GMC 1976 ZEO 6083 Maintenance Manual page 278

MECHANICAL
Choke mechanism faulty .
Lack of engine compression.
Incorrect valve timing .
Inaccurate speedometer (Gives impression
of lack of performance) .
ENGINE FAILS TO REACH OPERATING TEMPERATURE
GENERAL
Thermostat removed.
GENERAL
Scale or rust deposits .
Slipping fan belt .
Low coolant. (Leaky system-internal or
external .)
Pre-Ignition .
Detonation .
Excessive friction in engine or elsewhere
in power transmitting units. (Brakes
dragging, etc.)
Excessive back pressure in exhaust system .
Overloading vehicle.
High altitude .
Hot climate operation.
Insufficient oil in crankcase.
FUEL
Carburetor mixture too lean .
MECHANICAL
Cylinder head bolts loose.
Warped or damaged head or block.
Wrong head gasket .
ENGINE OVERHEATS
Valve spring weak, broken valves
sticking when hot.
Valve timing incorrect.
Worn camshaft lobes.
Blown cylinder head gasket .
Burned, warped or pitted valves .
COOLING
Defective thermostat (stuck open).
Faulty temperature sending unit or dash
unit .
NOTE :
Coolant is used to cool the engine and air is used to cool the coolant.
Anything which prevents the coolant air system from working properly will
cause engine to overheat . (Air, oil or grease in the coolant will reduce the ability
of the coolant to absorb heat from the block and to transfer heat to the coolant
in the radiator.)
IGNITION
Timing late .
Distributor advance faulty .
Valve timing off or late .
COOLING
ENGINE
6A- 1 3
or valves
Restricted flow of coolant. (Defective
components-dirt, rust and scale.)
Leaking head gasket . (Permits air in
cooling system and coolant in engine .)
Thermostat fails or wrong thermostat .
Hoses defective .
Exterior of radiator clogged with dirt,
leaves, or insects.
Water pump defective or loose.
Wrong type of coolant.
Wrong fan or hydraulic fan inoperative,
or defective.
Wrong pressure cap or faulty cap.
Radiator fins bent or mutilated.