GMC 1976 ZEO 6083 Maintenance Manual page 281

6A- 1 6 ENGINE
WHITE
Condensing water vapor which is a normal
product of combustion-no problem- usually seen
on cold days .
BLACK
Excessively rich fuel mixture .
(See "Excessive Fuel Comsumption" .)
EXCESSIVE FUEL CONSUMPTION (1)
GENERAL
"Jack Rabbit" starts .
High speed.
Short drives .
Restricted Choke (partly closed) .
Clogged air cleaner.
Loss of compression .
Excessive rolling resistance from low
tires, dragging brakes, wheel misalignment,
etc.
Restricted exhaust.
Engine overheating.
Crankcase ventilating system faulty .
Trailer towing .
Worn-out or badly tuned engine .
IGNITION
Faulty ignition system .
GENERAL
Low oil level.
Clogged oil filter .
Thin or diluted oil (frequent stops in
cold weather) .
Viscosity (too light grade) .
Oil has foam from water (condensation or
leaking head gasket).
Overheating.
SMOKE
LOW OIL PRESSURE
BLUE
(Or Bluish White)
Excessive oil consumption
(See "Excessive Oil Consumption")
FUEL
Excessive fuel pump pressure .
Float level high . (4)
Faulty carburetor .
Leakage or loose fittings .
Idle speed settings incorrect .
Accelerator pump improperly adjusted .
MECHANICAL
Faulty valves or valve train.
Faulty rings.
Choke mechanism binding or improperly
adjusted .
Accelerator linkage binding or improperly
adjusted .
Fuel tank cap missing.
MECHANICAL
Faulty pressure sending unit, line, or
gauge.
Worn oil pump .
Excessive bearing clearance.
Oil pump relief valve dirty, worn,
weak or worn .
Oil pump suction tube loose or cracked.
Screen clogged (ice, gummy, sludge or
dirt).
Air leak in oil pump (loose cover or too
thick gasket).
Loose connections in oil lines .
spring