This is an odd one. I'm currently working on a 98 XJ. It has had a random misfire with no codes for as long as I can remember. Originally, Cyl 3 and 4 had low compression, so a new engine was built. The cylinder head was rebuilt as well. Know that when I state rebuilt, I mean everything, but the rods were replaced. It was stroked and bored. The cylinder head came back with a de burr, new valves, and a light porting. The fuel system is completely new, as is the sensors, manifold, and throttle body. This thing still has the same misfire! I even swapped the PCM to no avail! Fuel pressure and voltage is at spec. I added block to frame woven grounding straps and confirmed ground on the battery and block. Ignition cap, wires, and rotor are new. Distro is original, but shows no play.any ideas?98 4.098 4.0​​​​​​​

 

A little more information is needed. Does it always misfire? Does it misfire only when hot or only when cold? What's the scenario here.

 

Was the engine a stroker before rebuild? Do you have the correct sized injectors..My stroked 98 has a long history of misfires until i learned that i screwed up the math in the injector size calculations and i was running too lean. Also many sellers mislabel the injector flow rate so do a several source comparison to verify the injectors have the proper flow rate for your rig

 

It was stock before the build. The misfire was a constant random then. The build came from Clegg Engines as a Stage 1 with a few upgrades. The random misfire is identical to what was prior. Temperature has no effect. It baffles me that even though I see misfires on live data, no CEL is generated.

 

I have the same problem with my 99 4.0. No solution found so far.

The only thing you did not change is the wiring harness. Especialy those cables over the intake manifold are a PITA. They are 25 years old and brittle because of the heat from the exhaust manifold. I am currently rebuilding the harness from my Jeep because the cables were completely baked together. Might be the reason why there are some gremlins in the electricity. All major engine sensors and injectors are affected by this part of the harness.

However, with the facelift model, removing the harness is not as hard as with the pre facelift.

Regards,

​​​​​​Johnny

​

 

I had a new spark plug with a tiny crack in the insulator. It arced to the cylinderhead just under the sparkplug boot. Could see the spark at night when it was very dark, and you could hear a very very faint tick at idle if you listened closely, a faint faint tick each time that bolt of electric shorted to the head. New plugs mean nothing if it cracked.

Also,have you examine the sparkplugs for color and carbon build up, and gap? That can be a clue as to what is bad, identify a dead cylinder, etc...

 

I like the wiring harness thought. I can unwrap and check for chafing. Am I correct that the distributor houses a hall effect sensor? What is the likelihood that could cause my symptoms?

 

Don't know if it's a hall sensor but it shares the same ground with all other sensors. Also the ones over the manifold. Checking the harness could be worth your time.

Also, when you are already working on the harness it my be a good idea to install "diagnostic" wires. Just add leads from the sensor signal wires, ground and 5V supply to an additional plug outside the harness. This way you can tap in with a multimeter later on without messing with the sensor wires themselves.

Regards
Johnny

 

Did you use anti seize compound on the spark plug threads? It is a long shot but if you did clean all the anti seize from the spark plug threads and head. Many anti seize compounds are not good electrical conductors. The spark plug body thread to head is part of the electrical ground path and if not good it can cause spark plug performance problems. Most spark plug manufactures specifically say not to use anti seize.

 

I do not apply anti sieze to spark plugs. The manufacturer has a chemical already applied to the threads. Additionally, this has been prevalent prior to the new engine being installed. I'm tearing down the engine compartment wiring harness to visually inspect for chafing.

 

It seems that there are a large number of "rough idle / misfire" complaints on these vehicles. Is there any commonality to the symptoms?

 

Yes, the engine is designed to run hotter than almost anything produced in modern times. That extra heat isn't friendly to ANYTHING in the engine bay.