Questions and Answers

Ford Truck Starts Hard And Runs Rough Q. I'm working on a 1989 Ford 5.0, automatic transmission, 4WD. This truck was in bad shape when she brought it to me, knocking, oil screen clogged, run bad. I replaced her oil pump screen, and did tune up. I tried to start it and it just cranks and cranks, I checked the fuel pressure at the pump under the drivers door, I get about 60 PSI.

I used starting fluid and got it running, runs very rough. When I remove the vacuum from the fuel pressure regulator it runs great. I replaced the fuel pressure regulator but runs the same, doesn't want to start and runs rough. I'm a diesel mechanic, the old kind and I'm really at a loss here.

I thought it might be the fuel pressure regulator, but that doesn't seem to be the problem. Drove it down the road yesterday, runs real smooth but when you set at idle runs rough again and still hard to start. I turned the key on and let the pump run and then it starts, Bad pump?

Thanks Vince,
Cathy (RET. USN)

A. First off, the fuel pressure is too high. it should be:

Key On Engine Off	30 - 45 psi
Key On Engine Running	30 - 45 psi

The other thing is the fuel pressure test should be taken at the test port on the fuel rail. This will give a more accurate reading.

From what you describe and the fuel pressure reading, this engine is getting way too much fuel. Try unplugging a vacuum line and starting it. If it starts up fairly well, then it is definitely getting too much fuel.

Now what is the fuel pressure when you unplug the vacuum line from the fuel pressure regulator? I think it is staying the same, just the vacuum leak is thinning out the fuel mixture and that's why it starts fairly well.

The fuel pressure regulator is supposed to maintain the proper fuel pressure. If the new he fuel pressure regulator is working correctly, then the fuel pump internal pressure relief valve is not working.