GMC 4000 Series Service Manual page 297

Sec. 6A
Page 286
GASO LINE ENGINES
SYMPTOM A - ERRATIC RUNNING OR SURGING (Cont'd.)
POSSIBLE CAUSES
4. Inoperative crankcase ventilation valves.
5. Cylinder head gaskets leaking, or cylinder head
cracked.
6. Worn valve guides.
SYMPTOM B - STALLING
POSSIBLE CAUSES
1. Engine idle speed set too slow.
2. Engine running too cool.
3. Exhaust restricted.
4. Carburetor flooding.
5. Defective fuel pump.
(SPARK KNO CK OR PING O N ACCELERATION)
POSSIBLE CAUSES
1. Ignition advance too far for fuel being used.
2. Wrong type (heat range) spark plug.
3. Excessive build-up of deposits in combustion
chambers.
4. Restricted coolant passages in cylinder head
causing "hot spots" in combustion chamber.
5. Overheated engine.
6. Lugging engine.
POSSIBLE CAUSES
1. Oil lines or connections leaking.
2. Leaking gaskets.
3. Crankcase oil level too high.
4. Crankshaft oil seals worn.
5. Pistons and/or rings worn, or pistons damaged;
cylinder bores scored or worn.
G M C S E R V I C E M A N U A L
ROUGH IDLE A N D /O R STALLING (CONT.)
DETONATION
HIGH LUBRICATING OIL CONSUMPTION
REMEDY
4. Clean or replace valves.
5. Replace cylinder head or gasket.
6. Overhaul cylinder head.
REMEDY
1. Adjust idle speed to specifications.
2. Install proper thermostat.
3. Repair exhaust system to eliminate excessive
back pressure.
4. Replace defective float valve, set carburetor
float level. Check fuel pump for excessive
pressure.
5. Replace pump.
REMEDY
1. Retard ignition timing.
2. Install correct spark plug.
3. Clean combustion chambers.
4. Remove cylinder head and clean passages.
5. Make corrections to lower the engine operating
temperature. (Refer to "Causes" and "Remedies"
previously covered under "Overheating.")
6. Use lower transmission gear to prevent over­
loading engine.
REMEDY
1. Tighten or replace defective parts.
2. Replace gaskets as necessary.
3. Drain crankcase and refill to correct level.
4. Replace oil seals.
5. Overhaul engine.