My rear tank pumps fuel into the front tank. I know the check valve is bad in one of the FDM's, but wich one? The front tank im assuming? If i were buying the parts to replace and fix the problem, what am i buying? Is the check valve built into the pump itself, and i just have to replace the pump? When ford refers to FDM, i assume they are refering to the entire assembly; Fuel pump, valves, filter, sending unit...? 1989 F-150 4x4 4.9 300 6cyl 5 speed manual. Thanks.

 

In a 1989 F-series truck you do not have the so called FDM. That started in 1990.
Your problem is not in the tank, it is on the frame.
Both fuel tanks feed this fuel tank selector valve and the item has a stuck "O" ring in it on the return valve.
You will have to special order this device from a Ford dealer as no auto part store carries them.
Selector valve assy, p/n F1UZ-9B263-B.

 

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• Thank you for your help.
• your response is very close to answering my question
• mine is a 95 so that means I do have an FDM ?
• fuel is feeding from front to back tank, so my back tank is likely the week unit ?
• 'FDM' "fuel delivery module" is the name of the whole fuel pump and filter unit ?
• If I can hear the selector valve on the frame cycling cycling when I operate the dashboard switch it is likely OK ?

Thank you, Sam

 

A 95' has an intake fuel pump/FDM. FDM is the abbreviation for the whole assembly (pump, sending unit and hanger). If fuel is returning to your rear tank when running of the front tank the check valve in your rear pump is bad. The check valve is not replaceable so you would need to replace the fuel pump or FDM. There is not a selector valve on the frame for 90-96 F150's.

 

yours looks like this in each tank

 

appreciated

 

I Have a 95 that will randomly die, and/or won't start occasionally. When this happens, I have determined that there is no fuel pressure at the fuel rail, but you can hear both fuel pumps prime (individually, when selected) when you cycle the ignition. The engine is usually warm when this happens, and if it sits for 10 minutes, it will usually start back up and run fine.

I though maybe the front pump was getting weak, so I replaced the rear pump (because it had stopped working a year ago), but this did not solve the problem.

Is it possible that the FDM check valve in the front tank is leaking just enough to cause occasional loss in pressure? I have not noticed a significant increase in fuel level in the front tank (when running the rear tank), but I haven't ruled out that some fuel may be getting into the front tank. These old gauges are not exactly 100% accurate.

Just trying to figure out this ghost issue.

TIA

 

When it will not restart does the MIL in the cluster turn off while cranking?
If not replace the PIP sensor in the distributor.
The PIP signal is what keeps the selected fuel pump pumping.
The ignition one second primping of the fuel pumps have nothing to do with PIP signal.