Im stumped. I have a 94 F-150 w/ 302 and it has given me hell for years. The truck runs fine most of the time, but sometimes it acts as though it has a clogged fuel filter. The truck hesitates, has little power,is jumpy and will fall flat on its face. Heres the kicker, it WILL run if you get it above 2000 rpm, drives fine above that, but let it fall and it will die on you and won't restart, or if it does it can barely manage a choppy slow idle. Either way your stuck where ever it is. Changing the filter seems logical, and it usually makes the problem go away. Funny thing is the filters are not very dirty. I always drain them into a glass jar so I can se what's in them. The gas is usually a bit cloudy and dirty looking. I let it stand for a few hours to check for any water, sometimes I see a drop or two, ussually none. I've even cut them open and can't find much inside either. The filters are not plugged up you can blow through them quite easily. Changing the filter gets me going again but it isn't the problem. I checked the pressure on the fuel rail about six months ago, 35 psi. I checked it yesterday 22 psi. I rigged up the gauge so that I could see it while I was driving. It read 22psi cruising, and fluttered aprox 10 psi under accelleration. With the pedal on the floor it fluttered wildly, and steadily dropped to about 5 psi(the needle would hit the peg when it fluttered, so I am estimating about 5psi). The check valve in the pump is good, the system will hold full pressure after it has been shut off for a few hours, but not over night. I made an adapter from an old fuel filter and tested the pump pressure at the filter fitting, nowhere for the gas to go but to the gauge. It read 16 psi, and held that pressure for about 30 min. Imagine my supprise when I got 16 deadheaded at the pump, 22 running at the rail, and 25 after I shut the truck off. I am completely baffled. Could the low pressure in the system cause the ECU to widen the injector pulse width so much that it is out of its opperating parameters, and actually be causing the truck to run rich? It does run rich, you can smell it, bad. The pressure regulator does not leak into the diaphram or manifold. I installed a clear tube between it and the manifold, I never saw any liquid pass through it. I am begining to think my truck is possesed. It seems to defy everything I know about electronics or hydraulics. How would you get more pressure than what you started with at the pump?

 

I dont claim to know much about fuel injection or computer controlled motors, but it sounds to me like you may have some crud in your fuel tank. This would explain why the problem gets worse as you drive. The contaminates settle to the bottom of the tank over night, then as the fuel sturs it up going down the road, the pickup screen clogs and starves the pump. As for the differance in pressures throughout the fuel system, does the fuel line reduce in size towards the motor? Same volume of fuel in smaller line=higher pressure. Im not sure how much pressure your system should have, but i think it should be much higher than it is. This could cause your ecm to function improperly. Unfortunately, you would probably have to pull the tank out to check it, and the screen, for contamination.

I just remembered, I think some Ford have two fuel filters. If yours does, did you replace both?

Hope I could help

Nathan

 

SP ,
If you only have 16 Psi at the filter , you have either a clogged screen or the pump is about half dead . You should have 40 psi at the Shrader valve . I know its a pain ...but the only way to find out for sure is drop the tank ( Im assuming its a single tank and pump ) Run it till its almost out of gas so its easier to deal with . It does sound like you have some crud in the tank . Good Luck .
Paul

 

Very good thinking on the fuel injector pulse width according to fuel pressure.....you hit it on the nose! I couldn't have explained it better myself. You definently have a pressure problem ! Verify available voltage at fuel pumps,if good, verify no kinked lines,if good, all I can say is to replace high pressure pump. The fuel pressure should be at least 40 pounds with a couple of key cycles,minimum.
Phillip

 

I was afraid I was right about the pump being toasted. I'll drop the tank, give the pump a good once over and a final testing. Probably won't get it done for a day or two. I'll keep you posted. Thanks for all your help.
Stephen

 

>I was afraid I was right about
>the pump being toasted. I'll drop
>the tank, give the pump a
>good once over and a final
>testing. Probably won't get it done
>for a day or two. I'll
>keep you posted. Thanks for all
>your help. Stephen


Do you have a pump in the tank and a high pressure pump on the chassis rail ??? If so, need to check both. Should have about 45 psi at the rail all the time. Almost sounds like the high pressure pump is cutting out and only the tank pump (lower pressure) is working. Hope you share your success story with us. Good Luck

 

i have the same problem but i already spent $150.00 to find out that was not the problem i am not getting good power to the pump can anybody tell what to check please.

 

Can you explain "not good power to tank" is. Is it low voltage or no amperage,etc. You may want to start at the fuel pump relay and work your way back through the connectors.Also, rework the 2 small ground wires at the battery,they are for the PCM. Also,there should be a ground connection underneath for the fuel pumps,rework them all.In the worst case, your problem could be a weak positive feed wire leading to the fuel pump(s),one that doesn't carry amperage well enough which will show up as a voltage drop across the connection.
Phillip