I have a 96 ZJ w/ the 5.2L that suddenly developed a misfire on all 8 cylinders throwing codes 300-308 & 1762. With a noticeable miss at idle & while accelerating. (Seems like only one cylinder, sometimes more then one.)

The plugs were changed 2 weeks ago, so I replaced the cap, coil & rotor, cleared the codes and took it for a test drive. It threw 300, & 302 w/ 301, 303, 304, 305, 306, & 307 pending.

Replaced the wires, fuel filter, checked the fuel pressure (50lbs steady dropping to 49lbs at it's lowest) and removed then cleaned the fuel rail out, and did a noid light test.

Took it for another test drive still misfiring. This time only #2 was set. So I replaced the injector with a brand new one. Another test drive and still a noticeable miss and still there at idle. No code though.

Drove it around for a bit longer and it finally threw the same codes again. 300, 302 & 6 of the other 8 pending.

I've checked the timing, and it looks like the computer is trying to constantly adjust it. As it'll stay steady for a bit, then retard, then advance back to normal, etc. over and over with varying times staying at each amount. Vacuum is also at a steady 22" or so at idle, needle only twitches after a misfire.

Weird thing.. as I was pulling injector wires for the noid test there was no noticeable change in how the engine idled. The misfire didn't seem to get any worse, it's like it didn't even notice that it was down one more cylinder.

Other info, the heads were rebuilt about 8 months ago, so all new gaskets there including intake. The crank sensor was replaced about 3 months ago when it failed and the oxygen sensor was replaced about the same time as well.

I'm basically at a loss as to whats going on now. My next thought is PCM, but I don't want to spend that much $ if there could be something else possibly to blame.

 

Do a compression test before spending any more cash on parts. There is a chance the plenumn gasket at the bottom of the intake manifold could be leaking and causing a vacuum leak. And of course I must ask if you have the firing order correct?

 

I'll do a compression test tomorrow. The firing order is correct, I triple checked that. Plus it was 2 weeks prior that I changed the plugs 1 at a time before it started happening.

 

Welcome to CF. The misfire suddenly appears, and you changed the plugs 2 weeks ago. I don't think it's a coincidence. Which plugs did you use, and at what gap? I always used Champion plugs gapped at .040" in my '97 ZJ's 5.2L. Never had a misfire with them. Others have reported misfires caused by using other brands of plugs.

 

Yes pull the air tube off the TB and look to see if you can see any oil in the intake. The 5.2 and 5.9s are terible for that.

 

Haven't gotten around to the compression test. I have an 02 with a H.O. 4.7L that dropped a valve seat to deal with first.

The plugs are Champion's Copper core gapped at 0.040".

There is a slight amount of oil in the intake tube, but not much. Probably what I'd normally expect.

 

Look down into the intake manifold itself. That would indicate a leaking manifold gasket.

 

He has the 4.7, I assumed it was a 5.2 or a 5.9. I don't think the 4.7 has plenumn gasket issues, but I don't know much about the 4.7 though.

 

He states that it happened right after he changed the plugs. I'd recheck the obvious things first. Is the cap on tight? Are the wires fully seated on both ends?

 

Check his first post ,he states its a 96 5.2. He then goes on to say in another post that he also has a 4.7 that dropped a valve seat he is going to fix first.

OP,
Did you check the wires on the injectors themselves? How did the old plugs look when taken out? What is the fuel pressure while driving around? Have you tried resetting the PCM by disconnecting the battery for a few minutes?