Fuel pump will not prime
If your truck utilizes a mechanical fuel pump follow the testing procedure listed below. These trucks (1990 – 1996) employ a single, high pressure pump which is part of the modular, In-Tank Reservoir (ITR) assembly. Besides the pump, the ITR consists of a venturi jet pump, a supply check valve and a shuttle selector valve. All this is mounted on the fuel gauge sender flange. The sending unit is separate from the ITR module.
When the fuel system is opened allowing the pressure to bleed off will require the system to be primed once reassembled. This can be achieved by cycling the ignition switch on and off. The ignition key needs to be moved to the “ON” position for 5 seconds and then moved to “OFF” for 5 seconds. This needs to be repeated a minimum of 10 times. If fuel pressure has not built up, check for fuel leaks first. Then test your fuel pump relay function. Next test your fuel pump.
Mechanical Fuel Pump Testing
CAUTION
Never smoke when working around gasoline! Avoid all sources of sparks or ignition. Gasoline vapors are EXTREMELY volatile!
Incorrect fuel pump pressure and low volume (flow rate) are the two most likely fuel pump troubles that will affect engine performance. Low pressure will cause a lean mixture and fuel starvation at high speeds and excessive pressure will cause high fuel consumption and carburetor flooding.
To determine that the fuel pump is in satisfactory operating condition, tests for both fuel pump pressure and volume should be performed.
The test are performed with the fuel pump installed on the engine and the engine at normal operating temperature and at idle speed.
Before the test, make sure that the replaceable fuel filter has been changed at the proper mileage interval. If in doubt, install a new filter.
Pressure Test
- Remove the air cleaner assembly. Disconnect the fuel inlet line of the fuel filter at the carburetor. Use care to prevent fire, due to fuel spillage. Place an absorbent cloth under the connection before removing the line to catch any fuel that might flow out of the line.
- Connect a pressure gauge, a restrictor and a flexible hose between the fuel filter and the carburetor.
- Position the flexible hose and the restrictor so that the fuel can be discharged into a suitable, graduated container.
- Before taking a pressure reading, operate the engine at the specified idle rpm and vent the system into the container by opening the hose restrictor momentarily.
- Close the hose restrictor, allow the pressure to stabilize and note the reading. The pressure should be 5 psi. (34.5 kPa).
If the pump pressure is not within 4–6 psi (27.6–41.4 kPa) and the fuel lines and filter are in satisfactory condition, the pump is defective and should be replaced.
If the pump pressure is within the proper range, perform the test for fuel volume.
Volume Test
- Operate the engine at the specified idle rpm.
- Open the hose restrictor and catch the fuel in the container while observing the time it takes to pump 1 pint. 1 pint should be pumped in 20 seconds. If the pump does not pump to specifications, check for proper fuel tank venting or a restriction in the fuel line leading from the fuel tank to the carburetor before replacing the fuel pump.