Check engine light coming on intermittently, but each time it comes on the revs drop right down and if accelerator is touched revs come back up for another few kilometres. It is getting worse and worse and less time between check engine light and revs dropping.
Have replaced ignition coils, spark plugs. Have checked injectors, all passed A1, checked fuel pressure - okay, but fuel consumption getting worse and plugs sooting up from 50 to 100 km so badly have to be cleaned. I have exchanged the ECU with another with same numbers, shows no difference - maf airflow also been changed to no apparent benefit, lambda's have changed, but don't help. This car has done about 180 thousand km and this problem came up about 4000 km ago. Have also changed fuel filter, fuel pumps, inside of petrol tank is perfectly clean.
Can somebody please help me with this because whenever I have looked up check engine light on the internet, none of them have the same symptoms of the rev counter dropping right down.
Hopefully I can get to drive my beautiful car again.
I am a jag mechanic but have not worked on jags since V12s
Desperate Dave

 

Codes ?

 

One possibility, your Jag's fuel pump is only working intermittently. You might want to have a reader hook up to view live stream for fuel trim when this occurred.

 

With the super charged engine you have a # 2 fuel pump that normally does not come on until 4000 RPM

If you can't read fuel trims you can jumper the # 2 FP relay and test drive it

In jumpering relay socket it is hot at all times so will drain battery once exiting car with key

Easier try is to swap # 1 and # 2 relays

 

Catalyst collapsing?

 

Glad I appreciate same cars as you. What exact codes do you mean?
Thanks.

 

Thank you for your suggestion, but I replaced both fuel pumps with American-made, fairly recently Have also had fuel pressure checked, all a okay.
Thank you for your advice

 

The car is not here at present, but I will most certainly try your suggestion, because I always thought it was a fuel problem and that is why I replaced fuel pumps, so I will try your relay ideas - which are very much appreciated.

 

Catalyst were all removed about 35,000 miles ago, with no ill effects.
These are not necessary for warrant of fitness in New Zealand.
Thank you for your suggestion

 

Have you checked or replaced the crank position sensor on the front of the engine? From what I have read on this forum it can give intermittent problems like you describe. I replaced mine at 100000 miles as a matter of maintenance. You may even want to check the sensor wire and connector first, as they get very gunked up.

 

 

 

 

Hi Dave,

As Lady P and others have recommended, the fastest route to a diagnosis may be to have the car scanned for stored Diagnostic Trouble Codes (DTCs) via the On-Board Diagnostics (OBDII) Data Link Connector (DLC). I don't know if the same is true in New Zealand, but in the U.S. most auto parts stores will scan your car for free. Any issue that can trigger the CEL should also trigger one or more DTCs. If you will post any of these codes that are stored in your car exactly as they appear (the letter P followed by four numerals, typically), our members will try to help.

Cheers,

Don