i have a question for you i have a 1991 f-150 with the 4.9 inline engine.I had to park it back in february when the transmission let go well i have now replaced the tranny but the truck wont start.i checked and its getting spark but doesnt seem to be getting fuel..when i turn on the key and have it on the front gas tank i can hear the what i believe is the electric fuel pump running but it doesnt shut off unless i turn off the key.I know when i parked the truck it was low on fuel but it was running...is it possable that what was left in the tank for fuel evaporated out and its that empty(i have put in 3 gals of gas b4 trying to start) or do i have further problems?? any ideas or suggestions would be appreciated thanx tim

 

Turn the key to off, then on for 5 seconds, then off. The pump should run for 2 seconds each time you go to "on", then shut off. Repeat a couple of times. Now pop the hood and depress the plunger on the Shrader valve in the fuel rail. At least some fuel should spray out. If it doesn't, you have no fuel at all reaching the injectors.
You will need to track down why the pump isn't pressurizing the lines. Check the fusible links, fuel pump relay, inertia switch, etc.

If you have at least some fuel, and you have spark, you will need to buy a fuel pressure guage and check that you have enough pressure at the rails.

One diagnostic technique is to jumper the "FP" wire at the self-test connector to ground. This will energize the fuel pump relay coil and should force the pump to run (assuming the key is on). If the pump does not run, figure out why not.

 

What should that fuel pressure be? Is the pressure the same on a 4.9 inline and a 302 (5.0)?

Thanks,
GUZZLER96

 

The 4.9L is spec'd at 50-60 PSI KOEO, 45-60 PSI with engine at idle.

The 5.0L is spec'd at 35-45 PSI KOEO, 30-45 PSI with engine at idle.

There are some slight variations year-to-year.

For a no-start condition we don't need much accuracy. I thought he should test fuel pressure so he could tell the difference between 10PSI (which probably wouldn't start), and 30+ PSI (which probably would start).

 

It there is fuel under pressure at the scharder valve it should run. They will actually start and run with as little as 15lbs won't run very well but will run. Put 5 gallons in the tank that may be all that it needs. I had a 460 efi that got a big dose of water in the gas (had to drain everything and start over with new fuel) and the only way I got it to start after the clean up was with a shot of ether. If you use ether don't get crazy but a little shot in the throttle body will let you know if it will hit.

 

again i do hear the fuel pump running but it doesnt shut off would that be because its not getting the fuel or is it more serious???thanx

 

At least the high pressure fuel pump should shut off if the engine is not running. This is a basic safety issue. If the truck is in a wreck and breaks a fuel line, you don't want the battery-powered pump spewing gas out the leak at 60PSI. You also don't want the pump to keep running if the tank goes dry, because the motor is cooled by the liquid gasoline supposedly running through it.

In order to meet these requirements, the factory designed the PCM to control the fuel pump through a relay. The PCM grounds the relay coil, therby turnning on the fuel pump, for two seconds when the key goes from "off" to "run", it then ungrounds the relay coil (and shuts the pump off) until it sees the key go to "crank" or pulses coming in on PIP, or both. When you release the key from "crank" back to "run" you should hear the pump run for a second after the starter stops turning. Then the pump should be quiet.

I don't know if this configuration had the in-tank lift pump and a frame mounted high pressure pump, or just an in-tank high pressure pump. There may be a difference here, but I suspect there isn't and that the pumps operate (or don't operate) together.

There is no pressure feedback to the fuel pump. Lack of gas will not cause it to "keep trying" when the PCM tells it to shut off. This is exactly what you don't want if the tank goes dry.

You are either hearing something other than the fuel pump, the relay is sticking closed, or somebody has messed with the fuel pump wiring.

The 6 pin test connector is arranged in two rows. One row has 2 positions, the other row has 4 positions.

Hold the connector so you are facing into the business end, the row of two is on top, the row of 4 is on the bottom. The right hand connection point in the bottom row of 4 is the "FP" line coming from pin 22 on the PCM and going to the fuel pump relay coil. If you jumper this line to ground, and the key is in the "on" position, the fuel pump should run. Do this only briefly, but see if the sound you hear changes.

 

ok the fuel pump is running and wont shut off unless i shut it off via the ignition key...as soon as i turn the key on i can hear it running but still no gas to the engine...any ideas what to do next???its a 91 f-150 ext cab with the inline 6 and manual tranny i know the back tank hasnt worked in yrs but it was running fine till i parked it about 6 months ago...now it wont run and i have put about 4 gals of new gas in thinking the tank was empty...thanx for any help tim

 

If you are sure the pump is running, then you have two problems. The fuel pump runs when it should not. Fuel does not appear at the rails.

For problem one, remove the fuel pump relay. Does the pump still run? If so, find out where the voltage is coming from -- it's obviously not from the FP relay. Look for non-factory splices. If the pump shuts off without the relay, trace the control circuit and find out why "FP" is shorted to ground -- the PCM should be turning of the fuel pump.

Problem two could be a problem with the tank selector valve. It may not be switching over to the tank with the working pump. The pump will run (deadheaded), but there is no place for the fuel to go.

Alternatively, the filter "sock" in the tank may be blocked with varnish. Try putting some more gasoline in the tank.

If you have a frame rail mounted high pressure pump, then the low pressure lift pump in the tank may not be working. I don't know the layout of your particular combo.

The filter may be blocked with varnish from not running for 6 months. Try removing the filter and see if you get gasoline from the input line to the filter when the pump runs. At least that will help narrow down how far the fuel is flowing, if at all.

 

ok i dropped the tank and pulled off the 2 lines running from the pump. it is pumping plenty of fuel and i also had fuel returning in the other line but it still has none to the engine...what would you try or do next???

 

If fuel reaches the return line, I would guess the problem is in the fuel pressure regulator -- unless the tank switchover valve connects the tank back to itself when that tank is not selected.

The next step would be to pull the return line at the regulator. If fuel comes out of there, but not the fuel injector rail Schrader valve, then the problem pretty much has to be the fuel pressure regulator.

 

Do you have fuel in front of the fuel filter? Unhook the fuel line between the engine and filter at the filter and turn the key for 10 secs you should get about a pint of fuel.

 

where is the fuel pressure regulator located...i have a 1991 f-150 ext cab with dual tanks thanx for any help tim

 

On my 87 f150 4.9 its connected to the red plastic vacuum line. The lines come off the vacuum tree thats on top of your engines should have 5 or 6 rubber hoses all blach except for the red vaccum line. Its on the passanger side on the fuel rail, fuel lines come up behind and then change to metal lines that run under the upper manifold. Its round about 2 inches wide and 1 inch high.

 

Souds like it may be the pressure regulator(behind the drivers' side valve cover on the fuel rail 5.0). Pump is either not being told to shut off or is not making enough pressure. Hope fuel filter is changed before you spend too much money. Let em' sit too long and bad things happen. Dump some fuel into the intake/throttle body and see if it cranks up.