Mechanics

F150 FUEL PRESSURE

1988 Ford F-150

Engine Performance problem
1988 Ford F150 6 cyl Two Wheel Drive Manual 190000 miles

Hi,

engine is stalling, apparently due to low fuel pressure. Guage reads 45 -60 psi when first started, then as engine runs pressure gets lower until engine stalls from lack of pressure. (About 20 psi or so.) After a 30 second wait, pressure comes up and starts again.

with ground leads from rear fuel pumps disconnected from battery, there is no pressure or fuel. When connected, pressure comes up - I disconnected the ground lead from the h.P. Pump (so I can hear) and I do hear the pump in the tank whirring. Seems like a fuel volume issue. This truck has twin tanks - could the selector valve between the tanks be the culprit?

other causes for low fuel volume to the high pressure pump?

no apparent kinks or restrictions in the fuel lines, new fuel filter, new h.P. Pump (was making bearing noises with erratic fluctuations in pressure - sounded like it was on a failure path). Rear tank pumps replaced about 1.5 years ago.

need to get running as is a work truck and I am losing money by the hour.

thanks.
Avatar
Jblco
July 16, 2009.




If pressure drops that fast then either the filter is almost completely clogged, or the pumps need to be replaced, also there is a new switching valve for EFI type systems.


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Merlin2021
Jul 16, 2009.
Since the only two components up stream of the high pressure pump are the switch and the pump, how can I eliminate one or the other?

Fuel filter is downstream of the h.P. Pump and is brand new, so filter clog is low on my list (though not completely ruled out as if there is a lot of crap in the fuel, I can see where a new filter would clog up fast.)

also parked truck on incline of about 15 degrees (front end up). Starts, runs about 30 sec and dies (pressure drops from 55-60 psi to less than 30 psi and stalls from lack of fuel).

I don't mind spending money on the system to get it going again, but dollars are tight and I don't want to keep playing this game of replacing everything and hope I got the bad part in the process.

Any test to rule out the switch, since it appears to be the easier of the two to replace?

