GMC 1976 ZEO 6083 Maintenance Manual page 274

ENGINE HAS INCONSISTENT IDLE SPEED (LOPES) (1)
NOTE :
If idle speed is slow, unstable, rolling, frequent stalling, and oily engine
compartment, the positive crankcase ventilation valve may be completely
plugged, or the valve may be stuck in the "OPEN" position . A valve stuck in
the "CLOSED" position is indicated by breather back-flow at heavy throttle and
oily engine compartment. If the valve is stuck in the intermediate position it will
be indicated by rough, fast idle and stalling .
GENERAL
Restricted exhaust.
Vacuum leak (Intake valve stem leaking,
carburetor mounting gasket leaking, cabruetor
throttle shaft in carburetor leaking, intake
manifold or vacuum hoses leaking) . (9)
Timing and dwell not correct.
Restricted air cleaner.
Overheated engine (Refer to "Engine
Overheats" in this section) . (11)
Blown head gasket.
Low compression.
Quality of fuel .
Lean idle mixture. (1)
FUEL
Dirt and/or water infuel system .
Too rich or lean mixture.
Filter restricted .
Faulty fuel pump . (4)
Faulty carburetor .
Vacuum leak . (9)
Timing not correct.
Exhaust restriction .
Blown head gasket .
Low compression.
Fuel quality poor.
Air cleaner dirty.
MECHANICAL
Leaky or incorrect valve.
Worn or leaky valve guide.
Worn timing chain, gears, sprocket or
camshaft lobe .
Dirt in hydraulic lifter .
MECHANICAL
Throttle shaft, accelerator pedal and/or
throttle linkage sticking or binding.
Timing chain or gears and/or camshaft
lobes worn .
Burned, warped, pitted, leaky or sticking
valves .
Inoperative choke .
Sticking hydraulic lifter .
IGNITION
ENGINE 6A- 9
Excessive oil or dirt on ignition system .
Spark plugs damp or gap incorrectly set .
Excessive moisture on ignition wires and
caps .
Leaks in ignition wiring (Dirty, corroded,
or faulty wiring).
Ignition wires making poor contact.
Burned, pitted, or incorrectly set contact
points .
Faulty coil or condenser .
Worn distributor cam, or cracked distribu-
tor cap, radial contacts in distributor cap
burned or worn .
Faulty spark advance mechanism .
ENGINE RUNS - MISSES AT IDLE ONLY (1)
GENERAL IGNITION (2)
Spark plugs faulty or wrong gap.
Incorrect, worn, or gap incorrectly set.
Leaks in ignition wiring .
Burned, pitted, or incorrectly set contact
points .
Faulty coil and/or condenser.
Faulty spark advance mechanism.
Defective or worn rotor and/or cap.
FUEL (4)
Flooding in carburetor .
Refer to "Engine Has Inconsistent Idle Speeds" above.