Hello every body I'm new to posting in a forum but I think this the place for answers. I own a 1999 Cherokee xj with 4.0. It has close to 300k. Here's the problem it runs and drives good uses no oil and has a smooth running motor. But if you rev it above 2000 rpm it develops a pretty nasty random misfire in cylinder one. And it will have that misfire for about 10 days. About day 5 it starts to run a bit smoother but still a nasty misfire. And about 5 days later it runs nice and smooth again. So if I rev it around 2000 rpm it gets that nasty misfire again.
So I stay off the freeway and don't use passing gear . Then everything is fine the only code it spits out is a random misfire in cylinder one. And when it starts running good again the code disappears. So any suggestions you folks can send my way would be nice thanks

 

check #1 cyl. spark plug, boot and wire. If they are good, check the injector and its' wiring. If that's all good do a compression test on #1. Misfires can be tricky, especially when they're random. The 4.0 is a great engine and worth the time and $$$ for a valve job if needed.

 

Internal shorts are common with a coil rail and can be a PITA to diagnose. Many mechanics are so attuned to this that the first thing they do is replace it when a misfire occurs. I once bought a new set of Champions and had one with a cracked insulator. Try swapping that plug with another and see if your misfire moves to the cylinder you put it in, or just replace it, even with an old one. I've had better luck with NGK ZFR5N plugs in my 2000 WJ, but that's me.

 

I approach a single cylinder misfire as follows, and in this order:

1. Fresh Champion plugs gapped to .035, quality plug wires, distributor cap and rotor. Don't want to be chasing your tail for a simple tuneup.

2. Injector. Move the injector from #1 cylinder to another cylinder and see if the misfire follows with the swap or stays at cylinder #1

3. Compression. Can't ignore the possibility of internal engine issue with 300k. The compression spec for the 4.0 is 120-150 psi, with no more than a 30 psi variation between cylinders.

Good luck and keep us updated.

 

She's back on the road, running as good as ever. It was head gasket blown between 3 and 4. Good for another 300,000

 

Sorry