My 1985 Ford F250 pickup with a 460 and a holley double pumper carb

will start, but dies after 30-45 seconds. It seems to run out of

fuel. The truck ran fine one day, and the next day won't run longer

than the 30-45 seconds.

The truck has dual fuel tanks with a pump/filter in each tank, but the

problem occurs on both tanks, so I don't feel it is a pump problem.

Also, it does not have any other in line filters, or auxilary pumps

between the tanks and the engine.

The truck will immediately restart, but does the same thing again and

again.

Between myself and a mechanic I use, here are the things we have

tried:

- Replaced the in line fuel filter / check valve / return port.

- Removed the filters in the inlets of the carb.

- Replaced the fuel pump sensors.

- Tried a new electronic ignition module.

- Tried a new coil.

- Tested the spark with an extra spark plug - it appears to have spark

as it is dieing, just slowing down as the rpm's drop.

I am pretty well stumped at this point. Any suggestions would be

greatly appreciated.

Thanks!

 

Are you getting consistent fuel pressure at the carb? You did not mention a fuel line pressure test in your list. Try that immediately, before putting a bunch more time and money into it.

Brad

 

Sounds like a plugged exhaust system . . .

 

Help me out here - we didn't do a fuel pressure test based on the fact there are two pumps (two fuel tanks with in tank pumps), and the symptons are the same for each pump. Unless they both failed at the same time, it doesn't seem pressure could be a problem - am I missing something? What should the pressure be on a 460? I really appreciate the input.

Mark

 

Like I said, it sounds like a plugged exhaust system. The symptoms you said you are experiencing describe a typical plugged exhaust system to a "T".

Disconnect your head pipes at the manifolds, and see if it will stay running; if it will, then you most likely have a plugged catalytic converter.

 

I'm convinced the problem is a fuel supply problem. I disconnected the fuel line
from the pump, and hooked the carborator directly up to a gravity flow tank. The truck started and ran fine. While doing this, I had the fuel line diverted to a container. While cranking, the fuel pumps were putting out fuel. As soon as the truck started, there was no fuel - none - coming from the fuel line.

A user in another forum suggested an oil pressure shut off switch problem, but I evidently don't have one of those - they started in 86 and this truck is an 85. I also tried bypassing the safety shutoff switch, with no improvement.

Again, since the truck as two in tank pumps, I dont think it is a pump issue, as the problem happens on both tanks. Does anyone have any further ideas as to what would cause the fuel pumps to loose power after the ignition returns to the run position....

I'd sure appreciate any suggestions anyone could give...

 

Are the pumps losing power, or are you just losing pressure? Also, if it's common to both tanks, start at the first common point for the fuel system (the switching valve) and work your way forward.

Had the same problem on one of Dad's trucks. Turned out that a previous owner had installed an aftermarket inline fuel filter in a location not immediately obvious. Filter was clogged with debris (due to a failed stock filter which the previous owner had ovbiously not changed). I've also see a kinked fuel line cause the same problem. There is enough flow for residual line pressure to slowly fill the float bowls with the engine off, but not enough flow to keep it running. You might check for a collapsed line on the common side of the fuel circuit. If there are any rubber lines you might want to go ahead and replace them anyway. Cheap insurance, and one of them may have delaminated internally, which can cause a line blockage.

Brad

 

I'm presuming I'm losing power, but that may be incorrect. Since it happens with both pumps, which work independently of each other, and the output drops to nothing, I'm thinking I am losing power. Any suggestions about how to be sure?

 

If you have the fuel line to the carb disconnected, route it to a fuel-safe container and turn on the key. You should have a steady stream of fuel and you should be able to hear the pump running. With no buildup of pressure in the line (like when the floats in the carb rise to the top) the pump should continue running. If the pump runs but there is little or no fuel flow, you have a blocked line. If there is fuel flow but the pump shuts off and doesn't come back on then you have a power problem somewhere.

Brad

 

Electric pumps have two circuits--start & run. In start 12v goes to pump from start relay through a fusible link and a diode. In run 12v goes from ignition switch run terminal through a resistance wire to the pump so the pump operates on less than 12v.
Sounds like the resistance wire is bad. I think it should be about 1.2 ohms resistance. There may also be a fuel cutoff relay and impact and/or rollover switch in the run circuit that could cause a problem. You need a wiring diagram. If you don't have a Ford, Haynes or Chilton manual try your local library. Piffery1 ('83 F350)

 

One more thing, the Fuel pumps might not continue to run with out any pressure against the flow.
So squeeze the output a little to simulate pressure from the Carb, and see if the pump quits?

 

you should have fuel pressure at all times. if you dont have fuel coming out the lines while the truck is running your pump is weak. they dont just run at the beginning of start up. you really should put a fuel pressure gauge on this thing. cuts out the guess work. if this isnt an option, i would say buy a can of starting fluid. fire the truck up and keep spraying starting fluid into the carb, if the truck runs off that you got yourself a fuel problem.
i wouldnt really mess with anything thats ignition if the truck runs smooth during the 30-45 seconds that it stays running.

 

Piffery1 has to be right - the pumps work in the start position, and fill the fuel bowl of the carb. When the truck starts and the key is switched back to "ON", they do not receive any power and shut off. The truck continues to run untill all the fuel previously pumped into the fuel bowl is gone, and then dies.

Put a meter or a test light on the power input to either of the pumps and watch it. I bet you have power in "start" but none in "run"

Clint

 

There is a fuse for the fuel pumps in the fuse block, I beleive the tach is also run off that fuse. Your truck will do exactly what yours is doing if the fuse is loose or blown. I had the same problem with my '84 F250 460. Replaced the fuse and the tach worked and the engine stayed running.

 

I think I have found the problem. The truck is running, I'm just hope it keeps running. This is a 1985 pickup, but built late in the model year. Turns out the wiring harness is a 1986, and some of the components are 1986. Once I made this realization, I managed to get a wiring diagram which matched the truck - all the ones for 85 didnt follow through true. After identifying the power wire to the fuel pump relay, and the wire from the relay to the pumps I was able to jump the terminals, and the truck ran fine. I purchased a new relay, and tried it, but it still didnt work, so I started tracing the other circuits to the relay. On one of them I found reference to fuse 18, with said it was for the auxilary fuel tanks. When I looked at the diagram that came with the 1985 pickup, fuse 18 was labled for warning lights, but upon checking the fuse, I found it was blown. Replaced it, and the truck seems to run fine, but I havent had it out on the road....

More than a little frustrating, going through all this for a blown fuse. Evidently when they started changing over the model years, they changed the wiring, but not the schematic for the fuses....

Thanks everyone for your great input!