This is a bit odd. I seem to have a progressively worsening rough idle. However, the roughness disappears when the car is sitting in park. There is also a very intermittent high speed, full throttle stumble.

I've checked and tightened all the intake related bits, and am about to swap plugs. I suppose taking a look at the throttle body for gunk makes sense.

The odd part is the progressively rougher running. That would seem to indicate something wearing, or possible fuel starvation.

Ideas? Experiences?

 

I would clean the throttle bore and plate; clean the MAF sensor; check the air filter (replace); check the tube from filter to throttle for air/vacuum leaks. Might also clean the EGR and take any slack out of the accelerator cable. Change the fuel filter. If you haven't done so, replace the plugs. If those don't help, start checking the second fuel pump.

It would be useful to know if any OBD II codes are present. Also the year and mileage of the XJR.

 

Sorry, meant to post that info.

No OBD codes are present. The car is a 1999 XJR, 109,000 miles on her.

The throttle body has been cleaned, same with the MAF. Air filter is new. I've checked for air leaks, but haven't tried smoke yet. Nothing has helped so far. Plugs are next.

 

Sometimes the fuel system needs some cleaning. Any of the really good cleaners (Gumout's or Lucas' best; SeaFoam; or BK44 (if you can find it).

 

What do the fuel trims (LTFTs & STFTs) show, in the normal way of checking but in addition when the roughness occurs?

 

funny mine does it only at idle i can idle for like 10-20 minutes from cold (end of this last fall) without even realizing the engine is on there were times were i actually thought it died but its just really quiet and smooth