Hi
I own a 1990 F-350

I have been working on this truck for at least 3 weekends in a row. this truck once ran for about 5 minutes then quit runing. I dont understand. So I performed a fuel pressure test and came up with 5psi. I know thats way below specs so i decided since i have new parts around the shop i would install them. I installed a new fuel pump relay switch and a new fuel pressure regulator and still the fuel pressure is at 5 psi. Would anyone please help me out with the problem because i do not understand now where to go and both of the fuel pumps are fine they both work properly also I have installed a new fuel filter aswell any ideas please help me thanks Jaeger

 

well the fuel pumps cant be fine if you have replaced all other hardware with no change in pressure. you should have 38lbs at the rail. Unless you have some injectors that are stuck open but if that was the case you would have fuel in the oil probably.

I would install at least 1 new fuel pump and see if you get a different result.

 

Jaeger, hard to say for sure, but you should be able to find the problem fairly quickly. The tank pump picks up the fuel and supplies it to the frame mounted pump which boosts the pressure to the amount that the engine needs. The pressure regulator blocks the flow of fuel back to the tank until the pressure reaches a set point. If it fails open, then the pressure never builds up.
So, either you are never getting the pressure in the first place, or it is going somewhere. A bad booster pump might sound good, but not be pumping enough pressure. If you hook a gauge to the fuel rail, and then turn on the key, you should see the pressure come up. If it never does, then chances are the pump is bad. If it comes up but not all the way and then goes right back down, then it is going somewhere. If you do not see it running out on the ground, then it is either going back to the tank through a bad pressure regulator. ( even new ones might be bad) or into the engine through a stuck open injector.
See if the dipstick seems too full and smells of gas. Or, pull the plugs and see if one is wet with gas or you smell gas fumes coming out of the spark plug hole.
In really bad cases, you might notice the engine bogging slightly when it is cranking at one point because one cylinder is slightly hydrolocked with liquid gas. All those would be sure signs of a stuck injector. If so, DO NOT crank the engine, after fixing the injector, until you have changed the oil and filter, since the oil would be diluted with gas and would ruin your bearings shortly
My bet is on a bad frame mounted pump.

J.

 

Sounds like you are not grounding Pin #6 of the Test Connector with the key on.
You must do this to test the Fuel Pressure as the fuel pump will only run from one to two seconds after you turn the key to the run position.

Here is a diagram of the test connector.



Here is a diagram of your system.