ASE A1 cheat sheet

Cranking compression test conditions: engine warm, battery charged, throttle wide open, ignition and fuel disabled, and all spark plugs removed; each cylinder is recorded and compared.

Cylinder balance limit: a variation greater than about 10-15% between the highest and lowest cylinder indicates a mechanical problem such as worn rings, bad valves, or a head gasket leak.

Wet compression test decision rule: compression that rises significantly after adding oil points to worn piston rings; compression that remains low points to leaking valves or a head gasket.

Cylinder leakage test: with the cylinder at TDC compression and regulated air applied, escaping air at the oil filler indicates rings, at the intake indicates an intake valve, at the exhaust indicates an exhaust valve, and at the radiator indicates a head gasket or crack.

Vacuum gauge reading: a healthy engine typically holds 17-22 inHg at idle; a rhythmic fluctuation suggests a valve train or compression fault, while a low steady reading suggests an intake leak or late valve timing.

Fuel trim interpretation: positive long-term fuel trim means the PCM is adding fuel to correct a lean condition; negative long-term fuel trim means the PCM is removing fuel to correct a rich condition.

PCV blowby check: with the engine running, a card held over the oil filler should be pulled slightly inward; strong outward pressure indicates excessive crankcase blowby.

Oil pressure rule of thumb: minimum approximately 10 psi per 1000 rpm, with many engines specified at 20-60 psi at cruise; a low dash gauge reading is verified with a mechanical gauge.

Mechanical valve clearance is set to the manufacturer's specification, normally with the engine cold unless a hot setting is specified; hydraulic lifters normally use zero lash plus a specified preload, often 0.020-0.060 inch.

Valve seat width: intake seats are typically about 1/16-3/32 inch and exhaust seats about 3/32-1/8 inch; seats that are too narrow overheat and seats that are too wide carbon up.

Valve margin is the thickness of the valve head edge; margin below specification increases the risk of burned valves and requires valve replacement.

Valve guide clearance is checked with a dial indicator or wiggle test; excessive clearance causes oil consumption, blue exhaust smoke, and valve seat misalignment.

Cylinder head warpage is checked with a straightedge and feeler gauge lengthwise, crosswise, and diagonally; resurfacing is required when warpage exceeds the manufacturer's limit, often about 0.002-0.004 inch.

Valve spring service limits include free length, squareness, and installed height; weak or broken springs cause valve float, misfire at high rpm, and loss of compression.

Cam lobe lift equals maximum lobe diameter minus base circle diameter; valve lift equals cam lobe lift multiplied by the rocker arm ratio.

Torque-to-yield head bolts are tightened in sequence using torque plus angle and are replaced after use because they stretch permanently.

Valve stem seals on the intake side operate under manifold vacuum; worn seals commonly produce blue smoke after idling and during deceleration.

Cylinder bore taper and out-of-round are measured at the top, middle, and bottom in two directions with a bore gauge; boring or honing is required when wear exceeds specification.

Piston-to-wall clearance is measured at the specified point on the piston skirt; too little clearance causes scuffing and seizure, while too much clearance causes piston slap.

Ring end gap rule: top ring approximately 0.004 inch per inch of cylinder bore, second ring slightly less, and oil ring larger; gap is checked with the ring squared in the bore.

Ring side clearance is checked between the ring and its groove with a feeler gauge; excessive clearance causes ring flutter, poor sealing, and oil consumption.

Bearing oil clearance is measured with Plastigauge or by comparing journal diameter with bearing bore; typical clearance is 0.001-0.003 inch, with a rule of thumb of about 0.001 inch per inch of journal diameter.

Crankshaft endplay is measured with a dial indicator while the crankshaft is pried fore and aft; excessive endplay indicates worn thrust bearing surfaces or a damaged crankshaft thrust face.

Cylinder honing should produce a crosshatch pattern of about 45-60 degrees included angle, followed by a plateau finish and thorough cleaning.

Bearing failure analysis: a spun bearing shows a wiped and heat-discolored surface, fatigue shows cracked lining, and scoring indicates oil starvation or debris contamination.

Cooling system pressure cap: each 1 psi of pressure raises the coolant boiling point by about 3 degrees Fahrenheit, so a 15 psi cap raises boiling point about 45 degrees.

Coolant mixture: a 50/50 ethylene glycol and water mix typically protects to about minus 34 degrees Fahrenheit and raises boiling point; excessive glycol reduces heat transfer.

Thermostat operation: the thermostat begins opening at its rated temperature and is fully open about 15-20 degrees Fahrenheit higher; stuck open causes slow warm-up and stuck closed causes overheating.

Cooling system pressure test: the system is pumped to cap rating and watched for pressure drop; a drop indicates an external leak, internal leak, or faulty pressure cap.

Combustion gas test: block tester fluid changes from blue to yellow when hydrocarbons are present, indicating a leaking head gasket, cracked casting, or warped head.

Oil pressure diagnosis: low pressure can result from a worn oil pump, restricted pickup screen, excessive bearing clearance, diluted or wrong-viscosity oil, or a stuck-open relief valve.

Water pump diagnosis: coolant weeping from the weep hole indicates a failed shaft seal; bearing noise or a loose pulley also condemns the pump.

Cooling fan clutch: a good clutch has resistance when hot and spins more freely when cold; a worn clutch commonly causes overheating at idle or low speed.

Fuel pressure is compared with specification with the key on and with the engine running; low pressure causes lean misfire and high pressure causes rich running.

Fuel volume is tested separately from fuel pressure because a pump can produce specified pressure while delivering insufficient volume.

Injector balance testing compares each injector's contribution or resistance; injectors should be within specification and flow evenly.

Ignition spark testing uses a spark tester to confirm a consistent bright blue spark; weak or intermittent spark indicates a coil, wire, plug, or control circuit fault.

Spark plug reading: tan-gray indicates normal combustion, black soot indicates a rich mixture, white blistered porcelain indicates overheating or a lean mixture, and oil fouling indicates worn rings or valve guides.

Oxygen sensor operation: the upstream sensor should switch rapidly between about 0.1 and 0.9 volt; a slow or flat sensor affects fuel trim and may set a code.

Exhaust backpressure should be low, typically under 1-2 psi at idle; higher backpressure indicates a restricted catalytic converter or muffler.

Vacuum leak diagnosis: positive fuel trim at idle that improves at higher speed commonly indicates unmetered air entering through a vacuum leak.

Battery state of charge at rest: about 12.6 volts is 100%, 12.4 volts is 75%, 12.2 volts is 50%, and 12.0 volts is 25%.

Battery load test: apply half the rated cold cranking amps for 15 seconds; at 70 degrees Fahrenheit the battery should remain above about 9.6 volts.

Parasitic draw: after modules sleep, normal draw is usually less than 50 milliamps; excessive draw is located with an ammeter or by voltage-drop testing fuses.

Charging voltage: a good alternator typically produces 13.5-14.5 volts at the battery with the engine running and accessories off.

Alternator ripple: excessive AC ripple, often above about 0.5 volt AC, indicates failed diodes or a faulty stator.

Voltage drop testing: cranking circuit voltage drop should be less than about 0.5 volt on both the positive and ground sides.

Starter current draw: a typical starter draws about 150-250 amps during cranking; excessive draw indicates a seized starter, bad cable connection, or mechanical engine drag.

No-crank diagnosis: the circuit is checked at the battery, cables, starter relay, neutral safety or clutch switch, ignition switch, and starter solenoid.