Hi guys, lately my XJR has developed a lumpy idle intermittently and it is driving me crazy. Sometimes if you shut it off and restart it runs fines but it seems to be happening more frequently. It is throwing no codes, the tensioners were done a while ago and the throttle body, intercoolers and EGR valve were cleaned about 10K ago so they shouldn't be clogged up. It smooths out when you rev it and no misfires are detected. The MAF sensor was also cleaned in the last 5K, no smoke coming out of the tailpipe. Any ideas are appreciated, I'm thinking maybe TB, fuel pump or maybe the supercharger bypass valve.

 

What are STFTs and LTFTs when the vehicle is running rough?

 

Vacuum leaks - fuel filter - clogged injectors - MAF

 

Went through this w/a bogged down fuel filter recently.

 

I replaced the fuel filter recently with a K&N, The injectors were also recently cleaned and installed with new O-rings. There are no vacuum leaks detected with a smoke test, full and part load breather hoses were recently replaced and passages checked for clogs. From my experience with Jaguar V8s even a tiny vacuum leak can throw up a restricted performance and I'm not getting that. Most of this was done trying to solve the random lumpy idle.

I also had new bearings and coupler put on the supercharger and I'm running the 1.5lb upper pulley. It has been doing this weird idle every now and then for about 10K miles (supercharger was done 20K ago). I had recently cleaned the MAF and related connectors but just gave it another shot of cleaner and it has been idling fine for the last few days.

I'm just curious if this is a sign of the throttle body on it's way out or if maybe every now and then the ECM gets confused by extra supercharger boost. The way it comes and goes makes me feel it is an electrical/sensor issue. I love British cars but they strive to be quirky.

 

I haven't tested that yet but I hope scan gauge II can read it.

 

good day if you see my recent post I am going through the similar situation right now sent my throttle body for rebuild hopefully that will resolve the issue permanently

 

Without fuel trims you're guessing and that is costly very fast.

BTW it's not a British thing. It's a modern car thing, as dictated by laws (starting with CARB).

 

Without knowing the fuel trims you're just guessing. Guessing can get very expensive very fast of a Jaguar.

+1 on this having nothing to do with British cars, but with modern cars in general.