Fuel Injection System Diagnosis
Diagnosis of the fuel injection system includes the following diagnostic steps.
INJECTION PUMP
To check the fuel supply to the injection pump (figure 4-9):
1. Check that the output of the lift pump is correct:
• 1988-1993 Model Year Pumps (p/n 25115224):
• Volume of at least 0.24 liter (1/2 pint) in 15 seconds
• Pressure of 40 to 60 kPa (5.8 to 8.7 psi) (Tee in after filter at pump inlet, running at idle)
• 1994 Model Year Pumps (p/n 25117340):
• Volume of at least 0.24 liter (1/2 pint) in 15 seconds
• Pressure of 3 psi minimum with the engine cranking, the ESO disconnected, and the fuel line
open after the filter to an approved canister.
2. Check that the restriction of the fuel filter is not excessive:
• Lift pump volume and pressure should be present at the inlet of the injection pump.
3. Check the fuel entering the injection pump for the presence of air, using a transparent hose:
• If air bubbles appear, check the lift pump suction line for air leakage under a vacuum.
4. Check the quality of the fuel:
• If necessary, use a fuel with a known cetane rating.
INJECTION LINES AND NOZZLES
To check the operation of the injection lines and nozzles (figure 4-10):
1. Inspect the injection lines for external damage and evidence of fuel leakage.
2. Replace any damaged injection line.
3. Remove and test each injection nozzle for correct opening pressure, chatter, leakage and spray pattern.
• Refer to the Driveability and Emissions Service Manual section 4 for nozzle opening pressure.
4. Replace any faulty nozzle and install the nozzles with new compression gaskets.
Fuel Contamination Inspection and Cleaning
1. Remove the fuel filter element and inspect it:
• If water, gasoline or fungi/bacteria are not present, end the inspection.
• If water or fungi/bacteria are present, go to step 2.
• If gasoline is present, go to step 3.
2. Clean water from the fuel system in these steps:
A. Disconnect the batteries.
B. Drain the fuel tank.
C. Remove the fuel tank.
D. Remove the fuel sender unit.
E. Inspect the fuel tank and fuel sender for rust, fungi or bacteria:
• If no rust is present, clean the inside of the fuel tank and fuel sender with hot water, then
dry them with compressed air.
• If rust is present, replace affected parts.
F. Disconnect the ends of the following lines:
• Lift pump suction
• Lift pump feed
• Fuel filter outlet
• Fuel filter drain
• Fuel return
G. Inspect each of the lines and replace any rusted pipes.
H. Dry the inside of each line with low-pressure air.
I. Clean the inside of the fuel filter housing and dry it with compressed air.
J. On EFI pumps, disconnect the electrical connector for the engine shutoff (ESO) solenoid.
• On mechanical pumps, disconnect the pink wire from the fuel shut off solenoid.
K. Install a new fuel filter element.
L. Install the fuel pick-up/sending unit and fuel tank (add clean diesel fuel to one-quarter full).
M. Connect the following lines:
• Lift pump suction (both ends)
• Lift pump feed (both ends)
• Fuel filter drain
• Fuel return (at injection pump)
N. Connect the fuel filter outlet and the fuel return line at the pick-up/sending unit to hoses that flow
to metal containers.
O. Connect the batteries and crank the engine until clean fuel flows from the fuel filter outlet into a metal container (figure 4-11):
• Allow a maximum of 15 seconds cranking time, followed by 1 minute of cranking motor cooling time.
P. Connect the hose from the fuel filter outlet to the injection pump inlet.
Q. Open each injection line at its nozzle end and crank the engine until clean fuel flows from it:
• Use two wrenches when loosening the injection line fittings.
• Allow a maximum of 15 seconds cranking time, followed by 1 minute of cranking motor cooling time.
R. Tighten each injection line fitting at its nozzle:
• Use two wrenches when tightening the injection line fittings.
S. Connect the electrical connector for the ESO solenoid in the injection pump (electric pump) or the pink wire (mechanical pump).
T. Start and run the engine for 15 minutes while fuel flows from the fuel return line into a metal container (figure 4-11).
U. Stop the engine.
V. Connect the fuel return hose to the fuel sender.
W. Clean the engine of fuel spillage.
X. Fill the fuel tank and add a biocide, if needed.
3. Clean gasoline from the fuel system in these steps:
A. Determine a procedure:
• If the engine runs, follow steps B, C, and I.
• If the engine does not run, begin at step B.
B. Drain the fuel tank.
C. Fill the fuel tank.
D. On electric pumps, disconnect the electrical connector for the engine shutoff (ESO) solenoid in the injection pump.
• On mechanical pumps, disconnect the pink wire.
E. Remove the fuel filter outlet and connect it to a hose that flows to a metal container.
F. Crank the engine until clean fuel flows from the fuel filter outlet into a metal container:
• Allow a maximum of 15 seconds cranking time, followed by 1 minute of cranking motor cooling time.
G. Connect the hose from the fuel filter outlet to the injection pump inlet.
H. Connect the electrical connector for the ESO solenoid in the injection pump (electric pump) or the pink wire (mechanical pump).
Fuel Specific Gravity Check
1. Drain the fuel filter housing by doing these things:
• Stop the engine and place a container under the drain valve exit hose at the left front side of the
engine.
• Open the drain valve.
• Start the engine and idle until clear fuel appears at the drain valve exit hose.
• Fill a 1-liter (0.946-quart) container with a sample of clean fuel.
• Close the drain valve and stop the engine.
• Bring the fuel sample to 60°F (16°C).
2. Obtain a fuel quality hydrometer (special tool J 34352).
3. Fill the hydrometer with the fuel sample by doing these things:
• Squeeze the hydrometer bulb and submerse the hydrometer tip into the sample.
• Release the bulb, allowing fuel to enter the glass tube until it floats the glass bulb inside the tube.
• Gently spin the hydrometer to relieve the surface tension of the fuel sample.
4. Read the scale on the glass bulb at the point where the top of the fuel sample contacts it (figure 4-13):
• If the top of the fuel sample is in the yellow part of the glass bulb scale (above the green part), suspect diluted fuel.
• If the top of the fuel sample is in the green part of the glass bulb scale, the fuel has high quality
(approximate cetane rating of 46 to 50).
• If the top of the fuel sample is in the yellow part of the glass bulb scale (below the green part), the
fuel has moderate quality (approximate cetane rating of 41 to 45).
• If the top of the fuel sample is in the red part of the glass bulb scale, the fuel has low quality (approximate cetane rating of 38 to 40).
