Hi I have a 2001 XKR 62K miles. I have fairly high negative LTFT -7.2% and -10.2% STFT are negative 1-2%. So running Rich

So possible causes are MAF/Fuel injector jamming (did BG44K 2yrs ago)/Fuel Press Reg/Blocked CAT

MAF sensor is ok (changed 2 yrs ago i.e. 4K miles ago) air filter & fuel filter & both fuel pumps changed recently also. I would like to check the fuel pressure regulator is there a way to test that? Can I just check the fuel pressure? If so what should be the correct value across the rev range.

Does anyone have the MAF values for the XKR at idle and 2.5K revs e.t.c

I'm getting about 12.5 mpg and no codes yet P1111

 

The fuel pressure is the right thing to check and the pressure (I think) should be about 45psi at idle. As I recall, the 4.0 is a little different than the 4.2, so I'm not 100% sure what the value should be.

The fuel pressure regulator has a manifold reference because it tries to maintain a constant differential pressure between the fuel rail and the intake. At low load when there is high vacuum absolute pressure will go down, and under boost the pressure will go up on your external pressure gauge.

Since it's rich rather than lean, you might check the regulator manifold vacuum reference. If that is disconnected or the tube damaged then the fuel pressure will be too high (regulator is referencing atmosphere rather than manifold vacuum) so the fuel trims will try to compensate by going negative.

 

Ok so bben driving the car to work to get some miles on her as its been sitting for a while (I do about 1500K a year) now trim are looking better. LTFT Bank 1 is -0.78% and Bank 2 is -2.78% so still on the rich side and one bank probably has issues with an injector. Bank 2 has always been 2.5% diff to bank 1.

I will try some fuel injector cleaner at some point and see if that helps any.

 

If under about 5% they're good enough IMO.