nowthatsclassic

I have a post elsewhere on this board, but it's getting a little confusing, so I decided to break out this particular question into its own separate post.

My 2000 Jeep Cherokee has had a mysterious misfire since the day I bought it from a guy on Craigslist. Originally it was throwing a P0303 cylinder 3 code.

After trying literally everything that I can think of, 2 mechanics in Brooklyn could think of, and that my friend that is a Jeep expert back in WV can think of, we're all freakin' stumped.

A friend at the Jeep shop thinks the problem is caused by the valves but every test we've have done has negated that. My friend ran a compression lead down test on 2,3,4 cylinders and it passed with flying colors. Coolant is good, oil isn't murky. We're thinking there is nothing wrong internally with the engine.

The fuel injectors still pulse and since everything on the fuel side has been switched out (to rule out that the problem is on the fuel side) we still think that the problem is electrical with the ignition somewhere.

Here is what we're going to try next: we're going to swap out the ignition system with the older style, 97-98. They use a distributor instead of the coil rail. My friend has 90% of the parts laying around his shop - all he'll need to test it is a 98 model cpu, which is around 75.

It's out of the box thinking but my friend studied the wiring and it will eliminate the coil rail and wiring as a problem.

If this does NOT fix it, I really don't know what to do. Let me know if you all have any thoughts to share to help a brother out!

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Original post (for reference): https://www.cherokeeforum.com/f2/jus...-p0303-182259/

prcherokee

How did you check the coil pack?.

rompnXJ

wouldnt it be simpler and cheaper to replace the coil rail..?

nowthatsclassic

Yeah, we've checked the coil pack. My friend swapped it out with a coil rail from his Cherokee since it's 3 coils in one. As far as we can tell that's not the issue (my coil rail works fine in his Jeep too). But I do think it might be in that wiring...

belvedere

See the other thread for thoughts on valve springs.

Cherockee

I wasn't sure which thread I wanted to post in but I would ask something that hasn't been answered yet (at least not that I could find); is the mis consistant? Meaning, does it never fire? Or does it just miss every once and a while?

I have seen many people chasing their tails checking and replacing everything under the hood pinning thier hopes on the head not being cracked, only to be dissappointed to find that after they spent all that money on other things they still needed to buy a new head and have it replaced. You might want to take more than a few minutes to stop over looking the possibly obvious problem- a cracked head. Just a word of caution.

PocketsEmpty

x2. Don't do the distributor ignition conversion, you're gonna have a bad time.

jomps

you said it may be ignition, what about the camshaft position sensor...i got this one here for sale if you think you may want it

nowthatsclassic

Hi everyone,

I ended up separating my Jeep Cherokee misfire issue into two separate threads because I wanted the forum's opinion on swapping out my 2001 Cherokee ignition with a 97-98 ignition.

Either way, decided not to go down that route. My friend helped me out this weekend and tried a few more ideas, but unfortunately the misfire is still persistent. We've literally run out of ideas.

Below I've posted the latest update. If anyone on this thread has any idears to share, please send them my way!

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I have an update to share, and unfortunately it isn’t a good one.

My friend Matt and I have tried everything and sadly we’re not able to figure out what is causing this phantom miss. Overall the Jeep is in excellent condition, which is what makes this problem so frustrating.

Right now I need to figure out if this problem can eventually be fixed, and if so, how much more money I’m prepared to put into the thing. As of now I’m tempted to cut my (huge) losses.

Here’s a quick status update on the most recent things that were tried over the weekend:

• Checked and swapped valve springs with known good ones in my friend’s shop on ,3,4 cyl.
• spun (seated) 4 valves on 3,4 to ensure good seal and no buildup of carbon
• leak tested 3.4.5 cyls. All with passing #s! In fact problem cyls had better #s due to seating of valves.
• compression test for 3rd time…still great at 170lbs
• Set up and used jumper wires to go back to distributor. Check for spark on 3,4. Had strong spark. No change in operation (eliminating wiring, coil rail, electronic side problem)

Here’s an updated list of everything that we tried in the past:

• fixed oil pump housing leak
• drained and filled differentials, rear looked decent, front needed changed pretty bad.
• need different size sway bar bushings( will go back to parts store)
• fixed 4x4 indicator panel on the interior.
• found faulty Idle Air Control (IAC) sensor, had one laying around shop, swapped in, good now!
• replaced fuel injector plug
• swapped with known good coil rail
• swapped with known good throttle-body and sensors
• swapped fuel rail with injectors
• replaced #3 injector with known good one
• swapped 2,3,4 spark plugs
• ohmed wires for coil to pcm
• ohmed wires for injectors to pcm
• ohmed injectors (injectors fire while missing, eliminating fuel side problem)
• uel pressure test, passed 45#
• ohmed coil rail
• compression tested #3 again...still 165 as last time.
• swapped crank sensor (still on jeep), ohmed crank sensor old and new
• swapped cam sensor-orginal in jeep, noted above.
• installed used 635AG cpu (same as one before)

So, at this point all essentials are supposedly working (spark,fuel,compression). Everything I have done points to a internal engine problem, but all test for that pass with flying colors. Short of pulling the motor out and rebuilding it, or replacing whole wiring harness, etc, I really don’t know what else to do.

I don't want you to waste any more $$ knowing that it may or may not work. Hit me up with what you want to do.

If anyone out there has any ideas or suggestions to share, please let me know. I’m all ears!!!