I have a 1993 F-150 with a 302 and 5 speed 4x4. It has dual tanks. I have had a problem with my rear tank overflowing out the vent in the gas cap when my rear tank is full and I am running on the front tank. It is mainly a problem when the truck is sitting idling. Even when the rear tank is empty and I run on the front tank, it will fill up the rear tank. I went to my Ford dealer and they told me it was that Ford had problems with this and that they had to recall a few years of trucks and put these check valves on the fuel line (blue line). My year of truck was not under recall, but they were happy to sell me 2 of these things for $60 each plus tax. I installed these on each tank and I still have the same problem as before. Does anyone have any idea what is going on?

 

Yes the 1993-year was one of the years. Well documented but the 1993 year also had a run of bad fuel pressure regulators that would blow the check valve in the non-selected tank.
Click here.
https://www.ford-trucks.com/recalls/.../y-1993/t-1044
That said it may not have been the check valves, the shuttle valve will do the same thing. You said it is worst at idle and that would point more to the shuttle valve than the check valve.

The only way to fix it is to replace the rear FDM inside the tank and get rid of the check valves as when the FDM's check valve is working right some have reported that the second check valve will cause problems.


Here is a diagram of the FDM called an assembly in this diagram.


Your fuel system.


The Fuel Pressure Regulator.


The FDM.

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Subford is correct. I Googled my F-350 when I bought it and found quite a few recalls because of that valve allowing one tank to overfill the other tank. Your Ford dealer should fix it for free. Just have them run the VIN and it will show all the recalls and fixes for your truck. Since some dealers are not quite reputable, you may want to bring a printout of the link in subford's post.

 

Thank you both very much for your help and information.

 

subford, i have this problem and i would buy the new fdm if i was sure that it was going to be safe. what i mean by that is: if the fuel pressure regulator blew out the check valve in the old fdm, why wouldnt it do the same in the new fdm?

has anyone successfully fixed this problem by installing a new fuel delivery module? i think a lot of people like me just learn not to let fuel feed into a full tank.

 

As I understand it that 1993 was the only year that Ford had a bad run on the Fuel Pressure Regulator. These bad Regulator were put on when the truck was made and they would let the pressure go to high and take out the check valve in the non-selected (not running) FDM.
This would cause the cross flow problem.

Ford put out a recall on this problem just for 1993 and when the truck was taken in they would replace the fuel pressure regulator and put a check valve in each pressure side fuel line of each tank. They did not want to put out the money to do the job right by replacing the FDM.

Of course it is possible for regulators to go bad in other years on their own and the check valves can also stick open on their own.

If you are worried about the regulator get a fuel pressure gauge and check the fuel pressure at the fuel manifold with the pumps running and the engine off. Also pull the vacuum hose off of the regulator while the pumps are running and see if any fuel comes out of the nipple on it, if it does it is bad (a hole in the diaphragm).
When you stop the pumps watch the pressure and see if it falls fast, if it does then you have a check valve problem or a leaky injector. If it holds and you have this cross flow problem then you have a bad shuttle valve in the FDM of the filling tank.

To run the pumps with the engine not running just ground pin #6 (circuit 926, LB/O) of the test (VIP/DLC) connector with the key on.

A lot of the members of this site have fixed the problem by replacing the FDM.

Normal pressure is 50-60psi for a 4.9 with the engine off and the pumps running. For all other engines 40-45psi.

Here is a diagram of the VIP test connector.


The VIP test connector is call the Diagnostic self test plugs in the diagram below.


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subford, is the FDM the actual fuel pump itself? I am in Canada and I asked my both my local ford dealers and neither one knew what the fuel delivery module was.

 

subford, that is the best and most complete information i've gotten on this topic amidst the wonderous capabilities of the search feature to which im often refered. thank you. i'll check the pressure and buy fdm and fuel pressure regulator.

russ_06, yea its fuel pump itself. multiple components are housed there (including the fuel pump) and they call the whole thing a fuel delivery module. you cannot replace or service individual components in the fdm. the whole module is replaced is one component fails.

 

Thank you very much for your information.