GMC 4000 Series Service Manual page 298

(When checked with engine at normal operating temperature)
POSSIBLE CAUSES
1. Defective oil gauge or sending unit.
2. Oil viscosity too low.
3. Oil diluted with gasoline.
4. Suction lo ss.
5. Weak or broken relief valve spring in oil pump.
SYMPTOM A - VALVE MECHANISM NOISE
POSSIBLE CAUSES
1. Sticking valves.
2. Incorrect valve lash.
3. Bent push rod(s).
4. Worn rocker arms and/or shaft.
5. Broken valve spring.
6. Damaged valve lifter and/or camshaft.
SYMPTOM B - BEARING NOISE
POSSIBLE CAUSES
1. Insufficient oil supply.
2. Low oil pump pressure.
3. Thin or diluted oil.
4. Excessive bearing clearance.
5. Piston pins loose fit in connecting rod or piston.
6. Piston to cylinder bore clearance excessive
(piston slap).
NOTE: When diagnosing engine noise problems, be careful that noises caused by
accessories such as
engine noises. Removal of accessory drive belts will eliminate any noises caused
by these units.
G M C S E R V I C E M A N U A L
LOW ENGINE OIL PRESSURE
1. Check pressure with master gauge. Replace oil
gauge or sending unit if defective.
2. Fill crankcase with correct oil.
3. Check for indications of choke malfunction or
carburetor flooding allowing gasoline to enter
crankcase. Make necessary correction.
4. Check for loose intake pipe and screen in oil
pan. Also check for partially clogged inlet screen.
5. Inspect spring and replace if necessary.
ENGINE NOISE
1. Clean and lubricate valve stem s.
2. Adjust valve lash.
3. Determine and correct cause of push rod bend­
ing. Install new push rod.
4. Replace worn parts and make sure oil is reach­
ing valve rocker arm s.
5. Replace spring.
6. Replace lifter and/or camshaft.
1. Check oil level and add oil as required.
2. Remove and inspect oil pump and inlet screen.
Make necessary corrections.
3. Change oil. Use oil with proper viscosity.
4. Remove oil pan and make bearing replacement
or repairs.
5. Install new piston pins (oversize pins if req'd.).
6. Overhaul engine.
air com pressor and power take-off are not mistaken for
Page 287
GASOLINE ENGINES
REMEDY
REMEDY
REMEDY
Sec. 6A