Shipping Coast to Coast and International

Translate

Tech Links

Bulletins

Technical Bulletins

 Diagnosing Engine Problems


SYMPTOM: Engine misses at all speeds.

PROBABLE CAUSE: Leaking intake manifold gasket.
REMEDY: Torque intake manifold bolts to manufacturer's specs. If leak still exists, remove intake manifold and install a new gasket. On vehicles with an air pump emission control system, make sure that antibackfire valve is not stuck in open position.

PROBABLE CAUSE: Late ignition timing or valve timing.
REMEDY: Check ignition timing and adjust as necessary to manufacturer's specs. Check valve timing with a vacuum gauge. If reading is below 18 in. Hg., the valve timing is probably late. To correct this, the engine must be disassembled as necessary to gain access to the timing chain
and/or gears and camshaft. The timing chain and/or gears or camshaft must be replaced as necessary to correct the cause of late valve timing.

PROBABLE CAUSE: Low compression due to leakage past piston
rings.
REMEDY: Perform compression test. If it is determined that the cause is worn piston rings, the engine must be disassembled to gain access to the piston rings for replacement.

PROBABLE CAUSE: Partial or complete loss of power in one or more cylinders due to leaking head or intake manifold gasket, leaking valve(s) or defective valve springs.
REMEDY: Perform vacuum test. If needle fluctuates steadily as engine speed is increased, one or more of the problems is indicated. To check for leaking head gasket or valves, perform a compression test. If the head gasket or valves are at fault, disassemble the engine as necessary and replace the affected part(s). To check for leaking intake manifold gasket, squirt engine oil around parting surfaces of intake manifold and cylinder head(s). If engine idle
speed is affected as the oil is squirted, correct problem as described previously in step 1. To check for weak valve springs, remove springs and have them checked on a spring scale. If tension is not within manufacturer's specs, replace the springs.

PROBABLE CAUSE: Loss of power in one or more cylinders due
to defect(s) in ignition system.
REMEDY: Check point dwell and ignition timing and reset as necessary. If malfunction still exists, proceed as follows: Inspect condition of points, condenser, distributor cap, rotor, spark plugs, wires and ignition coil. Replace all parts which are defective or questionable.


PROBABLE CAUSE: Fuel mixture too lean or too rich.
REMEDY: Check carburetor float level and float drop and adjust as necessary to manufacturer's specs. If malfunction still exists, overhaul or replace carburetor.

PROBABLE CAUSE: Excessive back pressure in exhaust system.
REMEDY: Perform vacuum test. A slow drop of the needle while engine is idling indicates a restriction in the exhaust system. To correct, remove the exhaust system and check for obstructions or kinks and bends in the pipes and mufflers. Also check exhaust manifolds for obstructions.

PROBABLE CAUSE: Defective fuel pump.
REMEDY: Check fuel pump pressure. If pressure is not within manufacturer's specs, replace fuel pump.

 

SYMPTOM: Engine misses at low or idle speeds.

PROBABLE CAUSE: Valve Adjustment set too tight.
REMEDY: Reset valve adjustment to manufacturer's specs.

PROBABLE CAUSE: Worn camshaft lobes, bent push rods or worn
valve lifters.
REMEDY: Check valve lift using a dial indicator. If valve lift is not within manufacturer's specs, disassemble engine and check push rods, valve lifters and camshaft. Replace any affected parts.

PROBABLE CAUSE: Loss of power in one or more cylinders due
to defect in ignition system.
REMEDY: Check point dwell and ignition timing and reset as necessary to manufacturer's specs. If problem still exists,proceed as follows: Inspect condition of points, condenser, distributor cap, rotor, spark plugs, wires and ignition coil. Replace all parts which are defective or questionable.

PROBABLE CAUSE: Leaking intake manifold system.
REMEDY: Squirt engine oil around parting surfaces of intake manifold and cylinder head(s). If engine idle speed is affected as the oil is squirted, torque intake manifold bolts to specs and recheck for leaks. If leak still exists, replace the intake manifold gasket. On vehicles with air pump emission control system, make sure that the antibackfire valve is not stuck in the open position.

PROBABLE CAUSE: Dirty air cleaner.
REMEDY: Replace air cleaner.

PROBABLE CAUSE: Leaky valves.
REMEDY: Perform compression test. If test indicates that
valves are leaking, have a valve job performed.

PROBABLE CAUSE: Fuel mixture too lean or too rich.
REMEDY: Check carburetor float level and float drop and adjust as necessary to manufacturer's specs. If malfunction still exists, overhaul or replace carburetor.

PROBABLE CAUSE: Defective fuel pump.
REMEDY: Check fuel pump pressure. If pressure is not within manufacturer's specs, replace fuel pump.


SYMPTOM: Poor high speed performance or lack of power. (Note that the altitude at which the vehicle is operated will have an effect on engine performance. A vehicle that operates normally at low altitudes, will not necessarily perform satisfactorily at high altitudes and vice versa).

PROBABLE CAUSE: Incorrect ignition timing.
REMEDY: Reset ignition timing to manufacturer's specs.

