up both when the engine was cold but mostly it happens after driving it a while. Once the problem occurs if you rev the engine from idle it will backfire out the throttle body. The timing appears to be advanced. There are no codes stored except the usual 12, 33 (I dont have AC). I have listened to the injectors with a mechanics stethoscope and they are all just clicking away.
My mechanic thinks it's the computer, but from what I have read here computers dont go bad very often. I brought it to the dealer for a diagnostic they found a massive vacuum leak at the throttle body and replaced. They also suggested the cat converter may be bad.
The dealer said the compression was good and my mechanic said he checked the grounds.
I have replaced Rotor Distrib Cap Plug Wires Coil Plugs Radiator (it needed it lots of fins were gone) Coolant Temperature Sensor Vehicle Speed Sensor Throttle body Cat Converter Oil Oil Filter I have also added a bottle of fuel injector cleaner in the gas.
When its not acting up it runs like a dream, all kinds of power, well at least for a 4.2L. I really dont want to replace this one I just got it setup the way I wanted it.
The computer is next.
Does anyone have any ideas.