Hello everyone,

Did not post a lot of things so far. I was more a frequent reader of old posts and nearly everything that was wrong with my Jeep could be solved this way. The forum was very helpful.
Now i have a topic where i dont know what to do anymoere,. I have been chasing an issue with my '99 XJ 4.0 for 2 years now. I read a lot in the forum, tried everything i found but did not manage to solve it.

My Jeep has a slight stutter when idling. Its not significant but its getting on my nerves. Engine holds idle rpm, no codes are shown. But every few seconds i get a bump that me and passengers definitively notice. Vehicle shakes, steering wheel shakes, then back to normal. Seems like a misfire or something like it. Car is equipped with a LPG system. I can drive the vehicle with LPG as an alternative which is cheaper than fuel. Switching between fuel and LPG does not make a difference.

When i rev the engine in neutral i can still feel a slight stutter. But when i drive around (you could say when engine is under load) everything is fine.

Other symptoms: Engine starts perfectly when car was parked over night. When i park it for a few minutes after driving around and want to restart its harder to start. Sometimes i have to give it a little throttle to start it.

What i have done so far:

• CPS new (MOPAR)
• TPS new (Mopar)
• IAC new (Standard)
• cam sensor new (Standard)
• fluid temp sensor new
• cleaned throttle body
• new throttle body gasket
• new distributor, cap, rotor,
• new plugs (champion)
• new plug wires (blue streak)
• new ignition coil (tried 2, blue streak and NGK, no difference)
• new Oxygen sensor pre cat (tried 2, NGK, Denso, no difference)
• new oxygen sensor post cat (NGK)
• new cat
• new muffler
• new head gasket (fel-pro)
• installed other MAP sensor from junkyard unit. no difference
• new lifters and rocker arms (correctly adjusted to lifter preload of 3/4 turns)
• new manifold gasket (fel-pro)
• cleaned all ground points
• Compression Test -Cylinder 6 to 1
  • 130 PSI
  • 140 PSI
  • 120 PSI
  • 140 PSI
  • 150 PSI
  • 150 PSI
• new alternator (ACdelco GOLD)
• new battery
• checked for vakuum leak with a smoke machine. all good.
• cleaned big connectors in wiring loom.
• new crank case ventiliation valves and elbows
• new engine mounts
• replaced all injectors with reman 4 hole injectors from Ksuspension
• dumped two cans of cumbustion chamber cleaner through the intake

The last thing i did was the alternator because last week i noticed a drop of charging voltage to 13.4V at idle. I thought "finally i have it" and put a new alternator in it. Did not make a difference at all. Charging voltage is now around 13.8/13.9V but is not at 14.4 what i would assume to be the rated voltage. Maybe it has something to do with the problem.

Vehicle does not overheat (cooling system is all new), oilpressure is good, after 3000 miles oil still looks like new.
Average fuel consumption as around 14 mpg.


However, i am out of ideas. Any suggestions or help is higly appreciated.

Greetings from Germany,
Johnny

 

Done a fuel pressure test?

 

What happens to your ignition advance when the issue occurs? Obd2 reading and timing light.

 

I see a 120 psi did you have the valves checked when you did the head gasket ?

 

Thanks for the answers.

Good point with the fuel pressure and ignition advance. I will check both.
Ruled out the fuel pressure because symtoms are the same when running on LPG (Liquid Petrol Gas) and this is not affected by the fuel pump. Will check anyhow.

Valves were not checked when the head gasket was done. Maybe that was stupid not to do it. Garage said the valves look good.
Before i go that road again i want to check all other options because thats a lot of work.

 

Well you have replaced almost everything you could. Even got Mopar CPS and TPS!
Replace the ECU and you're done!

Gapped the sparkplugs?
Just trying to think for more/other causes here..

 

Yep sparkplugs are gapped to .035

i also was thinking about the ECU. Need to source a US one that does not require the key.

Is there any way to check an ECU if its bad?

 

Update:

Checked the ignition advance with an OBD bluetooth adapter.
At idle its between 14 to 17 degrees. Sometimes 11 degrees. During normal drive its between 33 to 37 degrees.

Throttle position at idle ist at14.5 degrees. Is that normal?

Regards,
Johnny
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With the stock lift and 235 tires shown in your profile 14 mpg is pretty bad, depending on how/where your driving it

Might be interesting to see what a vacuum gauge shows. You aren't getting a misfire code but if the miss shows up on the vacuum gauge at least you would be certain it is some sort of misfire rather than some mechanical weirdness.

 

Timing light

 

Are you happy there's no arcing between plug leads? I had a tricky misfire at idle once and it was down to that.

 

It took a while to get the tools and to perform the tests. Sorry for the late reply.

Fuel pressure is steady at 40PSI with the engine off and ignition on. With the engine running it is at 45 PSI. I think that is fine.

I am pretty sure that there is no arcing between the plug leads. The wires are brand new and have been replaced 2 years ago already. Did not solve the problem. I dont see arcs in the dark and i checked for enough distance between each wire.

Vacuum is in the green zone at idle but not as steady as i hoped for. I made a video. You can see that the needle is slightly flickering. It moves at the same time the engine makes the slight bumps.https://youtu.be/eI2brMk3ShQ

Does that mean there is a misfire?

Regards,
Johnny

 

Something else.

Any recommendations on how to place the timing light withing the engine compartment to reach the scale on the harminuc balancer?Its very tight in there.

 

Remove the E-fan

 

It’s a pita, but can you check the exhaust manifold for cracks? Any fresh air getting in can really goof on the O2 sensors.