Hesitation, Sag, Stumble

DEFINITION:
Engine power variation, under steady throttle or cruise. Feels like the vehicle speeds up and slows down, with no change in the accelerator pedal.




SYSTEM OR COMPONENT TO CHECK ACTION

COMPUTER INPUTS/OUTPUTS CHECK: ^ OXYGEN SENSOR (O2S) - Inspect sensor for
silicon contamination from fuel, or use of
improper RTV sealant.
The sensor will have a white, powdery coating,
and will result in a high but false signal
voltage (rich exhaust indication). The ECM/PCM
will then reduce the amount of fuel delivered
to the engine, causing a severe driveability
problem.
^ ECM/PCM grounds for being clean,
tight, and in their proper location.
NOTE: ^ Use a "Scan" tool to make sure reading
of VSS matches vehicle speedometer except
vehicles with 4L80E transmission where some
variation between VSS and speedometer is normal.




FUEL 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 block learn will help identify problem.
LEAN: Block learn more than 150
RICH: Block learn less than 115
CHECK: ^ FUEL PRESSURE
^ FUEL INJECTOR BALANCE TEST.
^ IN-LINE FUEL FILTER.
^ Contaminated Fuel.



IGNITION CHECK: ^ Proper ignition voltage output with
spark tester J 26792 or equivalent (ST-125).
^ Spark plugs, wet plugs, cracks, wear, improper
gap, burned electrodes or heavy deposits.
^ IGNITION TIMING.





ENGINE CHECK: ^ Vacuum lines for kinks and leaks.
^ Generator output voltage. Repair if less than
9 or more than 16 volts.
^ TCC operation.
NOTE: ^ Be sure driver understands
Torque Converter Clutch (TCC) and A/C
compressor operation in owner's manual.