1996 4.9, rear fuel tank primes, front fuel tank does not prime.

Cold start off of rear tank, no issues. Once the truck has been running and shut off attempting to start again immediately after will result in failure. If I turn the switch to the front tank, I can get the truck fired up, but have to switch over quickly to the rear to maintain fuel supply. Anyone encounter something like this before? Cold start is flawless. If the truck sits for roughly 20 minutes, it will start again normally.

I have a fuel pump relay, fuel filter, and fuel selector switch but 'm not sure which I'd like to try first. It doesn't seem like the fuel filter is the issue here. If the fuel pump relay was stuck open, could the rear pump flood the lines or is there a return on the fuel rail to prevent that? The inertia switch appears to be fine, but I haven't tested the voltage to it. It seems to be related to a fuel issue and not a spark issue based on the cold start, and the fact that when I manipulate the selector switch it allows me to get it running. I don't believe the selector switch is stock for this truck, as the labeling is reversed. I checked voltages at the fuel pumps, and listened under the truck to verify it is the rear tank operating, even though front is selected. Maybe I should swap that first? and if it doesn't fix things then swap the relay? If both of those are new and the problem persists, what else would you check?

Thanks. Typing out my problems has been giving me the opportunity to think on some things in a different way and I've been finding some of my answers by the end of my posts.

 

Has the rear pump been changed? Maybe it heats up and cuts off?

 

What is your definition of “prime”
Have you measured your fuel pressure from each tank?
It is always prudent to replace a fuel filter.
Suggest you add 2 yellow bottles of ISO HEET gas-line antifreeze to each tank to absorb the water.
It does sound like you have a Fuel Selector or wiring issue to your pumps, you need to verify that both pumps are working and providing the proper pressure.

 

 

Bringing this up again because I'm still having some issues and finally hooked up a fuel pressure gauge. New fuel filter.

Key on, rear pump selected, short prime @ 35 psi. Start the truck and it'll run @ 45 psi. pull the vacuum hose from the regulator and it'll jump to 55 psi. That sounds like a bad vacuum, correct?

But then I'll turn it off once it's warmed up and when I go to restart the truck it won't fire. I have to switch to the front tank, which gets voltage but doesn't prime (my guess is the pump is shot) and the engine will fire up, but I have to switch over to the rear tank to keep it running. Is this the FDM of the front tank syphoning gas from the rear?

Would plugging the lines to the front tank solve this?

Thanks.

 

This looks ok to me...

 

So, no fuel pressure when the front tank is selected ?

 

Correct, no fuel pressure when front tank is selected. But it will only start from the front tank selected once the engine is warm. (Running more than 10 min)

 

When it doesn't restart, what is the Fuel psi of the rear tank?

My 4.9l pump, replaced 10 years ago, it shows 50 psi with engine off, Key On, cold.

 

Next time this happens keep the rear selected & put the accelerator to the floor and hold it there and try to start.

 

Pedal to the floor, no change.

I had no way of releasing the pressure from my gauge (inexpensive harbor freight) between having the truck running and then restarting it so I'm not sure if that made any impact, but it went from cold start 35 psi prime, running at 45 psi, shut the truck down and on the restart from the rear it seemed to be between 55 - 60 psi after the switch toggle trick.

I saw a post about plugging the front tank lines with ground down bolts, have many of you tried that single tank conversion, and would you know the bolt sizes to be used?

 

I disconnected the front tank fuel lines, plugged them, and disconnected the plug jack. Still no change in starting, which leads me to believe it is not some freakish siphoning action between the rear and front pumps. Starts up great, runs for a while, I turn it off, try to start up again and it won't go until I flip the selector to the front tank (disconnected now), turns over and runs, then I switch to the rear for the fuel supply.

Anyone want to venture another guess as to what could be causing this?

 

Pull your selector switch and put in a jumper between the Y/LB and the Y/W wires. put a jumper between the BR/W and the R/W wires.
try that.

 

Same problem with the wires jumped except I don’t have the ability to swap to front to actually get it running.

For real, thank you for the support so far.

 

Strange. The only thing you are doing is turning off the rear fuel pump until she starts. This lowers the fuel pressure. Leads me to believe your engine is flooding on re-start.

Try the accelerator to the floor again before you turn the key on to re-start.

Maybe your Injectors are leaking into the cylinders after you shut down......