Jeep XJ 1994 Cutting out, help needed

samsterman99

Active Member
Joined
Jan 22, 2013
Posting some help for a friends jeep. it is a 1994 cherokee with the 4.0. Got it from a member on here and it was doing this already, but we thought we could fix it. (should have known better)
Replaced already:
Crank Pos sensor (2 times)
Cam position (ignition module) sensor
Coil
Cap
Rotor
Wires
Plugs (2 times).
No cat or muffler
PO also said he replaced fuel pump


Starts up and runs good, low RPM is good (below ~2500-3500 no tach on it). As soon as we hit high RPM's the engine cuts out, misses, shakes, and backfires out the exhaust. It isnt just one cylinder. it has to be happening on all cylinders. This happens in Drive or neutral and park. I have tried unplugging the O2, TPS, MAP, IAC, and plugging all vaccum lines. Still acts the same way at that certain RPM. It runs great until that point, part or full throttle, no hesitation or stuttering based on throttle position or load or gear. it seems to be only dependent on engine speed.

Going to try the fuel pressure test today. I thought I would have been able to solve this, but I really want to help my friend out before uwharrie opens
 
Changed tps and computer for known good units and fuel pressure is spot on as is regulator . No change. I'm now fully stumped. Any guesses on where to go next?
 
take off cap and check distributor shaft. those are known to get worn and lots of play in them which results in a hefty misfire but that's usually throughout all rpms but wont hurt to look. I would've started with tps (wish I caught this thread sooner) and then moved on to map sensor. however it's starting to sound like wiring issues though. you've got the main spark and fire stuff new so signals from something is still off. normally this falls under tps issue and then map sensor. I wish you were closer to me cause I have tons of these parts laying around. hope this helps.
 
What's the fuel volume? Disconnect the supply line at the rail and put in a large bottle while cranking. Pressure isn't the same as volume. It is possible to have clogged injectors or even a split hose inside the fuel pump assembly that can cause a low volume issue.

Also, have you verified the distributor installation?
 
leave the fuel pres gauge on it and go for a spin. make sure you can see the gauge. could be a clogged fuel filter, or fuel strainer.. (its easier to pull the tank in my opinion than to fuss with it under the jeep)
 
or i'd say time to pull out voltmeter and go to town on that sucker! every major sensor and ground to start with and work your way back to ecu. hope you get it running. post up what fixed it, I'm kind of stumped myself.
 
I have seen the fuel strainers get plugged with rust from the metal tanks (Did this a few times on my own.. I ended up pressure washing the inside of tank).. Also no mention of a new fuel filter (the filter will show you relative tank condition.. a bunch of orange crud.. probably rusty) Of course, this will also show with the volume test suggested by Jody
 
x2 on checking the fuel pump. I have had a filter come loose from the pump and float around in the tank. jeep ran great until the sock would suck up into the pump intake, then nada!. easy enough to pull and at least look at it.
 
when mine would clog up, it wouldn't even really look dirty (2nd or 3rd time), but sprayed it out anyway with brake cleaner, and it'd run great for another couple months.. I would have replaced the strainer but I can't find one to fit my super-quiet bosch..
 
So just to update and get some more feedback....
We checked the pressure while it was missing. Still holds strong and never falls off. If it's a fuel issue, then it's got to be the injectors. I may play with them
When I have time.

When out after a drive and it's hot, if you start it up it will sometimes stumble and run horrible at idle and want to die. Turning the engine off and on will cure the problem. I checked all ECM wires with a voltmeter. And the only issue is that the o2 sensor heater circuit is not getting 12 volts. But even with the 02 unplugged the problem still occurs. Also replaced the distributor and checked it, it's perfect. We have a tach on the motor now and the problem occurs between 2400-3000 rpm. Anyone in raleigh area want to have a look at this thing? I'm about to recommend bringing it to the jeep dealer
 
at 2400-3000, the o2 isn't even doing anything, the engine is running open loop. maybe check the MAP sensor hose to see if its got a clog in it.. replace the MAP sensor if you have one hanging around.. 2.4k - 3k it should not have much manifold vacuum left but if the map sensor/line isn't releasing vacuum, the computer would think it doesn't need so much fuel (if I understand the operation of the system properly.) thats the only sensor you haven't tried yet.. test the intake air temp sensor too.. other than that, i guess the injectors are left. (nothing else left ... really)
 
I tried unplugging those two sensors. And same thing occurs. All vaccum hoses are new. I am going to try and move crank position sensor closer to flywheel by enlarging holes to get a stronger signal..... Maybe? That's all I got left in my brain.
 
maybe a faulty driver in the computer for the ignition coil or injectors.. I've got a few computers laying around that are good.. I don't think moving the sensor is fix it. The distributor is spot on, right? But then why would power-cycling the computer make it clear up unless it was the computer?
 
samsterman99 said:
So just to update and get some more feedback....
We checked the pressure while it was missing. Still holds strong and never falls off. If it's a fuel issue, then it's got to be the injectors. I may play with them
When I have time.

When out after a drive and it's hot, if you start it up it will sometimes stumble and run horrible at idle and want to die. Turning the engine off and on will cure the problem. I checked all ECM wires with a voltmeter. And the only issue is that the o2 sensor heater circuit is not getting 12 volts. But even with the 02 unplugged the problem still occurs. Also replaced the distributor and checked it, it's perfect. We have a tach on the motor now and the problem occurs between 2400-3000 rpm. Anyone in raleigh area want to have a look at this thing? I'm about to recommend bringing it to the jeep dealer
Click to expand...


Jody Treadway said:
What's the fuel volume? Disconnect the supply line at the rail and put in a large bottle while cranking. Pressure isn't the same as volume. It is possible to have clogged injectors or even a split hose inside the fuel pump assembly that can cause a low volume issue.
Click to expand...
 
Yeah it sure is confusing. Usually at this point I would have found something that's the culprit. I am wondering if it is an internal problem at all or just totally sensor/fuel related and I can end up finding it one of these days.
 
I don't see any reply about the fuel volume or pump/strainer being checked, ya might want to rule that out somehow. I had similar symptoms, checked and changed about every sensor, spark plu, vacuum line, bypassed the fuel filter, swapped injectors...nothing changed. Finally pulled the fuel pump and found the filter sock was off and lil rubber cubes from a deteriorated fuel pump hose were all over the bottom of the tank. Put in a new advance auto airtek pump assy, and all was good. There are a lot of bad reviews about this pump, but I was going cheap and this xj sees little use.
 
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