holeshot9g

I know this is a lengthy post but hopefully it will shed some light to everyone else with the elusive solution to the intermittent miss.

I recently bought an 87 xj 4.0 with 228,000k and I have done a ton of repairs to it getting it in excellent working condition. I am careful not to fix more than one problem at a time so that I don't have to rack my brains figuring it out if something else messes up as a result of my repair. I know if it's something new or something I did wrong.
I recently had the notorious intermittent miss that so many others complain about in their RENIX xj's. I didn't notice it when I bought it two months ago but it became apparent as I did more and more work on the jeep and everything else was smoothing out. One of the first things I did to it was replace all the vacuum hoses, cap & rotor, plugs & wires, and cleaned out the breather baffles in the valve cover to stop it from blowing oil into the air box. My buddy brought over his Snap-On diagnostics computer and it showed everything to be working within specs with no trouble codes. The miss was very subtle and often I dismissed it as the A/C compressor cycling off and on as I was traveling on the freeway but I soon discovered that it did it with the A/C off. I had checked for vacuum leaks on the intake manifold where it bolts to the head by spraying wd-40 but didn't find any vacuum leaks but it did have a small exhaust leak. When I pulled the intake and exhaust to replace the gasket, I let the intake rest on the manifold and broke two of the pipes which may have been cracked to begin with but I didn't see any soot in the cracks. Anyway, I found a new one at a junk yard and while I had everything apart I decided to replace the O2 sensor since it had 228,000 on it and I seriously doubt it had been replaced anytime in the last 10 years. (I replaced the O2 sensor with a Bosch from Autozone). After I had everything back together I noticed a lot of the wire loom on the engine wiring harness was all dry rotted and crumbling apart. As I was taping the wiring harness back together I noticed that there was a wire that appeared to be a bare wire wrapped with light blue foil tape. This wire went all the way through the harness to somewhere down on the driver side of the engine. Where it went under the vacuum hose junction block the foil tape was peeling off from all the oil that had been on it for years. It looked like it could be grounding itself on the metal bracket so I taped it up really good and finished taping up the harness. After all was said and done it ran great with no more miss....untill.... a week later I got ready to replace the rear main seal and oil pan gasket and I sprayed degreaser all over the underside and the top side of the engine and hosed it all down with water. After I was finished with the rear main seal job the miss was back and it was worse. In a 20 mile trip down the freeway it missed eight times and hard enough to jerk the drivetrain. I figured water had gotten into a plug-in somewhere. A week later I was still trying to get rid of the miss by cleaning out plugs, doing the ground refresher, adding extra grounds, replacing both battery cables (which were horrible to begin with), replaced the CPS, cleaned up the C-101 connector, etc. I called my buddy (who is also a whiz in the ways of the jeep cherokee) and had him bring his diagnostic computer over again and told him what was going on. Before he came over I had pulled the AC Delco platinum plugs out and they all looked perfect. Just for s*#ts and giggles I went to Autozone and got a set of Champion copper plus plugs and put them in. I figured that the Jeep may be picky about plug brand just like most jap cars will miss and run like crap unless they have a set of NGK plugs in them. I didn't even start it until he came over the next day. The hood was open and he leaned in and pulled one of the plugs-ins off of the coil and said that his cherokee had done something similar once except that it missed several times in a row and then just went dead. He showed me the three pin connector and one of the pins looked to be slightly charred / carboned up. We cleaned it with brake parts cleaner and scraped it and cleaned it up with a brass pick and plugged it back in. The miss is gone. I drove it all over the place today, in town driving, highway driving and A/C on and off. Still no more miss. I am reluctant to think that the spark plugs got rid of the miss after seeing the dirty pin in the coil plug-in. So, when they say check ALL electrical connections, don't just check the sensors, check EVERYTHING. Don't overlook the obvious. If it looks like a duck, walks like a duck and quacks like a duck, then it's probably a duck. If it feels like an ignition miss then it probably is.