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Pretty sure the check valves are in the sender units to prevent cross-tank flow, not in the lines. Front tank in action, rear tank check valve in action, and vice versa.
Yes, the check valves are in the fuel pump assembly that sits in your tanks. There were some recalls and TSBs on this back in the day. Do a quick search online and it'll tell you which way to go on this.
Ok online searching brings me to these forums and there seems to be many “dead” threads on the subject.
as far as I can tell the front pump is it’s original but with a new fuel gauge sender (I fitted that)
the rear tank is a entirely new unit, I fitted that.
The gas is leaking out of the port on top of the tank. Where there is an incoming line to the port and and outgoing line to 18” of hose with a plug in the end.
kinda run out of ideas… either my new pump has no shuttle selector valve (apparently some manufacturers didn’t include one) or something else? Either way I can’t drive a truck that costs $4.50 a mile and makes the town smell like gas.
Here's the blurb from the recall 93V125000 (at least for my '93):
A MALFUNCTIONING CHECK VALVE WITHIN THE FUEL PUMP ASSEMBLY CAUSES A PORTION OF THE UNUSED FUEL FROM ONE TANK TO BE RETURNED TO THE SECOND TANK. WHEN THIS OCCURS, THE CAPACITY OF THE SECOND TANK CAN BE EXCEEDED, CAUSING FUEL SPILLAGE. SPILLED FUEL CAN RESULT IN A FIRE WHEN EXPOSED TO A SOURCE OF IGNITION. REPLACEMENT OF THE FUEL PRESSURE REGULATOR AND INSTALLATION OF REDUNDANT CHECK VALVES.
So basically, whichever tank you're spilling out of, it's the other tank's check valve that is the problem. There's only one high pressure feed line (the one with the fuel filter in it mounted to the frame) and one lower pressure return line. I'm looking through my manuals now to see if I can find a diagram showing where the check valve is exactly.
A MALFUNCTIONING CHECK VALVE WITHIN THE FUEL PUMP ASSEMBLY CAUSES A PORTION OF THE UNUSED FUEL FROM ONE TANK TO BE RETURNED TO THE SECOND TANK. WHEN THIS OCCURS, THE CAPACITY OF THE SECOND TANK CAN BE EXCEEDED, CAUSING FUEL SPILLAGE. SPILLED FUEL CAN RESULT IN A FIRE WHEN EXPOSED TO A SOURCE OF IGNITION. REPLACEMENT OF THE FUEL PRESSURE REGULATOR AND INSTALLATION OF REDUNDANT CHECK VALVES.
Also, the point at which the gas is coming out is the fuel tank vent. It's there to keep the fuel tank pressure from building up, either too high or too low. I recently had to drop my rear tank to replace the vent because some idiot at a shop decided that permatex grey was a good replacement for the seal that should've been used on the vent. Also, that vent hose is supposed to go all the way to the charcoal/evap canister, if you have one on your truck.
Here's the blurb from the recall 93V125000 (at least for my '93):
So basically, whichever tank you're spilling out of, it's the other tank's check valve that is the problem. There's only one high pressure feed line (the one with the fuel filter in it mounted to the frame) and one lower pressure return line. I'm looking through my manuals now to see if I can find a diagram showing where the check valve is exactly.
gotcha! So it’s likely my OE pump in the front tank is due for replacement! I’ll order one from Amazon now and report back, my seals on the vents are also toast (35 years of Arizona heat) so will order them too.
So basically, whichever tank you're spilling out of, it's the other tank's check valve that is the problem.
I disagree with that statement. The tank that is getting filled/over flowing has the bad valve. Typically the high pressure side check valve goes bad, yes even brand new fuel delivery modules. And we have seen more than a few "new" FDMs with no check valve. The return side shuttle valve typically is not the cause of the cross flow issue that plagues these trucks.
Originally Posted by adrianpglover
There's only one high pressure feed line (the one with the fuel filter in it mounted to the frame) and one lower pressure return line. I'm looking through my manuals now to see if I can find a diagram showing where the check valve is exactly.
As you found in the diagram you posted the check valve is on the high side output of the pump.
I disagree with that statement. The tank that is getting filled/over flowing has the bad valve. Typically the high pressure side check valve goes bad, yes even brand new fuel delivery modules. And we have seen more than a few "new" FDMs with no check valve. The return side shuttle valve typically is not the cause of the cross flow issue that plagues these trucks.
As you found in the diagram you posted the check valve is on the high side output of the pump.
A check valve stops reverse flow. As you stated, the check valve is on the output of the high pressure fuel pump. For the tank in which the fuel pump is currently operating, the check valve should be in the open position. If it's either stuck open or operating normally, the valve doesn't cause any issues. Now take the opposite scenario - the check valve in the tank with the pump off is stuck open. In this case, we'd get reverse flow through the pump back into the tank. We don't have to consider the scenarios of the check valve stuck closed, since that would cause there to be no fuel pressure at all and the engine would die shortly after starting, at least if there's some fuel in the fuel rails. Besides, I quoted the Ford factory service manual directly. Argue with me all you want, but I'm taking my conclusions word for word from the manual.
We don't have to consider the scenarios of the check valve stuck closed, since that would cause there to be no fuel pressure at all and the engine would die shortly after starting.
I can confirm that when I switch to the rear tank it will start but dies after 5 seconds or so, if I start on front, switch to rear, again - dies after 5 seconds or so.
this supports what your manual states. Clearly my front pump is goosed.
should be here tomorrow, if it arrives at a reasonable time I’ll fit and report back, otherwise I’m stuck till Thursday at the earliest.
I am not arguing, only pointing out this scenario happens dozens/hundreds of times on this forum. In every case the tank that is getting filled/over flowing has the bad valve or none at all.
Yes, I would say that it would be the front tank pump that would be at issue, but the comments from @rla2005 have me second guessing this.
I did a little more reading on this. There's both a check valve and a shuttle selector valve in the fuel pump assembly. The check valve is on the supply side and the shuttle selector valve is on the return side. I'm just not seeing how a bad check valve on the rear tank would cause it to fill up. As far as the shuttle valve is concerned, maybe that could be it, but I'd need to see a flow diagram on it first.
A bad check valve in the rear fuel delivery module allows fuel to flow back into the rear tank on the high side while the front pump is running. Very common problem on these trucks, Rarely see a bad shuttle valve on the return side, it happens but not as often as the check valve.
A little more info on how the fuel delivery modules work:
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