Ok so I have a 86 f150 302 efi. When I got the truck it wasn’t getting fuel from the tanks. So just to make it run so I can see what else it may need I replaced the tank and sending unit/ pump in the tank. I bypassed the fuel tank selector and then hooked up directly to the pump on the rail adding a filter between the pump and the tank. I can hear both pumps cutting on when I turn the key and the gas gauge works so I assume the in tank pump is working. I know the one on the rail works because I had the truck running on just it and a gas can before I got the new tank. After setting all this up it cranked and ran for about 1 min and now it won’t start anymore. Please help.

 

Shoot some brake cleaner or starting fluid down the intake and see if it tries to run. If it does, you still have fuel problems.

 

I know it will run. It ran fine when all I had was the pump on the frame rail and a hose from it going to a 5 gallon gas can. I just don’t understand if it’s now getting fuel from the tank what could be the problem. Has to be between the rail pump and the tank I suppose

 

You are going to need a pressure gauge to figure it out. The tank pump is a low pressure transfer pump and the rail pump is high pressure.

 

You said you are assuming the in tank pump is running. Just because it's new doesn't mean it's working, I would not assume anything.

 

Yes I know better to assume. But it does cut on and run for a second when you turn the key. Like it should. So it’s getting power I guess is what I should have said. I’m gonna pull the hose between the two pumps and make sure I’m getting fuel out of the tank. Also after trying to start it for a few seconds if I press the schrader valve on the fuel rail gas comes out but not with any pressure. It just kind of pours out until it’s empty.

 

Sounds like a good plan. I would get a 5 gallon bucket and see if it keeps running as you are cranking it over. There is a test port also on the test plug in the engine compartment where you pull the codes. If you ground that terminal the pumps should run with the key on.

 

Thank You for that. I will give that a try too.

 

The selector valve you bypassed is also a reservoir to supply fuel to the high pressure pump. The tank pump/HP pump combination you have will not work without the reservoir.

 

What is your theory on this? Besides possible fuel starvation when the tank is low and he goes around a turn, why would it not work, at least temporarily?

 

For reference, here's your post from the other day,

https://www.ford-trucks.com/forums/1...new-truck.html

 

I'll give it a shot but I too used to have my doubts about why the bypass shouldn't work.

In the scenario in which we believe the bypass should work, we think of the fuel circuit as a loop running from the tank pump, through the HP pump, fuel rail, flow valve then excess fuel back to the tank.

In reality, there are two loops, a primary and secondary loop. I think of the primary loop as starting at the reservoir. The HP pump picks up the fuel, pressurizes it at the fuel rail. After the flow (pressure regulator) valve, the excess in the primary loop is NOT returned to the tank as it seems. The fuel is returned to the reservoir where it is again picked up by the HP pump. A reciculating loop.

The tank pump merely replenishes the fuel used up in the primary system. It has its own loop pumping fuel to the reservoir with any excess returning to the tank. You have to think of these two loops as independent yet still connected. There is only one connection, the reservoir.

Now the theory part. The HP pump can pump more fuel at a higher volume than the tank pump. If it is allowed to pump unused fuel directly back to the tank, there is simply not enough fuel available from tank pump. For the HP pump to perform as it should, it requires much more fuel than the tank pump can supply. It requires the primary loop recirculated fuel.

The HP pump starving for fuel cannot build the 35-45psi pressure required to run the engine. We know that if the fuel pressure is below 30 psi that the truck fails to fire. Think of when the HP pump check valve fails and you are able to overcome that by cycling the key several times in quick succession building the pressure to something that will light the engine.

The tank pump is not capable of supplying enough fuel for the HP pump to build the necessary pressure.

Hope this helps.

 

Thank You to all who ha e commented. I understand the theory as to why it shouldn’t work without the reservoir but I also have another 86 f150 that I’ve had for almost 2 years now that only uses the front tank. The previous owner bypassed the reservoir and that truck runs amazing. I just copied what they did on this one. The trucks are the exact same combo of drivetrain.

but if your theory is true how can I just make the HP pump pull from the tank? Because as I said earlier I had the truck running for over 10 min strait with just the HP pump pulling gas from a 5gallon can and returning the gas back to another can. Ran very smoothly like this. Thanks again.

 

My explanation is only a reflection of my understanding of how the system is designed to work.

Running out of a can, I can see where plenty of fuel is available. On the truck that runs fine without a reservoir. I have no researched ideas.

I have seen where guys have run 1990 and later HP tank pumps. Not sure but I think the later tanks have a different size opening and the pump/ sending units, a different size flange.

My final word is that if you get the reservoir in order, you will have solved your problem.

I can probably dig up some helpful information on understanding/fixing reservoirs. The dual tank version is expensive to replace. It can possibly be repaired though.

EDIT: If you are inclined to run just one tank, a new reservoir can be delivered to your house for about $70. MaxCapacity can tell you where.

 

Yeah I have taken apart the fuel reservoir/tank selector and found what was wrong with it. I have fixed it but was told it won’t work without both tanks working. And didn’t want to spend money on another tank and sending unit until I know the truck has nothing else wrong like transmission or rearend or anything else. But that may be what I have to do.