Service manual

G420F(FE) Service Manual 262 Chapter 8. Basic Troubleshooting
Surges and/or Stumbles
Engine power varies under steady throttle or cruise. Feels like the vehicle speeds up and slows down with no
change in the acceleration pedal.
Preliminary Checks
Perform the visual checks as described at start of “ Basic Troubleshooting” chapter.
Be sure driver understands vehicle operation as explained in the operator manual.
PROBABLE CAUSE CORRECTIVE ACTION
Oxygen sensor
malfunction
The fuel management should maintain a stoichiometric air-fuel ratio under all
steady state operating conditions following engine warmup. Failure of the Pre-
catalyst O2 sensor should cause an O2 sensor fault that can be diagnosed with
the MIL lamp or Service Tool.
Fuel system malfunction
NOTE: To determine if the condition is caused by a rich or lean system, the
vehicle should be driven at the speed of the complaint. Monitoring pre-catalyst
O2 adapts*, dither valve duty cycle, or mechanical injector pulse width will help
identify problem.
Check fuel supply while condition exists.
Check in-line fuel filter. Replace if dirty or plugged.
Check fuel pressure.
Ignition system
malfunction
Check for proper ignition voltage output using spark tester.
Check spark plugs.
• Remove spark plugs, check for wet plugs, cracks, wear, improper gap, burned
electrodes, or heavy deposits.
• Repair or replace as necessary.
• Check condition of distributor cap, rotor and spark plug wires (where
applicable).
Check ignition timing. Refer to application manual for specs.
Component malfunction
Check vacuum lines for kinks or leaks.
Check alternator output voltage. Repair if less than 9 or more than 16 volts.
Exhaust backpressure
Check condition of exhaust system.
Check backpressure before catalyst. It should be less than 3.5 psig (24.13 kPa).
(*) Refer to Table 1 for description of gaseous and liquid O2 adapts.
Related MIL Faults:
Pre-catalyst O2 sensor errors / O2 control errors
Dither valve DC faults / EST faults / ETC faults