Diagnosis Of Poor Fuel Economy/Smoke/Oil/Odors - GMC G Service Manual

ECONOMY/SMOKE/OIL/ODORS
PROBLEM
1. Customer driving habits.
Poor Gas Mileage
* Black Smoke From
Tail Pipe
2. Wrong speedometer gear.
3. Low tire pressure or incorrect tire size.
4. Transmission malfunction or in wrong gear.
5. Fuel leaks.
6. ' Plugged air cleaner element.
7. ' Choke not operating properly.
8. ' Carburetor vacuum break assembly(s) mal­
9. Ignition system malfunction.
10. Ignition timing misadjusted.
11. Distributor vacuum or mechanical advance
12. (1MEF)
13. Engine in need of service.
14. Restricted exhaust system.
15. ' High fuel level in carburetor or flooding.
ENGINE. PRIVEABILITY AND DIAGNOSIS 6A-21
DIAGNOSIS OF POOR FUEL
POSSIBLE CAUSE
functioning or misadjusted.
malfunctioning.
Air leaks at carburetor flange/insulator gas-
ket(s), or at intake manifold gasket. Vacuum
hoses disconnected or improperly installed.
(M4MEF)
Air leaks at carburetor flange gasket(s), or at
intake manifold gasket(s). Vacuum hoses
disconnected or improperly installed.
CORRECTION
1. Run mileage test, with customer driving if
possible. Make sure engine has at least
2,000-3,000 miles (3 200-4 800 km) for the
"break-in" period.
2. Check odometer against measured mile. Re­
place speedometer gear if necessary. Refer
to TRANSMISSION AND CLUTCH (SEC. 7).
3. Inflate tires to specifications and use correct
tire sizes. Refer to label on driver' s door.
4. Refer to TRANSMISSION AND CLUTCH
(SEC. 7).
5. Inspect fuel tank, fuel lines and fuel pump for
any fuel leakage.
6. Replace element.
7. Check for complete opening and closing of
choke valve. See "Choke Checking Pro­
cedure (Engine Off)" and "Checking Electric
Choke" in CARBURETORS (SEC. 6C1).
8. See "Vacuum Break Checking Procedure" in
CARBURETORS (SEC. 6C1). Adjust vac­
uum break assembly(s) to specification.
9. Check ignition system. Refer to ENGINE
ELECTRICAL (SEC. 6D).
10. Adjust timing. See Emission Control Informa­
tion Label.
11. Check operation. Refer to ENGINE ELEC­
TRICAL (SEC. 6D).
12. Use a pressure oil can to spray light oil or
kerosene around carburetor throttle body (do
not spray at throttle shaft ends), and manifold
to head mounting surfaces.
If engine RPM changes, torque carburetor
to manifold bolts and/or intake manifold bolts
to specification. If necessary, replace the
carburetor flange/insulator gasket(s) and/or
intake manifold gasket(s). Check condition
and routing of vacuum hoses. Correct or re­
place as necessary. Refer to Emission Con­
trol Information Label on vehicle for correct
routing.
13. Check engine compression.
14. Check for restrictions and correct as neces­
sary. See diagnosis for "Restricted Exhaust
System" in this chart.
15. Inspect float for restricted movement or
being loaded.
Check float needle and seat for wear, dirt
or chips.
If foreign material is found in carburetor,
clean fuel system and carburetor. Replace
fuel filter(s) as necessary.
Check float level and adjust if necessary.
(M4MEF)
Also check float needle pull clip location.
Open end of clip should be installed over float
lever cross bar, facing away from pontoon.
GMTB-0920-2L