Low Fuel Pressure 10PSI
 

I have an 87 944 NA. I did a fuel pressure check at the end of the rail and got 10psi. After stopping the fuel pump the gauge looses pressure almost instantly. Need some suggestions. I was thinking maybe an injector stuck open... I was going to try doing a pressure check before the fuel damper by removing the supply line and running the pump. I did the test with the DME removed and jumped so the pump runs when the key is in on position. I got the car as a non-runner that had fuel supply issues. I've been working through them starting at the back and replaced everything with new Porsche parts from tank filter to hard lines. Original in tank filter was in shreds when I pulled it out. Also stripped the plastic tank filter and had to go with the custom in-tank boss replacement as I wanted this fixed right with no worried of leaks from epoxy leaking.
So again, what I've done is replaced intank filter, fuel pump, fuel filter twice, and after talking to an expert checked the fuel line pressure which registered 10psi after running the pump for 15 seconds. The engine has been running but only if I start it with the pedal floored and then very poorly and it dies if I let up. There has been white smoke out the back and oil accumulated at the tailpipe but that may have been from the wet compression tests I did which ranged from 145 to 160psi. So I'm not worried about rings or blown head gasket. Any advice on what to do next? I've thought maybe it's a leaky injector or pressure regulator or bad fuel damper.

fuel pump check valve faulty? (backwards?)

fuel pressure regulator (on the return side of the rail) could be hosed just letting all the fuel bypass.

I pinched the return line after fuel regulator and the pressure was the same.
 

There's no way you're losing 30psi through a leaking injector. If it's not your regulator, then you have some kind of obstruction in the fuel lines. Check for kinked or crushed lines. Or something back at the filter area. Maybe even the screen in the tank.

Yes Injector in Cyl. 1 is leaking
 

Ok, so I pulled the fuel rail up and put the injectors inside 4 little glass spice jars and ran the fuel pump by jumpering the DME relay socket and turning the key to the on position. In jar #1 which is in cylinder 1 there was fuel draining into it. Not much, but it was definitely leaking. That's probably the weak link in the fuel line that returns the gas pressure to zero (I think) (I guess it could be a check valve in the fuel pump, but it's a new pump and the check valve was already installed.)

Next step is this, I will put the jars under the injectors again and crank the engine (with the ignition wire disconnected) to see if there is good spray in all the jars. I'm thinking at this point to just replace all the injectors at once and be done with this, but I want to check the results first. After that I may try measuring the psi of the supply line fuel before the fuel damper because it doesn't make sense that I have only 10 psi and only one injector is leaking slowly. So where else could it be clogged? I did have a destroyed in-tank filter, so it could be dirt and crud got through the lines but I'm making guesses at this point. I need to pull the supply line and check pressure again. Not sure where the low pressure begins so I have to work backward and figure it out.

you might have to cobble some fittings together, but see if you can figure a way to measure fuel pressure at the outlet of the pump (back of the car) since you're only seeing 10psi at the rail (front of the car).
might help find a potential clog.

How about disconnecting the fuel line at the fuel rail and disconnecting the other end at the filter. Using compressed air at the front (fuel rail side) blow out the line to see if you have an obstruction. While you are at it, follow the line all the way back and inspect for kinks or damage. Do the same for the return line as well. If that is clear, put a bucket under the filter and turn on the fuel pump to ensure flow from the filter. You may find your issue in one of these areas.

Here's another thought-

What is the voltage AT the pump when it is RUNNING?

Found the problem...
 

I think you are right about the fuel pressure regulator. I took off the throttle control last night and was looking in the intake manifold thinking I could clean it up a bit when I saw there was about 2" of gas sloshing around in there. How did that happen? Still working on a solution, but looking like faulty FPR is the problem. I'll be looking to replace that next. Has anyone had an intake manifold full of gas? How does it get in there. Last time I smelled the oil it smelled like oil, no sign of gas. I'm also not getting any ohm readings on the Throttle Position Sensor so I'm thinking that's bad too. Pretty much replaced everything fuel line component so far, down to all injectors, damper and regulator. (Regulator is next) Thanks for your help.

Bad Fuel Pressure Regulator, no fuel pressure after pump is off.
 

I got the pressure up, line was pinched in back. The other issue was the instant drop in fuel pressure from 35psi to zero in about a second flat. I have a new fuel pump, so now I'm thinking it's the Fuel Pressure Regulator that is at fault.

Loosing fuel pressure after shutdown indicates your check valve is bad. It's back at the pump, I would replace that before the regulator.

Still want to change things.
 

Did the new fuel pump come with a new check valve, they're two separate parts and part #'s?
I don't know what's causing fuel to get into your intake manifold but a bad check valve is a common cause for fuel pressure to drop after shutting down the car.
Still not a bad idea to replace the regulator and damper if original.