Hello folks: My 1998 XK8 has been acting up for several months now. It has always been reliable and trouble free until now. 70,000 miles. My problem began with the check engine light warning accompanied by a text showing Restricted Performance. Codes indicated both cylinder banks running rich. I had the throttle body rebuilt, and replaced the mass air flow meter but still not solved. Measured fuel pressure and found the culprit. The fuel pressure was reading 90 lbs therefore I assumed it's faulty so ordered a new one. In the meantime while waiting for my new regulator to arrive I tapped on the old one with a wrench, erased the codes and since then the cel has not returned. Again I'm assuming the diaphragm was stuck and tapping it may have freed it up. Car runs fine again with no codes. However the pressure reads 60 lbs at idle. Is this going to give me more problems or issues? What should the pressure be?
Thanks

Paul
1998 XK8. BRG

 

I'm wondering the same thing - what should the fuel pressure be on a naturally aspirated 4.0?

 

2.9bar/42psi if i am not mistaken, no doubt someone will correct me if am👍

 

gauge reading of 90 then 60 psi? I'd get the gauge checked as I'm certain that the pump cant reach 90psi ,unless its not a standard item. I'm not even sure it can attain 60 psi?

 

Electric fuel pumps can put out some pressure!!
I remember back in the 1980s when CIS systems were popular with Rolls Royce and German cars we 'pinched-off' the return line and the pressure shot up to 10 BAR (150PSI) with a BOSCH roller pump.
The engine started running RICH and we quickly removed the pliers from the return line!!!!!!!!

The normal fuel pressure is around 36 to 42 PSI on the regulated return type system.
The fuel pressure is about 55 PSI for the 4.2 liter AJ34 and it varies because it is a RETURNLESS system that varies the pump speed/pressure via the ECM and pump control module.

The pump won't do 90 PSI for very long as that is a strain on the pump/circuit.
Pump will burn out or the fuse/wiring will fail.

bob