Hi all. I have a 91 F150 4.9. Woke up on Sunday morning about 100 miles from home, to find my fuel pump wouldn't turn on. Found the existing one to be defective so I replaced it. When I went to drive home, I found I could only apply about 1/8 throttle before the engine would sputter and buck and produce no torque. After driving home at 30 mph, I thought I'd start at the fuel filter. When I went to remove the old one, I found the return line fitting at the back of the block disconnected, and I fixed that, but there was no residual pressure in the fuel line when I disconnected the filter. I'm thinking that the regulator valve is stuck open, which killed the fuel pump by changing its duty cycle to 100%, and helped open the return line fitting which was already somewhat damaged, by allowing the full fuel pump pressure to continuously circulate through the return line. Any thoughts on what my problem might be?

 

The fuel pump runs all the time anyway. What fuel pressure does the system build now? It should be 55-60psi with the engine running, any less and the motor will be down on power.

 

I don't know what the pressure is now, as I don't have a gauge and am looking for something that will work. However, I did find the old fuel filter was perfectly fine and clear, so I doubt that was the problem. Anyways, with a good fuel filter I'm still getting very low pressure- barely enough for a smooth idle, certainly not enough to drive on. By the way I forgot to mention I have only one tank (rear), the truck came that way from the factory. I didn't realize the pump was always on, so is there some factor or event that would both fry a fuel pump and lead to low pressure after pump replacement? I've always had symptoms of low fuel pressure (limited RPM range, hesitation), but I attributed them to bad timing or worn ignition components and now I realize they may have been from a fuel issue.

 

Update: Problem solved! I got some time today and worked on this problem a lil. Electrical tests on everything that could f*** with fuel delivery turned out good. New pressure regulator did not help, although apparently the old one was about to go (gas in vacuum system). I didn't use a gauge to test the fuel pressure, but I did poke the Schrader valve with KOEO, EEC fuel pump pin grounded, and nothing but a drip. Found a full 12v going to the pump when it should, so I did a volume test before the fuel filter. It failed miserably, putting out about 6 drops in 10 seconds. I reasoned that the pump was either defective or clogged and in either case I'd have to drop the tank, so I did that and found the supply line to be pinched between the tank and the underside of the bed. That was the issue, the engine runs perfectly now. I just have to wait until daylight to put the tank back in- it's full and I refuse to dump it or remove the bed, so I'd rather not do it by myself in the dark.