nowthatsclassic

Here's another update. I'm still trying to figure out this miss, and it's proving to be really hard to get to the bottom of things.

My friend ordered a tool he needed and did a “compression leak down test” on 2,3,4 cylds. They all passed with flying colors less than 7% loss which is great. The down side is again that proves there should be nothing wrong internally with the engine.

The miss is still there and still getting slightly more frequent. 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 wap 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.

This is one of our final guess’s. 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. A friend at the Jeep shop thinks the problem is caused by the valves but every test we've have done has negated that.

Let me know if you all have any thoughts to share to help a brother out.

captainwoot

Not to burst your bubble , but i would be greatly surprised if it is an electrical issue. When you swap or reset the pcm and the light stayed off for awhile it was because the computer takes a little while to learn adaptive values. When it learns these values it can then run a "misfire" monitor and then trigger the engine lamp when it detects the problem.
Maybe Salad will chime in here, but i think the problem will be one of 2 things. Either a cracked cyl head, or a valve issue. I would be tempted to run the compression test again and check results. If you truly have a cyl with 70lbs should be fairly consistant misfire.....
Good luck

redmuck

you got the coil packs on the right plugs? on my v6 dodge, I had a hiccup/miss/tick, after a day or two of wtf's, we finally found out I had two plug wires mixed up.

Radi

Did you check the valve spring(s)? It's a known cause of misfire. I believe there is a TSB out there related to multi-cylinder misfire and bad springs. No reason just one cylinder couldn't do it.

belvedere

Yeah, this caused a #1 cyl misfire on mine. The TSB is for '96-'99, I believe, but that doesn't mean your springs couldn't be bad. Put a vacuum gauge on the engine and see if it's steady at idle. If there's any question about their condition, they make a tool to check springs on the engine, but I've never used one.

BlueRidgeMark

Yes, and Mechanic #2 just doesn't know what he's talking about.


You have a misfire and he tells you to dump the Jeep? He's an idiot.


Since the beast passed the leakdown test, the 70# compression check is... suspect. That might be pilot error rather than a mechanical issue.

Or maybe not.

Still, you need to verify that cracked head scenario with a coolant test. It's quite possible that there is a crack but you can't see it in the coolant with your naked eye.

I'd be chasing that before an electrical problem.

belvedere

This is true. I once had a Triumph motorcycle that would lose a very small amount of coolant. The oil looked perfectly fine, but since the warranty was about to expire, I sent off an oil sample for peace of mind. The lab called: the oil tested positive for coolant. Head gasket replaced under warranty.

PocketsEmpty

I don't know much about the newer ignition system, but this sounds like a BAD move to me, and probably would cause even more headaches for you. Why not just replace the coil packs or something?

Also, you do need to completely rule out the head issue. Just looking at the oil and coolant is not good enough to rule out a cracked head.

captainwoot

Any update??

Denimfake

Picture of spark plug from cylinder #3

Turbo X_J

Better than nothing, just stay clear so you don't get zapped!

nowthatsclassic

my friend matt is helping me work through this issue. unfortunately he's had to work back to back 12 hour days this week, so he hasn't had time to explore the latest recommendations that you all have shared.

thanks for all of your input! i'm hoping to have some additional info to share with yall in the next few days...

PocketsEmpty

Try searching for topics related to "Heat Soak". Basically this is a condition when the manifolds cause the fuel system right above them to get too hot. I've heard about this issue on the newer XJ's, but don't know enough about it other than you might have a symptom of it..

nowthatsclassic

Okay, 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!!!

Parsnip

Question. Is it random multiple misfire or is it cyl 3 solid?