Fuel pressure with FMU? (SN89)

[Polecat_Racing]

I have a 89 Mustang LX with an SN89, Paxton AFPR, 24 lb/hr injectors, and FMU.

The aformentioned parts were installed by the previous owner and I've been slowly but surely undoing his installation and tuning mistakes.

Today I finally got around to checking the fuel pressure at idle and it was 62psi

I dialed it back to 40 psi and the car seemed to idle slightly smoother. My question is why would the base fuel pressure need to be that high? I don't have any info on the FMU, but it looks like it has a small release valve/knob on it.

Any thoughts are appereciated. thanks

[speedytang]

The FMU unit on that model can be adjusted for how soon the WOT pressure will spike.
I myself would take the belt off and get the car running excellent before doing anything else. When it is running well then put on the belt and start tuning. To adjust the FMU use a hand vacuum/pressure pump and set the screw so that it opens when your at 1-2psi

[Polecat_Racing]

Thanks for the reply.

If I disconnect the SC belt for tuning purposes will I have to put the intake back to a stock style set-up for the car to run correctly? (ie: remove the SC from the intake tract)

[brian88lx]

No you dont. You can just unhook the belt.

[speedytang]

Like brian88lx said just remove the belt but also thinking it over make sure the plugs are gapped at .032-.035.

[Polecat_Racing]

Thanks again everyone for the suggestions.

I just installed a permanent fuel pressure gauge and took the car for a ride with the S/C reconnected. The base fuel pressure is 42psi.

Under acceleration (not under boost) the fuel pressure slowly rises. Is this normal?

Also, as boost increases, the fuel pressure immediately spikes to 50psi and stays there. I didn't go any higher than 3psi boost at this point but it didn't appear that the fuel pressure was going to increase higher than 50psi.

Does this sound like the FMU is functioning correctly? What is a safe fuel pressure reading it should increase to at 5psi?

The car is running excellent at this point, but I'm afraid of it going lean under boost. I want to acheive an initial safe setting on the FMU so I can put it on a dyno and further fine tune it with a wideband.

Any help would be greatly appreciated.