6 PERFORMANCE DIAGNOSTICS 225
4. Start the engine and measure fuel pressure with
the shut-off valve closed. Open the shut-off valve
to check for aeration.
NOTE: Breaking any fuel system joint will indu ce
air into the fuel system. The air should pass in a
short period of time. As fuel pressure is relieved, a
steady stream of fuel without air bubbles indicates
the fuel is not aerated.
NOTE: IfaFuelPressureGaugewithshut-off
valveandclear3/8”diameterhoseisnot
available to check for aeration, see alternative
test “Checking for Aerated Fuel using Spare Fuel
Line.”
5. Record results on Diagnostic Form.
• If fuel pressure is below specification and fuel
is not aerated, replace the fuel filter and clean
the strainer. Test the fuel pressure again.
• If fuel is aerated, see “Aerated Fuel” in
Section 4.
• If fuel pressure is still low and fuel is not
aerated after replacing the fuel filter and
cleaning the strainer, do “Operation of the
Fuel Pump.”
• If fue l pressure is in speci fi cation and fuel is
not aerated, continue with step 6.
6. Run the engine at high idle. Measure the fuel
pressure with the shut-off valve closed. Open the
shut-off valve to check for aeration.
7. Record results on Diagnostic Form.
• If fuel pressure is below specification, replace
the fuel filter and clean the strainer. Test the
fuel pressure again.
• If fuel pressure is still low after replacing the
fuel filter and cleaning the strainer, do the
“Operation of the Fuel Pump Test.”
• If fuel pressure stays in specification, continue
to step 8
8. Does the vehicle have an automatic
transmission?
• If yes, continue to st ep 9.
• If no, continue to the next diagnostic test.
9. Set the parking brake and apply service brake.
WARNING: To avoid serious personal
injury, possible death or damage to the
engine or vehicle, make sure brakes are
correctly adjusted and in good condition.
This procedure should be done in an open lot.
CAUTION: Avoid damage to the drive train. Do
not do this test for more than 10 seconds at a time
or more than twice back to back. (If doing twice –
wait 2 minutes between tests.)
10. Put transmission in drive.
11. Press accelerator to the floor for no longer than
10 seconds.
12. Record results on Diagnostic Form.
• If fuel pressure is below specification, replace
fuel filter and clean the strainer. Test the fuel
pressure again.
• If fuel pressure is still low after replacing the
fuel filter and cleaning the strainer, do the
“Operation of the Fuel Pump Test.”
• If fuel pressure is in spec i fication, continue to
the next diagnostic test.
Possible Causes
No fuel
• Low fuel level in fuel tank
• Debris in tank can cause high-restriction and low
fuel pressure.
• Inline fuel valve (if equipped) could be shut-off
• Failed seals or fuel lines between fuel tanks
• Ice in fuel lines
• Inoperative fuel tank transfer pump
• Fuel tank pickup tube cracked
Low fuel pressure
•Dirtyfilter element
• Debris or rust in fuel strainer
• Restriction from the fuel tank to the fuel filter
housing inlet can cause high-restriction and low
fuel pressure.
• Plugged supplemental filters or water separators
can cause high-restriction and low fuel pressure.
EGES-270-1
Read all safety instructions in the "Safety Information" section of this manual before doing any procedures.
Follow all warnings, cautions, and notes.
©August 2008 Navistar, Inc.
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