Truck runs rough. I tested 5psi at the fuel rail with the key on and engine off. Once I started the engine it built up to 30psi and stayed there. One I killed the engine pressure bled of at around 5psi per minute.

Is this enough info to tell what is most likely the problem?

 

Could be FPR. Had a similar situation with mine. But testing needs to be done to prevent wasted money and good parts.

 

You should get about 32psi within 2-3 primes of the pump engine not started, and the same pressure running at idle. When vacuum is removed from the regulator(engine running) pressure should jump to 39-45psi and you should get similar result with vacuum attached to regulator if the throttle is stabbed. Pressure should never drop below the base pressure or 30-35psi at any engine speed and load.

 

What other tests do I need to do on the FPR? Its strange it's only 5 psi KOEO and 30 at idle. 30 at idle would make it sound like it's not a pump issue

 

Go back and read my post above again, if you don't get at least 40psi with vacuum removed from the regulator or if pressure drops when the engine is revved or put under load you have a pump problem.

 

Ok went for another test just now. Keeo, the pressure barely moved past 0psi. Engine idling, it stayed around 20psi. Pulling the vacuum line gained a little less than 10psi. Revving the motor had no effect on fuel pressure. I will say that the only difference between my last readings and my current readings is that today it is below freezing.

Any ideas? Could it simply be a dirt buildup or water in the fuel?

 

Sounds like a bad gauge connection. The engine should not run or start @ 20psi.

 

My bad. Pressure KOEO is 5psi. Pressure KEER is 30 with vaccum line on, 40 with it off. Revving motor increases pressure by around 2psi

 

Could this be a blockage issue?

 

Is or was this a dual tank truck? If it was dual exactly what was done with the fuel lines when the 2nd tank deleted?

 

+2psi from what? (30 or 40)
It should never go above 40psi as the spring tension inside the regulator is for 40psi (39.6).

 

Went up 2 psi from 30, with the vacuum line on. I think. I can double check in the morning. In regards to the two tank question; nothing was done to the truck, I just stopped using it Because it leaked. I don't think it's sending any fuel to the rear tank I never smell any fuel around or see any leaks

 

Is there there any chance a weak alternator could be contributing to low fuel pressure?

 

Did you figure out what your problem was? Having similar problems with my truck

 

Update: Since last post I have changed out the FPR and fuel filter. Truck seems to run a little better, but still has the surging/hesitation when on the throttle. It doesnt seem to matter if the truck is pinned or lightly accelerating; the rate of misfire seems the same. Fuel pressure after three ignition key turns is about 25psi with motor off key on. Engine running at idle is 28 psi. Pulling the vaccum line off the regulator gains about 10 psi.

I noticed when I had it out the other night when I switched to he rear tank (which I never use) the truck ran just like it did on the front tank. Im pretty certain there is no fuel transfer going on; the gauge said the rear tank was on fumes. Does this correctly lead me to believe that neither pump in the tanks are weak, if they both operate the same? And I thought I heard these trucks have a main fuel pump outside the tanks, could this be my issue?

One last thing to note; today when i was driving the truck a little, before I put it away, I revved the engine to about 3k a number of times. I noticed that there was a period between when the engine settled back down to idle and where the truck started idling rough again, for about 3-4 seconds the truck would idle smooth as glass. So after revving the motor, once the rpms fell back down to 900 there was a period of 3-4 seconds where the truck ran great. Im not sure if that tells anyone anything, but it seemed like once the system got a big rush of fuel it was fine until that subsided. Just my thoughts.