Thanks

Tiny
Jblco
Jul 16, 2009.
ARTICLE BEGINNING DESCRIPTION & OPERATION When ignition is turned on, EEC power relay is energized, closing its contacts. Power is provided to both fuel pump relay and timing device in EEC module. Fuel pump runs through contacts of fuel pump relay. If ignition is not turned on within one second, timing device in EEC module will create an open ground circuit. Opening ground circuit of fuel pump relay de-energizes fuel pump. This circuit provides for pre- pressurization of fuel system. When ignition switch is turned to "START" position. EEC module operates fuel pump relay to provide fuel for starting engine while cranking. If vehicle is involved in a collision, inertia switch will stop fuel flow. Engine will run until residual fuel is exhausted. If inertia switch has been tripped, switch must be reset before vehicle can be restarted. Inertia switch location on Bronco II and Ranger is on floorboard to right of transmission hump. Aerostar inertia switch is located in the right kick panel. On "E" series, switch location is passenger's side cowl panel, forward of right front door. On Bronco and "F" series, switch location is on left kick panel, near emergency brake pedal. To reset inertia switch, turn ignition off. Check for fuel leaks in engine compartment, fuel lines, and at tank. If no leaks are found, reset switch by depressing button on top of inertia switch. If gasoline fumes or liquid are present, DO NOT reset switch. A high pressure pump is used on vehicles with MPFI equipped engines. A pressure relief valve, mounted internally, protects against over-pressurization of system if fuel flow is restricted. Aerostar has an in-tank high pressure pump. All other models use a frame mounted high pressure pump. A low pressure electric in-tank fuel pump is used on all models except Aerostar, to provide pressurized fuel to inlet of high pressure pump, reducing pump noise and pump overheating problems. HIGH PRESSURE FUEL PUMP 1. Check fuel tank(s) for adequate fuel supply. Check for fuel leaks at fittings and lines. Install jumper lead to fuel pump test terminal of VIP test connector. See Fig. 1. Use a test lead long enough to reach under vehicle. Install fuel pressure gauge. Turn ignition switch to the "RUN" position. ground jumper lead and refer to FUEL PUMP PRESSURE WITH ENGINE RUNNING table. If fuel pressure is within specification, go to step 5). If fuel pressure is not CAUTION: Fuel system is under pressure. Before removing any fuel lines, depressurize fuel system. Disconnect inertia switch and crank engine for a minimum of 15 seconds to depressurize system.within specification, go to step 2).2. If fuel pressure is low but greater than 3 psi (indicates pump is working), go to step 3). If not, go to step 4). 3. Check fuel system for plugged fuel filter or restricted fuel lines. Check for low voltage at the fuel pump (voltage should be with .5 volt of battery voltage). Disconnect fuel return line. If fuel is returning , adjust or replace pressure regulator. If filters are clear, voltage correct and no fuel return, replace fuel pump. 4. Ensure that battery voltage and ground are present at fuel pump connector. If circuits and voltages are correct, replace fuel pump. 5. Disconnect grounded test lead and note fuel pressure. Fuel pressure should remain within 2 psi for 3 minutes after disconnecting test lead. If fuel pressure holds, go to step 7). If not, go to step 6). 6. Inspect for fuel leaks. If no leaks are found, disconnect and plug fuel return line. ground test lead and raise pressure to normal level. If fuel pressure now holds, replace pressure regulator. If not, replace fuel pump. 7. Disconnect vacuum to fuel pressure regulator and start engine. Fuel pressure should within specification of FUEL PRESSURE WITH ENGINE RUNNING table. If fuel pressure is correct, go to step 9). If fuel pressure is not, go to step 8). 8. If fuel pressure is low, check fuel filter, fuel pressure regulator adjustment, fuel line restrictions or voltage to fuel pump. If all systems are normal, replace fuel pump. 9. With engine idling and fuel pressure regulator vacuum hose disconnected, measure fuel pressure. Accelerate rapidly and note fuel pressure. If fuel pressure drops more than 5 psi, repeat step 8). If fuel pressure does not drop, fuel system is operating correctly. LOW PRESSURE IN-TANK FUEL PUMP 1. Check fuel tank(s) for adequate fuel supply. Check for fuel leaks at fittings and lines. Install jumper lead to fuel pump test terminal of VIP test connector. See Fig. 1. Use a test lead long enough to reach under vehicle. Raise vehicle and ground test lead. Use a stethoscope and listen for pump operation. If low pressure pump is running, go to step 2). If not, go to step 3). 2. Disconnect test lead from ground. Remove reservoir inlet hose at reservoir. Place fuel hose in one quart calibrated container and ground test lead for 5 seconds. Fuel level should be at least 6 ounces. If fuel pump volume is correct, fuel pump is okay. If not, restrict fuel line momentarily to prime pump. If no flow develops, check for blocked fuel lines. If no blockage is found, replace pump. 3. If pump does not run, inspect all electrical connections. Check inertia switch and fuel pump relay for operation. If high pressure pump runs, but low pressure pump does not and voltage is present, replace low pressure fuel pump. REMOVAL & INSTALLATION FUEL PUMP CAUTION: Fuel system is under pressure. Before removing and fuel lines, depressurize fuel system. Disconnect inertia switch and crank engine for a minimum of 15 seconds to depressurize system.MPFI Frame-Mounted Fuel Pump Relieve fuel system pressure by opening electrical circuit to fuel pump and cranking engine for at least 20 seconds. Disconnect wiring at pump. Disconnect fuel lines. Unbolt 3 bolts attaching pump to frame. To install, reverse removal procedure. Start engine and check for leaks. Removal & Installation (In-Tank Fuel Pump) 1. Disconnect battery negative cable. Raise and support vehicle. Remove fuel tank. Turn sending unit lock ring counterclockwise and pull ring away from tank. 2. Lift fuel pump/sending unit assembly from tank. Note position of alignment tabs (if used). To install, reverse removal procedure. Start engine and check for leaks. Fig. 2: Electric Fuel Pump/Sending Unit Assembly Assembly shown is for vehicles with 7.5L engines. All models similar.


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Merlin2021
Jul 18, 2009.

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