Poor Fuel Economy

DEFINITION
Fuel economy, as measured by an actual road test, is noticeably lower than expected. Also, economy is noticeably lower than it was on this vehicle at one time, as previously shown by an actual road test.

PRELIMINARY CHECKS
^ Perform the careful visual checks as described at start of "Symptoms".
^ Visually (physically) check: Vacuum hoses for splits, kinks, and proper connections as shown on "Vehicle Emission Control Information" label found under the hood.
^ Check owner's driving habits.
- Is A/C "ON" full time (Defroster mode "ON")?
- Are tires at correct pressure?
- Are excessively heavy loads being carried?
- Is acceleration too much, too often?
^ Check for a dirty or plugged air cleaner.
- Fuel leaks.

ADDITIONAL CHECKS
^ [1][2]Torque Converter Clutch (TCC) operation. Use Chart C-8a for 4T60E and Chart C-8c for 3T40. Refer to Diagnostic Charts / C Charts. A Tech 1 scan tool should indicate an RPM drop, when the TCC is commanded "ON." Testing and Inspection
^ Exhaust system restriction. See Diagnostic Charts / B Charts / Chart B-1 Restricted Exhaust System Check. B Charts
^ Proper calibration of speedometer.
^ Induction system and crankcase for air leaks.

IGNITION SYSTEM CHECKS
^ Spark plugs. Remove spark plugs, check for wet plugs, cracks, wear, improper gap, burned electrodes, or heavy deposits. Repair or replace as necessary.
^ For open ignition control CKT 423. Refer to Diagnostic Charts / Diagnostic Trouble Code Charts / Code 41 Ignition Control (IC) Timing Circuit Error. Powertrain Controls - ECM/PCM

COOLING SYSTEM CHECKS
^ Engine coolant level.
^ Engine thermostat for faulty part (always open) or for wrong heat range.

ENGINE MECHANICAL CHECKS
^ Check Compression.