countryhick

I recently bought a 1991 f150 5.0 E4OD 4x4 single cab short bed. Its pretty hard to start cold. TPS and ECT seem to be functioning properly. Timing is set at 10 degrees btdc. When I turn the key on, I hear the fuel pump kick on, but I have to cycle the key several times to build up any pressure. When I get it running, the pressure hangs out right at 30psi. I have changed the fuel filter. Any thoughts?

justintendo

30 psi is low. more importantly, what does the psi do when driving? under load?
even still, 30 psi is low sitting still.
pump is likely weak/shot.

fltdriver

Yep, sounds like it's time for a new pump. I would just check the FPR first to make sure it's not stuck open or has a hole in the diaphragm, both of which can also cause low pressure. Easiest test is to just pull the vacuum line, if it smells like gas then you have a problem, apply vacuum to the FPR and it should hold vacuum. If the FPR test good then your pump is the most likely culprit.

jdinner

In addition to the above post. Leave your gauge on the fuel rail and slowly pinch the return fuel line (smooth jaw C-clamp) to see if the pressure increases. If it does, change your regulator. If it does not increase, change your pump.

countryhick

How do I test that the FPR will hold vacuum? I've never done any vacuum testing and I don't have any kind of vacuum pump.

jdinner

I wouldn't worry about that because the vacuum is used to close down the flow of fuel back to the tank.
High Vacuum - Idle - Low Fuel Flow and Low Pressure.
Zero Vacuum - Wide Open Throttle - High Fuel Flow and High Pressure.

Removing and plugging the vacuum line should give you the highest fuel pressure on a properly functioning pressure regulator. If the pressure regulator is stuck in one position, the test method of clamping the return line will determine if the regulator is bad.

countryhick

I didn't have anything to close the return line off, but I did take the vacuum line off to see how it would act. Pressure was running about 29psi and when I took the vacuum off, it would go up to about 31-32psi. This was at idle. I couldn't hardly get the rpms up with the vacuum line off. Also, with the FPR hooked up and holding the brakes with the truck in gear, when I would give it some gas, the pressure would go down to about 25-27psi. Does this help rule in or out the fuel pressure regulator or fuel pump as the problem?

Chris_1

iTrader: (1)

You don't say your engine size but I'll assume a v8. A v8 requires 30 to 45 psi. It will not run well at all at 30 or below.
Mine works properly, so here's what it does.
Turn the key on I immediately see 42 to 43 psi. I start the truck, the vacuum at idle opens the fuel pressure regulator (FPR) and pressure drops to about 38 psi.
As I open the throttle a little, vacuum drops off, the FPR closes and pressure goes back up to 42 psi.
Foot off the gas, back to idle and the vacuum increases, the FPR opens and lets some of the pressure go back to the tank so pressure drops down to 38 again.
That's about how your truck should be working. It's not, so it's pretty safe to assume you have a problem with your pump at the top of the most likely list.

jdinner

I still would not rule out the FPR until you block the return line and test the pressure.
How made would you be if you changed the fuel pump and then found out it was broken spring inside the FPR?

fltdriver

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