Poor Fuel Economy; Dieseling, Run On - GMC G Service Manual

6E1-16 CARBURETOR EMISSIONS
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.
• 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?
— Suggest driver read "Important Facts on Fuel
Economy" in Owner's Manual.
• Check air cleaner dam per door operation
(THERMAC). Refer to "Diagnosis of THERMAC" in
this section.
• Check air cleaner element (filter) for dirt or being
plugged.
• Check for proper calibration of speedometer.
• Visual check:
Definition: Engine continues to run after key is turned
"OFF", but runs very roughly. If engine runs smoothly,
check ignition switch and adjustment.
• Visual check:
— Vacuum hoses for splits, kinks and proper con­
nections as shown on Vehicle Emission Control
Information label.
— Carburetor choke, throttle linkage and fast idle
cam for sticking.
• Check carburetor choke, vacuum break linkage,
throttle linkage and fast idle cam for proper adjust­
ment. Refer to CARBURETORS (SEC. 6C1).
• Check cruise control for proper adjustment.
• Check engine idle speed.

POOR FUEL ECONOMY

DIESELING, RUN-ON
— Vacuum hoses for splits, kinks and proper con­
nections as shown on Vehicle Emission Control
Information label.
— Ignition wires for cracking, hardness and proper
connections at distributor and coil.
• Check ignition timing. Refer to Vehicle Emission Con­
trol Information label.
• Remove spark plugs. Check for cracks, wear,
improper gap, burned electrodes or heavy deposits.
Repair or replace as necessary.
• C h e ck c o m p re s s io n . R e fe r to E N G IN E ,
DRIVEABILITY AND DIAGNOSIS (SEC. 6A).
• Check TCC for proper operation.
• Check for dragging brakes.
• Suggest owner fill fuel tank and recheck fuel
economy.
• Check for exhaust system restriction. Refer to Chart
B-1.
• Check ignition timing. Refer to Vehicle Emission Con­
trol Information label.
• Remove carbon with top engine cleaner. Follow
instructions on can.
• Check for engine overheating. Normal coolant tem­
p eratu re is 85 to 100 degrees C (185 to
215 degrees F).
— Low coolant.
— Loose water pump belt.
— Restricted air flow to radiator, or restricted water
flow through radiator.
— Inoperative fan clutch.