Hey all.

I have a 2000 non-modified xj with 116k. Recently it's been idling rough at stops and throwing a P0305 code occasionally while coasting downhill.

I've replaced the ignition coil, replaced the plugs and injectors which helped for awhile but today the CEL blinked on again with a P0305 code.

After I did all of this, I took it to a mechanic I trust and he couldn't track down the issue either.

Any ideas welcome.

 

Plug wires?

I'd start with a timing light on the #5 wire to see if it is actually sparking during use. If it is sparking, then the misfire could be caused by fueling or compression issues.

If you do something to prevent fuel flow you can crank it for a longish while to listen if your relative compression sounds even or not.

Using a fuel pressure gauge you can use a tester to fire the #5 injector and watch if pressure drops. Best to compare that drop to another cylinder that is working better.

A full compression test done well should be able to show you if you have a flattened cam lobe on #5 as well as showing up valve problems other than an intermitant sticking valve.

 

**I for sure agree that if your mechanic has not yet done this, perform a compression test on all cylinders. Can't ignore the possibility that an internal engine problem is the cause. The compression spec for the 4.0 engine is 120-150 psi, with no more than a 30 psi variation between cylinders.

**Verify injector "pulse" when this misfire is seen. Use of a noid light can do that for you.

**Verify fuel pressure. The fuel pressure spec for the 2000 4.0 engine is 49 psi, plus or minus 5.

Process of elimination at this point. Finding out what is good is a big part of determining root cause. Good luck and keep us updated!