Hey All,

I recently replaced my secondary tensioners, valve cover gaskets, spark plug gaskets, and spark plugs. I also repaired my abs module by soldering the joints. My problem is now the car (00' XJR) stumbles at high speed. When doing about 80 mph, I punched it and it started missing really badly, but doesn't do it at low speed.

Any ideas?

Oh yeah, I installed new NGK iridiums. The spec called for them to be gapped at .052", which I thought was a bit much.

 

I just had my 99 XJ8 in the shop last week for scensor checks, and stability contol and traction control failure. When they got all that fixed, the performance limit was acting up.

They said that a Knock scensor was sending a false reading and skipped a valve all together. He cleaned the fuel filter, and injectors. He also changed a bad knock scensor. It doesn't have that problem anymore.

Start small. 93 octaine fuel, new fuel filter, diagnostics check on the knock scensor.

 

Just a couple of thoughts-
Are you absolutely sure you did the valve timing by the procedure? Are you sure you did not distort the flywheel sensor wheel by using the peg to hold the engine while torqueing the upper emd? Is the crank sensor in right? Other than than, I would look for loose plugs or hoses.
Are you getting any codes?

 

Turned out to be the fuel filter. The pressure was too low at higher speeds and a new filter corrected the problem.

 

Did you have to disconnect the brake lines to get the module off or were you able to get the module past them without disconnection?

Thanks,
Dan

 

I had to disconnect the brake lines.

 

Crap! Car started to stumble again. I'm beginning to think the NGK's I installed are junk!!!

Now the car misses while idling... Anyone else try the NGK's?

 

I have the NGKs and they work fine

 

Thanks for the disconnection info.

Have you checked for air leaks in the air duct from the air filter to throttle body? Cracks in the air duct?
What do the codes say now?

 

There are no codes present. I did however find a crack in the full load breather hose. Ordered a new one last night. Hopefully this will cure the problem.

 

NGKs are great plugs. Breather hose will definitely cause the poor idle and performance woes you describe. The engine needs to be totally sealed.

 

This is driving me crazy. I replaced the hose, but now the stumbling/missing happens when the car is put into gear. Is it possible for the s/cto be casuing this?

 

Well, the supercharger to intercooler plenum seals are another known induction leak point. They are sort of a pain to remove and inspect, but you might find a leak there. what are your LTFT and STFT doing?

 

If you are getting an air leak you should be getting some codes. If no codes are present then your fuel pump might be starting to fail. Check your pressure at the fuel rail. When reading your post I thought of the fuel filter right away but you said it's been changed out and the problem got worse.

 

Turned out to be the spark plugs. I gapped them at .052" instead of the .035-.039" Pulled the plugs and re-gapped. All is well again.