Resolved Fuel Pressure High at Idle

[Mokkan]

My 1990 AWD has been suffering from cut out at high RPM under full load for a little while and have been trying to find the cause. I've replaced the entire ignition system (every piece tested bad - wow) and the situation has improved considerably. I still get strong cut out as I pull through 2nd gear, so I started testing the fuel system.

Pressure reading at the fuel filter outlet:
32psi with vacuum hose attached
38psi w/o hose

32 is 5 over the recommended 27, but the 38 is spot on. Two different fuel pressure regulators gave me the exact same pressures. Is the 5psi overshoot enough to indicate a deeper problem?

Also, the pressure drops to 0 immediately when the ignition is turned off. Normally, I would think the pump's check valve is bad, but this one is new.

The fuel system is stock and the pump & filter have less than 3k miles on them. I've done a boost leak test and everything looks OK.

Thanks for your responses.

 

[Soccerguy136]

Do you have a manual or auto trans?

There are 3 different fprs on a 1g.

Non-Turbo, Manual (Turbo), and Auto (Turbo).

They all create different readings.

http://i343.photobucket.com/albums/o472/gofer1985/1gfuelpressure.jpg

 

[GRAVEDIGGER]

Fuel pressure seems good. How much boost pressure are you running when it cuts out? Sounds like fuel cut from overrunning the maf to me.

 

[Mokkan]

It's a manual and I am using the factory service manual as my reference for target pressure.

I'm running the stock 12lbs of boost. The only modification I have made to the intake/boost system is Dejon piping.

I'm going to replace the vacuum lines going to the FPR since they look original. A small leak would explain why it under reacts to vacuum/boost.

 

[Mokkan]

I've had time to work on this issue and have more information now, but no solution.

I replaced the vacuum lines going to the FPR and did not see any change in pressure. I tried another turbo AWD FPR and again, no change.

I finally had the opportunity to drive it around (this isn't the only problem I'm working on ) and the the pressure drops to 20psi at WOT. It steadily decreases to 20 by 3500RPM give or take 500RPM. It stays pretty steady after that, and the stumbling starts when the fuel pressure drops to around 22psi.

My first thought is that the fuel pump is bad, but it is a brand new DENSO OE replacement. I have rewired the pump and it is receiving 12v at the pump connector. The filter is new as well. What else would cause such a loss of pressure?

 

[Mokkan]

Finally figured out the problem. The oring that sits at the top of the fuel pump was damaged such that it would hold pressure at idle, but not in any other situation. It feels like a whole new car with a proper oring in place!

 

[lil corb]

Thanks for letting us know the resolution just in case anyone else gets the same symptom! Glad you got it figured out.

 

[jed344]

I was just about to say when it is shut off it should not drop to 0 press it takes about 30min for my gauge to return to 0 so had to be a leak.