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Series of MJ problems


NAXJA Forum User
The Mudwest
I am having a series of MJ issues/problems!!!

88 MJ 4.0l, BA-10, 231 t-case, 253k on the odometer.

First, when cold it takes forever to fire, like it is not getting fuel, when it does start at first it fires erratically then revs up to cold start rpm. If you drive it to the store, and shut it off it starts normally.

Now, it acts like it is losing spark or fuel at around 1500-2000 rpm in 3rd and fourth gears. It does it randomly, and doesn't seen to have a pattern to it. You can watch the tach drop from 1750 rpm to about 1200 or 1300 rpm, and go back up.

It also will die, when you push the clutch in at a stop, but it starts right back up.

I haven't changed any of the typical starting problem culprits. It could also be fuel related, as a friend borrowed it and put Mobil gas in it, I never use Mobil, my XJ seems to run like crap also on it.
(a) How old is the oxygen sensor (miles, and years/months)?

(b) How old is the catalytic converter?

(c) How old is the fuel filter?

(d) How old are the spark plug wires and what kind of condition are they in?

(e) How old is the CPS (crankshaft position sensor)?

(f) How old are the distributor cap and rotor?

(g) Have you ever changed/replaced the ignition coil?

With a manual tranny, there is no way the engine can suddenly lose 600 RPM (1/3 of its rotational speed using your numbers) without also losing 1/3 of vehicle speed. So, since the tach is electronic and triggered by the ignition, more than likely your rough running and stuttering are ignition-related. Accordingly, while all the things I've listed above could contribute, I would concentrate on the ignition items first and look at fuel and fuel delivery items only if ignition has been ruled out.
I don't know how old any of the ignition parts are. I just bought the truck in March, and haven't done anything but basic maintenence to it.

The guy I bought it from was the Original owner, and was pretty particular about how he maintained it.
Could also be EGR related if you have one. Try diconecting/plugging the vac line to it, or disco the power to the solenoid and drive it around to see if that makes a difference. I had the same problem w/ my MJ (2.5L) and I agree it was very sporadic, especailly right after I put fuel in it. I pulled the EGR and cleaned it, and replaced the solenoid. All seems well now.
Disconnecting the power to the EGR solenoid on the fender isn't going to help. I discovered that when you turn the key the ECU kicks that solenoid "ON" this makes the EGR valve close. It's completely opposite from what you'd think.

If you disconnect the power to that solenoid the EGR will fully open from the git-go. I discovered that the motor does not like running at all this way. I pulled the power from that solenoid while the engine was running and watched the EGR go full open and stall the motor. I just disconnected and plugged the vacuum line and left that solenoid alone.

It does sound like it could be faulty/sticking EGR or solenoid though. It's amazing how many problems the EGR valve can make you think you have. Mine had me going in all directions. Turns out the EGR was the LAST thing I checked and ended up fixing my stalling problem.

Also check the tube from the manifold that goes to the MAP sensor on firewall. If this tube is broken or has leaks it will make the engine run like crap.
Update to the problem:

I had a chance to drive it more today.

It sounds almost as the motor is completely quitting, but the transmission turning, is in effect bumpstarting it every time.
Huh? Wouldn't the key have to be turned to the "start" position to bumpstart it? Don't quote me, but it sounds like a failing CPS or a bad coil. Good luck! Keep us posted.
Jeep on!
Hey, peter

Checked out project pics. Your 86 looks almost like the twin of the MJ I am having problems with.
Okay, here is another update to the problem.

Plugs, wires, cap rotor, all check okay

The problem has gotten worse, but only under normal light load conditions

I had a heavy load (1000 lbs of cement, and sand) and was traveling on rolling hills, with almost no occurences of the problem. The next day, I was driving under normal conditions with an empty bed, and it was almost undriveable with the stuttering, and misfiring.

It is cutting out completely, but the transmission is engaged and turning the motor over, and it kicks back in.

Now, it also will sometimes rev up to 2500 rpm when you start, with no warning. Sometimes turning off and back on will cure, sometimes not.
WHowey, check out the MAP sensor located on the firewall, driver's side. It has an electrical connector and vacuum hose connected to it. Try spraying the connector with WD-40, and check the vacuum hose for leaks.
Did you try disabling the EGR? I'd bet that's the culprit....
And the winner of the Gold-tone plated, wrench is: Eagle!!!!

Turns out it was the CPS. Kind of!

The wiring from the CPS, had come in contact with the exhaust manifold. Melted the outer coating on the wiring, and it was shorting out against the manifold.

Changed CPS, repositioned the wiring, all problems gone!!

The once again proves, the number one 'project' axiom.