Hello, I'm trying to troubleshoot a hesitation under mid-throttle acceleration on my 2002 XJR, so I took it for a drive with my Ultragauge hooked up to display the MAP reading, MAF flow, and fuel trims.

The absolute manifold pressure reads 14.65 PSI with the engine off. It reads 6 PSI at idle, even lower on overrun, and around 12 PSI under mid-throttle, which is where I have the hesitation. I read online that 17 to 20 inches Hg of vacuum is normal at idle, and I have about 17.3 by my reckoning. Is that normal?

The fuel trims are varying from -4 to zero at idle, and from 2 to 10 under load. Long term trims are reading about 8. Both banks are quite similar.

The MAF flow is about 6 gm/s at idle, and I saw up to 50 when accelerating.

Does this all seem normal?

Further to my earlier thread, I took it for a drive with a fuel pressure gauge hooked up, and it was around 38 PSI, rising a little under load.

 

I'm chasing the same problem with a 2002 XKR100 Coupe. The car accelerates quickly then all of a sudden holds back. Full throttle seems to override the problem but I think I'm about 90HP short. I bought an Auto Engenuity program , but the new laptop promptly died. I want to check fuel pressure at full load, so how do you check this? Thanks....AT

 

I hooked a fuel gauge up to the rail, with a long enough pipe that I could hang the gauge out of the side of the hood, where I could see it while driving.

 

Thanks for the info. I keep hearing 42 PSI is the sweet spot.

 

WOT g/s should be ~280 if everything stock.


ECU manual says:
"Fuel pressure, measured on a test gauge, will vary between 2.7 bar (39 psi) at idle to 3.1 bar (45 psi) at full load
to compensate for intake manifold absolute pressure. ".

 

So I think in effect the rising fuel pressure with load is intended to enrichen the mixture during acceleration.