So I'm up to my nerves end with this engine. A few days ago it started sputtering, misfiring, shaking, and backfiring out of the blue. Codes say random misfires on Cylinders 5 & 6, and open fuel injector circuts on 1,2,&5. I've checked and tested everything I can think of.

I needed a tune up anyway so I replaced: spark plugs, head gasket, cap gasket, thermostat, thermostat housing/sensor, radiator ( after this bad *** engine blew the one I had on the way home.); lifters, rods, dizzy and rotor. Spark wires are correctly set on the dizzy.

I've cleaned and tested: TPS, CPS, fuel injectors (swapped them around to see if the problem followed, no joy), wiring harness ( everything is getting optimal voltages) ignition coil, all cylinders are getting spark, all fuel injectors are firing in sequence, all rockers, valve springs, and valves are operational (I checked them while cranked with the cap off) I also physically made sure to time the distributor by the Hynes manual. Disconnected both o2 sensors, nothing.

Only thing is. Every morning cold start she runs and idles perfectly fine for a few minutes, and even for a few miles if I take off right away. Then the misfires, sputtering, and shaking comes back. Could it be the actual PCM?

I have no idea on what to do next. Compression test would tell me what I already know: 5 & 6 haz no combustion. Any Ideas? Anyone have a spare 4.0 engine laying around?

 

 

1999 XJ
175,000 miles Odo
4.0 6 cylinder
Auto magic transmission

 

Man, you've been thorough!!

Ever used contact cleaner on the PCM plug?

 

Yes I have, the plugs are cleaner than anything else.

 

Kinda figured you did....
Might be worth it to snag a PCM from the junkyard........

 

Thought about hang testing one.

I've never had an issue with this jeep that I couldn't fix. I won't let this problem be the etching on the tombstone.

 

Great attitude. LOL.

 

Put back together, I've repinned the injector plugs for both the 5 and 6

 

Did you do anything to the head while you had it off?

You familiar with the weak valve spring issue?

 

Funny you ask, I ruled out all of the valves, springs, and rockers last night by installing a donor cylinder head I got from the junk yard.

The problem continued even with the newish head.

 

I'd say grab another PCM and any chance it's severe heat soak in conjunction with the PCM?

 

Not probable, the heat soak issue usully affects cylinder 3. And wouldn't cause 5&6 to be screwy at the same time.

 

I'd say I agree with you. But with everything you've gone through I wouldn't overlook it. It's worth the 25 bucks to wrap the fuel rail and injectors. I'm just as stumped as you. I really hope you find the issue soon

 

I ruled out heat soak when I switched around all my injectots