We have a 2000 XJ 4.0L with ~160,000 miles and it is repeatedly throwing random P0300, P0303, & P0304 misfire codes. The heep seems to be running fine and starts fine. It just throws these codes and I clear them out and they come back. We bought the heep with 140,000 miles so not sure of when it had a tuneup so I figured I would start there since it could probably use it.

1. Give the throttle body a good cleaning.
2. Plugs & wires - what are good replacements for a stock 4.0L?
3. Is there a good coil rail I should get?

We will start with the above items but doing some reading, it sounds like it could also possibly be the crank position sensor. I am dreading having to replace that since it sounds like it will be a pain in the you know what to get at....

Your input is greatly appreciated!

Thanks in advance - Mike

 

Hate to say it but it could be a cracked head. You having unexplained coolant loss? Milk shake oil under the cap?

Change the plugs first before you replace your coil pack. Champion coppers gapped to .35.

 

Its not that much of a Pain. However that P0300 would make me suspicious of the Coil. http://www.jeep4x4center.com/ignitio...6041476aa.html

 

Haven't noticed any obvious coolant loss but I just put a permanent marker line on the coolant reservoir (cold) and will keep an eye on it. Oil looks fine and no milk shape on oil cap.

I am going to start with plugs and see if that remedies the recurring random codes we get. If not, maybe the coil rail next?

 

No wires on the 00 and I would rule everything else out before replacing the coil pack. Start with checking/cleaning your grounds first.

 

Well I finally got around to working on the XJ and started with replacing the plugs. The plug tips were really bad. The gaps were worn to the point that the gap was probably double to triple the .035" spec'd gap. I did not do a compression check since time was not available to me. I figured since the plugs were so bad that I'd just replace them and see if I still get the cylinder misfire codes.

 

Nice, hope that solves the misfires.