DEFINITION:
Gas mileage, as measured by an actual road test, is noticeably lower than expected. Gas mileage is noticeably lower than it was during a previous actual road test.

Perform careful visual and physical check as lescribed at the beginning of Section "B". Perform "Diagnostic Circuit Check."

• CHECK
  • Proper operation of EST See Section "C4".
  • For possibility of misfiring, crossfirng, or cutting out under load or at idle. If present, refer to the "Ignition System Check" Chart in Section ''C4."
  • Spark plugs. Look for wetness, cracks, improper gap, burned electrodes, and heavy deposits.
  • Spark plugs for correct heat range.
  • Fuel for poor quality, "stale" fuel, and water contamination.
  • Fuel system for restricted filter or improper pressure. Use CHART A-7.
  • Injectors for leakage. Pressurize system by energizing fuel pump through the underhood fuel pump test connecter.
  • EGR operation. Use Code 32 chart.
  • Distributor for worn shaft.
  • For vacuum leaks at intake manifold gasket.
  • Air cleaner element (filter) for dirt or plugging.
  • Idle Air Control system. Use Code 35 chart.
  • Canister purge system for proper operation. Refer to EVAPORATIVE EMISSION CONTROL SYSTEM (SEC. C3) in this manual.
  • PROM for correct application (Consult Service Bulletins.)
  • Throttle shaft or TPS for sticking or binding. TPS voltage should read less than 1.25 volts on a "Scan" tool with the throttle closed.
  • MAP sensor output. Use CHART C1-D and/or check sensor by comparing it to the output on a similar vehicle if possible.
  • Oxygen sensor for silicone contamination from contaminated fuel or use of improper RTV sealant. The sensor will have a white, powdery coating and will cause a high but false signal voltage (rich exhaust indication). The ECM will reduce the amount of fuel delivered to the engine, causing a severe driveability problem.
  • Coolant sensor for a shift in calibration. Use Code 14 or Code 15 chart.
  • Vehicle Speed Sensor (VSS) input with a "Scan" tool to make sure reading of VSS matches that of vehicle speedometer. Refer to "Diagnostic Information" in Section "6E."
  • A/C relay operation. A/C should cut out at wide open throttle. Use CHART C-i 0
  • A/C refrigerant pressure for high pressure. Check for overcharging or faulty pressure switch.
  • Generator output voltage. Repair if less than 9 volts or more than 16 volts.
  • Cooling fan operation. Use CHART C-12.
  • Transmission torque converter operation. Refer to AUTOMATIC TRANSMISSION (SEC. 7A1) in the 1991 Light Truck SIT Models Service Manual.
  • Transmission for proper shift points. Refer to AUTOMATIC TRANSMISSION (SEC. 7A1) in the 1991 Light Truck SIT Models Service Manual.
  • Transmission torque converter clutch operation. Use CHART C-8.
  • Engine valve timing and compression. Refer to ENGINE (SEC. 6A) in the 1991 Light Truck SIT Models Service Manual.
  • For worn or incorrect basic engine parts si'ch as cam, heads, pistons, etc. Also check for bent pushrods, worn rocker arms, and broken or weak valve springs. Refer to 4.3 LITER V6 (SEC. 6A3) in the 1991 Light Truck SIT Models Service Manual.
  • For the possibility of an exhaust restriction or improper valve timing by performing the following test.
    1. With engine at normal operating temperature, connect a vacuum gauge to any convenient vacuum port on intake manifold.
    2. Run engine at 1000 rpm and record vacuum reading.
    3. Increase engine speed slowly to 2500 rpm. Note vacuum reading at steady 2500 rpm.
    4. If vacuum at 2500 rpm decreases more than 3" Hg from reading at 1000 rpm, the exhaust system should be inspected for restrictions.
    5. Disconnect exhaust pipe from engine and repeat Steps 3 & 4. If vacuum still drops more than 3" Hg with exhaust disconnected, check valve timing.

  • Thermostat for incorrect heat range or being inoperative

• Check driver's driving habits and vehicle conditions which affect gas mileage.
  • Suggest driver read "Important Facts on Fuel Economy" in Owner's Manual.
  • Is A/C "ON" full time (Defroster mode "ON")?
  • Are tires at correct pressure?
  • Are excessively heavy loads being carried?
  • Is acceleration often heavy?
  • Are the wheels aligned correctly?
  • Is the speedometer calibrated correctly?
  • Are the vehicle brakes dragging?
  • Is the brake switch applying excessive force on the brake pedal?

• If the system is running RICH, (block learn less than 118), refer to "Diagnostic Aids" on facing page of Code 45.