GMC 1976 ZEO 6083 Maintenance Manual page 283

6A- 1 8
ENGINE
EXCESSIVE OIL CONSUMPTION
NOTE :
Check the PCV valve for proper operation before checking causes of leak .
A clogged crankcase vent valve can build up pressure in the crankcase which
will cause seals and gaskets to leak .
EXTERNAL LEAKAGE
Oil pan drain plug loose or gasket missing.
Crack or hole in oil pan .
Oil pan gasket leaks due to :
(a) Loose screws ; (b) Damaged gasket ;
(c) Improperly installed gasket ;
(d) Bent oil pan flange .
Oil pan gasket leaks due to :
(a) Loose screws; (b) Damaged gasket ;
(c) Improperly installed gasket :
(d) Bent oil pan flange .
Timing case cover gasket leaks due to :
(a) Loose screws ; (b) Damaged gasket ;
(c) Improperly installed gasket ;
(d) Bent cover flange ;
Front crankshaft oil seal leaks due to :
(a) Worn oil seal ; (b) Seal not
properly installed; (c) Rough surface on
crankshaft, or fan pulley or damper ;
(d) Damper or pulley loose; (e) Seal or
cover not centered on crankshaft ; (f) Oil
return passage to crankcase clogged up .
Rear main bearing oil seal leaks due to :
(a) Worn oil seal ; (b) Improper oil seal
installation ; (c) Worn rear main bearing;
(d) Rough crankshaft . Oil passage to
crankcase clogged.
Expansion plug in block at rear of camshaft
leaks due to poor fit, careless installation,
or corrosion.
Leakage at any external piping .
Plugs at ends of oil passages in cylinder
block leak .
Oil filter leaks.
Leakage at distributor .housing .
Valve cover leaks due to loose screws,
defective gasket, improperly installed gasket
or bent cover flange .
Rocker arm cover or push rod cover leaks
due to loose screws, defective gasket, im-
proper gasket installation or bent cover flange .
Pipe connections loose on oil gauge or oil
filter lines .
Improperly seated or broken fuel pump
gasket .
Broken push rod cover gasket, oil filter
gasket, or timing chain cover gasket .
Worn timing chain cover oil seal .
Worn or improperly seated rear main
bearing oil seal .
Loose oil line plugs.
Rear camshaft bearing drain hole plugged.
Loose rocker arm cover, gasket broken, or
cover distorted or bent.
Rear main bearing side seal improperly
installed.
INTERNAL LEAKAGE
Carbon in oil ring slot .
Rings fitted too tight in grooves.
Leaky piston rings due to wear, scuffs or
broken .
Leaky piston rings due to sticking caused
by gummy deposit. Try to free up with
suitable solvent poured in fuel tank, Blue
smoke at tail pipe indicates badly
leaking rings.
Worn pistons and cylinders.
Cylinder block distorted by tightening
cylinder head bolts unevenly .
Excessive clearance between intake valve
stems and guides allows oil mist to be sucked
into cylinders.
Worn main or rod bearings allow excessive
leakage from bearings .
Result in cylinder walls are flooded with
oil.
Oil pressure too high due to faulty action
of oil pressure relief valve, or clogged
relief passage.
If pressure lubricated, loose piston pins
may permit excessive leakage to cylinder
walls.
Grade of oil used is too light. A poor
quality oil may become far too thin when
engine is hot. Hard driving on hot days will
also consume more oil.
Clogged crankcase ventilator system .
Intake valve seals damaged or missing.
Plugged drain back holes in head .
Intake manifold gasket leak in conjunction
with rocker cover gasket leak .
Ring grooves or oil return slots clogged.
Rings sticking in ring grooves of piston .
Ring grooves worn excessively in piston .
Compression rings installed upside down .
Excessively worn or scored cylinder walls.
Cylinder walls not properly honed or
finished .
Oil too thin (diluted).
Oil level too high.
Excessive main or connecting rod bearing
clearance.
Piston ring gaps not staggered.
Incorrect size rings installed.
Piston rings out-of-round, broken or
scored .
Insufficient piston ring tension due to
engine overheating.