460 Ford fuel pressure problems

[675081]

Hi Guys and Gals.
I am new here and from what I see so far looks helpful.

I have a problem with the fuel pressure on a 94 Class A Ford motorhome chassis.

As I am driving fuel pressure drops untill it reaches about 25 psi and starts backfiring. If I shut the engine off and back on. Pressure jumps to 35 to 40 psi and it runs fine for a while.

When I step down on the accelorator it also backfires

I replaced the fuel filter and spark plugs as well as removed the fuel pump and cleaned the filters in the tank.

The fuel pump was replaced in 2009.

I checked the fuel pressure regulator and it seems to work fine.
The check engine light is not on.

Anyone with similar problems?

[bad1032]

I had a 1989 allegro that would do the same thing but eventually it would vapor lock and cut off, I ended up replacing the fuel pump with a holley hi volume pump and never had another bit of trouble. good luck

[Nick-B]

Maybe someone will have a better suggestion but it sounds like a typical weak/failing fuel pump - again.

Was the replacement in 2009 with a Ford part or a "generic" replacement pump. I've read about other fuel injected Ford motorhomes with non-Ford replacement pumps failing after a few years.

BTW, there is no "hi-volume" pump available for the in-tank, fuel injection applications.

Good Luck!

[675081]

Yes it is a ford pump. I am wondering if part of the problem could be low voltage to the pump. It is a long way from the pump to the battery so there could be a large voltage drop if the wiring isnt adequate??

[mpaton]

The fuel injection computer controls the injectors by energizing them for the correct length of time while ASSUMING that the fuel pressure is always correct. So if the pressure is too low, your mixture will be too lean. A lean backfire is therefore no surprise, and not an additional symptom.

That pressure should be 39psi ABOVE THE PRESSURE in the intake manifold. As the pressure in the intake varies with engine rpm and throttle position, the regulator has a lot to do. It also means you need to see the fuel pressure change from 39 above ambient with the ignition on and the engine off, and the pressure will reduce as the engine starts.

So if the regulator is OK, then it probably is the pump, the filter, or else possibly a leak or an obstruction.

This OBD1 system rarely sets the check engine light, but it may well have internal codes set without the light.

Quote:

Originally Posted by 675081 Hi Guys and Gals. I am new here and from what I see so far looks helpful. I have a problem with the fuel pressure on a 94 Class A Ford motorhome chassis. As I am driving fuel pressure drops untill it reaches about 25 psi and starts backfiring. If I shut the engine off and back on. Pressure jumps to 35 to 40 psi and it runs fine for a while. When I step down on the accelorator it also backfires I replaced the fuel filter and spark plugs as well as removed the fuel pump and cleaned the filters in the tank. The fuel pump was replaced in 2009. I checked the fuel pressure regulator and it seems to work fine. The check engine light is not on. Anyone with similar problems?

[subford]

Quote:

Originally Posted by 675081 Yes it is a ford pump. I am wondering if part of the problem could be low voltage to the pump. It is a long way from the pump to the battery so there could be a large voltage drop if the wiring isnt adequate??

I would think the wiring to be OK. You could have contacts getting warm in the Inertia switch with a voltage drop across them.
They seem to go bad about as often as the fuel pump relay.

Try putting a fuel pressure gauge on the fuel rail test point. Then ground pin #6 of the EEC Self-Test plug and turn on the key.
You should get about 40-45 PSI with just the key on.
Let the fuel pump run for a while and watch the gauge, it should hold the same pressure all the time you watch it.
Then you can start the engine and the pressure should be about 32-34 PSI at idle.
At WOT the pressure should go up to about 40 PSI and then drop back to around 32 PSI when you close the throttle.

The new fuel pump from Ford talked about above is in the PDF below.

[675081]

Thanks Bill I will give this a try

[mustangcar]

you have to update ur pump with the intank pump with 97year model F# serial id numbers early design always drop pressure,,I did a frame mounted pump conversion,using the early efi econoline van style setup on my f53

[justimagination]

..........Welcome to IRV2......Looks like a fuel pump is in order. Like the TSB that Bill posted, you probably have the older style 3 wire pump and connector. Keep us posted once you get it fixed. Good luck and travel safe.
David G.

[675081]

I have a 4 wire pump dated 2009. So I assume it has been converted. The wiring has been spliced to a wire harness.