PROBABLE CAUSE: Advance mechanism not operating properly.
REMEDY: Check operation of centrifugal and vacuum mechanisms using a timing light. If either is malfunctioning, replace as necessary.

PROBABLE CAUSE: Defect(s) in ignition system.
REMEDY: Inspect condition of points, condenser, distributor cap, rotor, spark plugs, wires and ignition coil. Replace all parts which are defective or seem questionable.

PROBABLE CAUSE: Fuel mixture too lean or too rich.
REMEDY: Check carburetor float level and float drop and adjust as necessary to manufacturer's specs. If malfunction still exists, overhaul or replace carburetor.

PROBABLE CAUSE: Defective fuel pump.
REMEDY: Check fuel pump pressure. If pressure is not within manufacturer's specs, replace the fuel pump.

PROBABLE CAUSE: Dirty air cleaner.
REMEDY: Replace air cleaner.

PROBABLE CAUSE: Excessive back pressure in exhaust system.

REMEDY: Perform vacuum test. A slow drop of the needle indicates a restriction in the exhaust system. To correct this problem, remove the exhaust system and check for restrictions or kinks and bends in the muffler and pipes.
Also check exhaust manifold for obstructions.

PROBABLE CAUSE: Positive crankcase ventilation system
clogged.
REMEDY: Replace PCV valve and clean out or replace hoses as
necessary.

PROBABLE CAUSE: Worn lifters, camshaft lobes or bent push
rods.
REMEDY: Check valve lift using a dial indicator. If valve lift is not within manufacturer's specs, disassemble engine and check push rods, valve lifters and camshaft and replace any affected parts.

PROBABLE CAUSE: Manifold heat control valve stuck in closed
position.
REMEDY: Free up valve with solvent.

PROBABLE CAUSE: Sticking valves or low compression.
REMEDY: Perform compression test. If test indicates valves are leaking, disassemble engine as necessary and performe valve job.


SYMPTOM: Valve noise.

PROBABLE CAUSE: Defective lifter(s) or valve clearance
incorrectly adjusted.
REMEDY: Replace lifter(s) or adjust valve clearance to manufacturer's specs.


PROBABLE CAUSE: Weak or broken valve springs or excessively
worn valve guides.
REMEDY: Replace valve springs if broken or weak. If valve guides are excessively worn, ream valve guides to next oversize and install corresponding oversize valves.

PROBABLE CAUSE: Oil level too low or too high.

REMEDY: Add or drain oil as necessary to bring oil level to specs.


SYMPTOM: Detonation (Ping or spark knock).

PROBABLE CAUSE: Overheated engine.
REMEDY: Determine cause of overheating and correct as necessary.

PROBABLE CAUSE: Octane rating of fuel too low for engine.
REMEDY: Change to a fuel of higher octane rating.

PROBABLE CAUSE: Timing advanced excessively.
REMEDY: Reset timing to manufacturer's specs.

PROBABLE CAUSE: Excessive carbon deposits in combustion
chamber.
REMEDY: Add combustion chamber conditioner to engine. If problem still exists, remove cylinder head(s) and clean out the deposits.


SYMPTOM: External engine oil leakage.

PROBABLE CAUSE: Improperly seated or broken gaskets at timing chain cover, fuel pump, oil filter, oil pan or valve covers.
REMEDY: Torque the bolts retaining the affected parts to specs. If leak still exists, replace the affected gasket.

PROBABLE CAUSE: Worn timing chain cover oil seal or rear main seal.
REMEDY: Disassemble engine as necessary and replace the affected seal.

PROBABLE CAUSE: Leaking distributor housing seal or gasket.
REMEDY: Replace the gasket or seal.

PROBABLE CAUSE: Oil pan plug cocked or loose.
REMEDY: Torque plug to specs, or replace with new self-tapping plug, if oil pan threads are stripped.


SYMPTOM: Internal engine oil leakage. (Indicated by blue smoke
emitting from the tailpipe.)

PROBABLE CAUSE: Worn or damaged valve stem oil seals.
REMEDY: If blue smoke emits from the tailpipe when the engine is first started, the valve stem oil seals are probably defective. To correct, replace the valve stem oil seals.

PROBABLE CAUSE: Valve guides and/or valve stems excessively
worn.
REMEDY: Ream valve guides to next oversize and install the corresponding oversize valves.


PROBABLE CAUSE: Plugged drain back holes in cylinder head.
REMEDY: Clear out drain back holes.

PROBABLE CAUSE: Oil being sucked in past the intake
manifold gasket.
REMEDY: Torque intake manifold bolts to manufacturer's specs. If leak still exists, replace intake manifold gasket.

PROBABLE CAUSE: Punctured automatic transmission modulator
diaphragm.
REMEDY: Replace modulator.

PROBABLE CAUSE: Worn piston rings.
REMEDY: Replace piston rings.

PROBABLE CAUSE: Excessively worn connecting rod bearings.
REMEDY: Replace connecting rod bearings.

PROBABLE CAUSE: Too low engine oil viscosity.
REMEDY: Drain and add the correct type and amount of oil.

The AERA Technical Committee