My 2001 Cherokee Sport (2WD, 4.0L, auto, 158K miles) started running just a tad rough today. The check engine light came on and when I got home found a P0304 code was stored.

This is the second time this has happened in the last couple of months. Same P0304 code. Previously, I replaced the plugs with new Champion RC12ECCs, cleared the code and all was well...until it happened again about a month and 500 miles later.

I pulled the plugs and they all look fine, no wet plugs and no odd patterns. I replaced the plug boots and contact springs about 4000 miles ago and they look like new.

The Cherokee's been really well maintained and has been running like a top until now, no problems whatsoever. The P0304 is the only code...nothing else.

Could this be a bad coil? How would I test for a bad coil? Or should I be looking at something else? I'd like to test and/or fix whatever I can before taking it to the $hop.

Thanks.

 

swap the coil from that cylinder to a different cylinder. if the misfire moves with it, it's the coil.

 

Hmm...I didn't even look at them when I had the rail off to see if they were removable. Guess I figured they were built in to the ignition rail. I'll see if I can take it off again tonight and give that a shot.

 

So.....I took the ignition rail back off...NO, the coils can't be swapped. They're not removable at all.

While I have it off, anyone know how to test this coil rail? Even though it's my #1 suspect right now, they cost over $100 and I don't want to throw that kind of cash at something that "might" be defective.

Thank you.

 

I tested mine for resistance to start. It should be below 5 Ohms.

After that you can take the coil rail off and put the spark plugs in the coil. Then have someone try to start the engine while you have the spark plug touching a bolt or something on the block to see if your getting a spark. But be careful!

The second one isn't the most accurate because it isn't giving you the exact amount of volts. But gives you a place to start looking.

 

Also it could be with something on the air/fuel mixture. Check you EGR valve and see if it is working right. I have even heard of O2 sensors causing a misfire. Check things on that side of the motor because that us just as likely to throw a misfire.

 

I would suggest replacing the coil pack. No guarantees that this will fix your problem but, because you’ve already replaced the spark plugs, I would start with the coil pack. At 158K mile it’s due to be changed anyway (IMO). A resistance check on the coil pack won’t show you any problems unless it is a shorted or open coil. This isn’t likely in your case because you only get a misfire on one cylinder, Cyl# 4. The coil pack only contains 3 coils (even though your engine is a 6 cylinder), each coil is shared between two cylinders. If the insulation is breaking down on the secondary side of the coil for cylinder #4 the spark can ark over to a nearby ground or even internal to the coil. If it’s a constant misfire you can usually hear a popping sound coming from the area of the misfire.

If the coil pack replacement doesn’t fix it, then check the compression on all cylinders and compare them. If Cyl# 4 is lower than the others, pull the valve cover off and check the head for cracks. 2000 & 2001 models are notorious for cracking. Look for a crack between or around the valves.

 

I cleaned up all the contacts and connections, put it back together and cleared the P0304 code a couple of nights ago...and it's been running like new. About 3.5 hours of driving, no problems.

I'm thinking it may be a hot-soak issue since this misfire started after I drove about 45 minutes, then stopped at a store for a few minutes and restarted it. I vaguely remember the same scenario last time I got a misfire code.

Doesn't seem like coils go bad very often and it's hard to imagine one going bad so intermittently. Plus, I replaced the boots & contacts just a couple of years ago.

I'll keep driving it to see if it crops up again. If it throws a different code or gives me a new clue, I'm leaning toward replacing the injectors....they're the originals, never been serviced, have 158K miles on them, are made mostly of plastic...and it gets really hot under that hood.