I checked the voltage and there is 1.5 volts drop with the engine running.
The pressure is 40 psi with the engine off and drops to 30 with it running.
It goes back up to 40 at WOT and then slowly drops to about 25 psi. If I shut the key off and then back on the presure goes up to 35 and then starts droping back to 25.

I like your Idea of the addition of an inline pump, so I will give that a try. Thanks everyone.

[mpaton]

Quote:

Originally Posted by 675081 The pressure is 40 psi with the engine off and drops to 30 with it running.

It's hard to know if 30 when running is OK without knowing what you mean by "running". If you meant idling, then 30 is higher than I would expect, and could indicate a regulator problem.
If you were driving under light load, 30 could be OK.

Quote:

Originally Posted by 675081 It goes back up to 40 at WOT and then slowly drops to about 25 psi. If I shut the key off and then back on the presure goes up to 35 and then starts droping back to 25.

40 at WOT is OK, but what are you doing with the gas pedal as it drops to 25? If you were able to maintain WOT (not always easy), then dropping to 25 would be a sign of the supply not keeping up with demand.

If you were moving, and had to lift off from WOT, then dropping to 25 is likely OK.

I heard recently of an instance of a new pump having a small leak at one of the supply connections inside the tank, which made the pump unable to supply the proper pressure at the fuel rail at WOT.

[675081]

It drops to 25 or less at WOT after going to 40 when I step it down. Also at crusing speed (pulling a car) it will slowly drop.

When it drops this low it starts backfiring. When I turn the key off and back on again the pressure will come back higher and will run well, even at WOT, for a while.

When I say running I mean sitting ideling.

[subford]

At idle the fuel pressure should be between 32-34 PSI, anything less is a problem.
WOT 40 PSI is what it should be but it should hold at 40 PSI all the time it is at WOT with no drop.

When the fuel pressure drops below 28-30 PSI backfiring is normal and below 25-28 PSI the engine should die.

The fuel pump will put out about 95-100 PSI into a blocked head and the Fuel Pressure Regulator reduces the pressure down to 40-45 PSI at no vacuum and down to 32-34 PSI at max vacuum.

You either have a bad fuel pump or a restriction between the fuel tank and the Fuel Rail.
Sounds like the Fuel pressure regulator is OK but to make sure take the vacuum hose off the nipple on top and turn the key on and then off with a cold engine.
Put something above the nipple in case fuel shoots out of the nipple so it will not go all over inside the home.

If any fuel comes out at all the fuel pressure regulator is bad.

Also check for any fuel going on the ground under the home with the engine running.

[mustangcar]

Quote:

Originally Posted by 675081 I have a 4 wire pump dated 2009. So I assume it has been converted. The wiring has been spliced to a wire harness. I checked the voltage and there is 1.5 volts drop with the engine running. The pressure is 40 psi with the engine off and drops to 30 with it running. It goes back up to 40 at WOT and then slowly drops to about 25 psi. If I shut the key off and then back on the presure goes up to 35 and then starts droping back to 25. I like your Idea of the addition of an inline pump, so I will give that a try. Thanks everyone.

have you checked your fuel pump relay, i eliminated mine altogether,i get 12v straight from the battery, the accident fuel shutoff can be wired in series with it, the only reason for the fuel pump relay to to protect thinner guage wires in series with the ignition, but agian its not necessary since i have a direct 12v bypassing the ignition, using a 120v house switch for the pump as those switchs dont melt like the 12v switches you get at auto partstore,,putting an inline pump is a bit more complicated,, you actually need two frame mounted pumps, a low 4psi suction pump that pulls the fuel out of the tank and then the high pressure 45 psi that pushes,,, or you can adapt a low 4psi pressure intank pump ,like the87-91 460efi econolines used and then the highpressure pump the econolines used that was located at the frame close to the motor near under the driver section, those econolines used the two pump method, i eliminated the F53 75 gallon stock tank went for a smaller 45 gal poly plastic tank, to me dont need a fuel tank that large,condensation and rust, Iwould have to always keep the tank pretty full keep those issues down,you may have a bad pump or fuel regulator,does the numbers on the pump start with F7? might of gave you a new old pump design E7 or f0, get a BBK brand adjustable fuel pressure regulalator from a 5.0liter mustang application,they use same pressure as the 460 efi,, i guess u could try adding an inline frame pump with the stock high pressure pump still in the tank,might work, im thinking ur strainer/filter/sock in the tank might also be dirty