Hey guys got this XJ40 as a daily and oil sensor went out so I replaced that to be on the safe side. Once everything was reassemble car began to run rough idles at 1k to 1500 then surges dramatically to 3k rpm. I cleaned up the throttle body but no improvements. Couldn’t find a way to adjust idle. It’s also throwing a fuel fail 12 code but unsure of what could have caused it to throw the code. Sounds like it’s cammed but unaware of what to look for to make an improvement.
Any suggestions is greatly appreciated. Cheers.

 

You need to find and download the S91_AJ6 4.0 Engine Management.pdf for explanation and diagnosis of the AJ6 ECM.
I originally got it as a paper document in the 1990s at the dealer but Jaguar did offer it as a .pdf in the early 2000s.

The file is too large to include in this post but Gus will have it on his site because I sent it to him decades ago.

 

Code 12 is the MAF sensor. Check the pins and plug. Oh and as motorcarman says -post your VIN or at least the model year, big differences across the range

 

91 Jaguar XJ6
VIN SAJFY1746MC638003

 

My apologies bout that. This is the condition of theMAF sensor atm.

 

Check all the air intake hose connections and vacuum lines, sounds like you have a vacuum leak. The big corrugated hose to the throttle body can get pinched easily, check that at the bottom too. Overtightening the other end can collapse the plastic elbow also letting unmetered air in.

 

Turned out to be an intake manifold gasket. Air was seeping through heavily replaced that all good now!