Hello, I have a 1990 f-150 6cyl 4.9 fuel injected. It is 4wd with dual tanks. I'm having an issue with rear tank bleeding to front tank and the truck not running under load from the front tank. Seems to run well on rear tank..I know seems weird. I've tested the fuel pump pressures and they are around 40#. Also the problem seems to worsen when running on front tank as the weather gets hotter. I think there is definately a heat corrilation. I cant find anywhere a fuel tranfer switch/valve mounted on frame rails near tanks ect. I've followed fuel lines it just isnt there i bought a valve from dealer 3 ports so i would know what i was looking for no luck. Is it possible there isnt one? If anyone can help i would appreciate it. Bill

 

If you have dual tanks, you have a DFR (Dual function reservoir) located on the left frame rail just aft of the frame rail (high pressure pump) . Your fuel pressure sounds low should be above 45 closer to 55. The only way your DFR is not there is if some one removed it and somehow figured out a way to re-plumb the system. If your front tank is filling up, the DFR is bypassing. Here's a thread from a page or 2 back read the whole thread.

https://www.f150forum.com/f10/89-5-0...163981/index2/

There are tons of threads on this as this is probably THE most common problem with our trucks. Ford changed the system in 90 probably because of the weakness.

 

Thank you for your reply. So I was unaware of a high pressure pump on frame rail. I have 2 pumps 1 in each tank is there another? Also my truck is early 1990 Feb is the DFR also what i was referring to as the 3 port switching valve? I'm an engineer in the fire dept and drive a cummins diesel which has the high pressure pump on the rail was unaware my gas motor had that set-up but im all ears this problem has been haunting me for some time.. Thanks again Bill

 

My bad... I thought you said 1989. 1990 has 1 high pressure pump in each tank. No DFR. Each pump has its own check valve. The tank that is filling has the bad FDM (fuel delivery module) the check valve allows fuel to bypass. You should have greater then 40 psi, check your pressure from the schrader valve on the rail. I believe it should be in the 45 to 60 range. Low pressure would indicate a bad pump or a blockage/kink somewhere.

Check out this thread:
https://www.f150forum.com/f10/two-fu...adache-164084/

The contrast between the 89 to 90 fuel systems are striking. This system is among the least understood, most prone to fail, simple easy to fix system there is.

 

ok Thanks Is FDM inside tank? Is it all 1 unit or should I be looking to change check Valves also? Oh and would the FDM cause the power problem ? When running on front tank that is filling truck runs like crap no power and worse when trying to A/C or it is hot out.. Thanks again Bill

 

Well.. yes and yes... check valve is part of the fdm inside the tank and is replaced as a unit. There are external check valves available that are aftermarket but then the problem of plumbing it in comes into play. Additionally since your truck runs bad when on the front tank and your pressure is low. The fuel pump itself is suspect. So... in your case if you want to use the front tank you may want to replace the fdm. Also. I don't want for you to just throw parts at it. Make sure electrically you good there. If you can hear it run.. electrically you should be good. I am assuminjg you have checked the pressure at the rail with a gauge. What confuses me is you run good off the rear tank? And the pressure there is low as well. The only thing I can think of to explain the rear pump having low pressure as well yet the truck runs from it is the pump is good but the system loses pressure when the check valve in the front tank bypasses. The pumps serve another purpose as they pressurize the fuel, this is to prevent vapor lock which is where the fuel vaporizes (boils) under high heat condition. This also lends to the thought the front pump is bad.

I am unaware of the switching valve maybe one of the 90 and later gurus can chime in on this. I have a 90 factory manual I can check it tomorrow.

Check Rock Auto if you think you need parts. They are straight shooters and offer a fair price.

 

Thanks for your time on this I'll wait and see what you come up with. Bill

 

I came up with the part # of the Valve I bought from the dealer E5TZ 9189 A hope this helps dont think I have one though on the truck..

 

Interestingly enough, the 1990 Ford shop manual doesnt mention any kind of "transfer valve". In fact alot of the manaul regarding fuel system is devoted to the older system with the DFR, the low pressure intanks and the high pressure frame rail pumps. So I'm not sure what the E5TZ 9189 A is. IMO and I may be wrong, my truck is an 88 but I dont think there can be anything more then a T-Block for the routing of pressurized fuel and return fuel. The switching is electrical, sending power to one pump or the other. Heres and electrical schematic for the 90:



I also want to make a correction from an earlier statement, there is the possibility of a low voltage condition at the pump that can cause it to run slow, voltage should be no more the .5v from battery volts.

Your pressure is low. Engine running should be 45-60 psi, Key on engine off should be 50-60 psi.

Finally, there is a Fuel Pressure Regulator at the front of the fuel rail on the engine. There is a possibility this could cause both pressures to be low. Check its vacuum line for the presence of raw fuel which would indicate a ruptured diaphram in the FPR.

 

Thank you Very much for your time and info I'll let you know what I find.....Bill