BNJeepsta

So, I've got a 99 4.0 AX-15 that has been driving me absolutely insane. Every couple of starts, the jeep will run like crap. It runs super rich, stutters, bucks, backfires, idles like it has a monster cam and is almost undriveable. This has been going on for about a month and a half now and has NEVER thrown a code. When it happens, I'll be cruising along and all of a sudden it will fall flat on its face then pick up and fall off which starts the bucking. If I hammer on it, it will bog and back fire for a second and then accelerate in a lethargic fashion. I've cleaned the throttle body twice, changed the tps to a brand new NAPA unit and checked all of the plugs and wires. Today, I plugged it into a MAC scanner and all off the values bounced back and forth from minimum value to maximum value. Then I tried a snap on scanner and while that actually showed correct values, it would only read for a few seconds then it would say the connection was lost. Right after that, it would start reading again. I am seriously at my wits end. It seems to happen randomly and then go away randomly. Any help would be GREATLY appreciated.

BNJeepsta

Also, the jeep has 211,xxx miles, bone stock and does have a small crack in the exhaust right above the upstream O2 sensor. I've had several other XJ's with cracked manifolds that were way worse than this one and none of them ran this way so I doubt that is the problem. Both o2 sensors are hooked up and fairly new as well.

Crash_Course

I hate to point to an ECM right away as a possible cause of problems, and it's a rare occasion that I do, but man I'm having a hard time not doing so in your case. The intermittent nature of the symptoms and the fact that the scanner loses connection just screams computer problem to me.

Might start by checking chassis grounds and wiring harness connections.

Good luck!

BNJeepsta

I have been thinking the same thing, honestly. The grounds are all good and all of the wiring harness connections are good as well. I wasn't thinking ECM at all until today when I hooked the scanners up. I've been working on cars for a long time, especially jeeps, and I've never seen something exactly like this before.

CCKen

I would suspect a faulty ignition coil. Inspect it closely for cracks.


This cracked coil caused backfiring, chugging, etc.





You can also test the coil.

CCKen

Does your CHECK ENGINE bulb work?


Key to RUN the CEL should illuminate.

CCKen

If the coil checks good, consider a faulty Camshaft Position Sensor (in the distributor) as being the culprit.


Always check engine sensor wiring for chafing.

BNJeepsta

I never thought to check the coil, I'll do that when I get home from work. My check engine light definitely works. I have considered the CPS but, I was under the assumption that if the cps was bad, the jeep wouldn't run at all? It starts great every time.

VTJeep

Haha, that's my old coil that Ken posted.. It was an ongoing gremlin that took me way, way too long to find.. Good luck!

CCKen

I do steal photos occasionally. LOL

VTJeep

Haha, completely acceptable! It's nice to have a photographic representation sometimes and open source content is instrumental in achieving this!

CCKen

By CPS, are your referring to the Crankshaft Position Sensor or the Camshaft Position Sensor?


The Crank Sensor can cause big problems but the Cam Sensor (in the distributor) can be sneaky. It too can cause the engine not to fire up if its completely shot.


The Cam Sensor, in addition to providing the PCM with cylinder/piston position info for injector timing, it provides the PCM the same info that it uses for ignition/spark timing.


If the Cam Sensor is squirrelly it can randomly give false info to the PCM, causing the injector and spark timing to be off - misfires.


If the distributor shaft has excessive up & down play it will throw the timing off (shaft climbs up the helical splines on the shaft/oil pump drive and causes the rotor to be off the mark). Same if there's too much rotational play in the shaft. This should be checked as a routine maintenance task when the distributor cap is changed.

TwistedWrench

This is very true, my 328is was behaving much like you described, cam sensor fixed her right up. BUT, the cam sensor(or any other sensor) taking a dump should trigger a code in your ecu. If you're getting this kind of issue and no codes are showing up on a scan, I'd certainly suspect the computer.

If you know anybody with the same unit in their XJ, maybe sweet talk your way into borrowing it?

BNJeepsta

Well I tested the coil and it is well within spec and has no cracks at all. By cps I was referring to crank position sensor. I'm still leaning toward the computer since there aren't any codes. I will check the cam sensor but,I have the feeling it's going to be fine. As far as borrowing a computer goes, I don't know anyone with a 99 5 speed so I'm sol on that one.

BNJeepsta

Also, I have has absolutely ZERO problems with starting