ASE A8 cheat sheet

Compression test preparation: disable fuel and ignition, hold throttle wide open, crank equal strokes per cylinder; variation greater than about 10 to 15 percent or pressure below specification indicates a mechanical fault.

Wet compression test: add a small amount of oil to the cylinder and repeat; a significant pressure increase indicates worn rings or cylinder walls, while little change points to valve or head-gasket leakage.

Cylinder leakage test: apply regulated shop air at TDC; hiss at throttle body = intake valve, at tailpipe = exhaust valve, at oil filler = rings, bubbles in coolant = head gasket or crack.

Vacuum gauge at idle: steady 17 to 22 inHg at sea level is normal; a low steady reading suggests intake leakage or late valve timing; a fluctuating needle suggests valve or ignition problems.

Exhaust smoke color: blue-gray indicates burning oil; black indicates rich mixture; white with sweet odor indicates coolant; white vapor at cold start can be normal condensation.

Rod bearing knock is low-pitched and load-related; lifter or valve tick is top-end and rpm-related; spark knock is a sharp rattle under acceleration and may be corrected by timing or fuel quality.

Power balance test: disable one cylinder at a time and note rpm drop; a cylinder with little or no drop is weak and requires spark, fuel, and compression checks.

Timing chain stretch: symptoms include cam/crank correlation codes, erratic cam timing, rattling at startup, and poor idle; verify with timing marks and correlation data.

Three-wire sensor circuit: reference is commonly 5 volts, signal varies, and ground completes the circuit; measure reference and ground under load before condemning the sensor.

Fuel trim interpretation: positive STFT/LTFT means the PCM is adding fuel for a lean condition; negative means subtracting fuel for rich; total trim near 0 is ideal, and beyond about plus or minus 25 percent commonly sets a code.

O2 sensor voltage: upstream high (about 0.8-0.9V) = rich, low (about 0.1-0.2V) = lean; downstream should be relatively steady after the converter is warm.

MAP sensor signal: low voltage at idle (high vacuum) and higher voltage near wide-open throttle (low vacuum); compare signal to vacuum gauge and throttle position.

MAF sensor: contamination or failure can underreport airflow and cause lean fuel trims; verify grams per second at idle and known rpm, and inspect for unmetered air downstream.

CAN bus: ignition off resistance between CAN high and CAN low is about 60 ohms; an open or short changes the value; verify power, ground, and module communication.

Voltage drop: measure across a cable or connection with the circuit loaded; guideline is less than 0.1 volt per connection and less than 0.5 volt for a ground circuit.

Adaptive memory: battery disconnect or PCM replacement can erase learned fuel trims and idle; drive cycles or a relearn procedure restore readiness and driveability.

Spark test: use an adjustable spark tester; strong blue spark confirms secondary output; no spark requires checking coil, control, power, and ground.

Ignition coil primary resistance is typically low, often under 2 ohms; secondary is typically thousands of ohms; compare to specification and inspect for cracks or carbon tracking.

Spark plug reading: tan/gray normal; dry black carbon = rich or fouled; wet black = oil; blistered white = overheating or lean.

Waste spark system: one coil fires two plugs simultaneously; paired cylinders are 360 degrees apart in firing order; one plug fires with reverse polarity.

CKP/CMP: CKP provides rpm and crankshaft position; CMP identifies cylinder 1 and cam timing; loss of CMP can cause long crank/no-start or misfire.

Secondary ignition waveform: burn time is roughly 1.5 to 2.0 ms; a short burn time suggests lean mixture or high resistance, while a long burn time suggests rich mixture or fouled plug.

Hall effect sensor produces a square wave; a magnetic reluctor sensor produces an AC sine wave whose amplitude increases with speed; the PCM converts the signal to rpm and position.

Knock sensor: detects detonation and the PCM retards timing; check for a loose sensor, wiring damage, and excessive mechanical noise before replacement.

Fuel pressure: compare key-on and engine-running pressure to specification; low pressure can cause a lean condition, high pressure can cause rich; also check volume and pressure drop.

Fuel injector balance: compare injector resistance and pressure drop or flow; a clogged injector causes a lean misfire in one cylinder.

Vacuum leak: propane or carb cleaner around the intake changes rpm; positive fuel trims and lean codes may result; check hoses, PCV, intake gaskets, and brake booster.

Exhaust backpressure: measure at the upstream O2 port; excessive backpressure, often above about 1.5 to 2 psi at idle, indicates a restricted converter or exhaust.

Catalyst temperature: outlet should be hotter than inlet under normal operation; outlet equal to or cooler than inlet suggests reduced catalyst efficiency.

Air filter restriction: a restricted filter reduces airflow and can cause a rich mixture at high load; use a restriction gauge or vacuum gauge for verification.

Unmetered air: any air entering after the MAF is unmeasured; it causes a lean mixture and positive fuel trims; inspect intake ducts, gaskets, and PCV.

Fuel pump current: low current can indicate high resistance or an open; high current can indicate a seized pump or short; compare to specification and check voltage drop.

PCV: uses intake vacuum to pull crankcase vapors into the intake; stuck closed causes crankcase pressure and oil leaks; stuck open causes a lean rough idle.

EVAP purge: the PCM opens the purge valve to draw stored vapor into the intake; the vent valve seals the system for leak testing; a smoke machine locates leaks.

EVAP leak thresholds: OBD II monitors a small leak of about 0.020 inch and a large leak of about 0.040 inch; a loose or damaged fuel cap is a common cause.

EGR: recirculates exhaust to lower combustion temperature and NOx; stuck open causes rough idle or stall; inoperative can cause spark knock and high NOx.

Catalyst: oxidizes HC and CO and reduces NOx; stores and releases oxygen; downstream O2 should be steady; misfire can overheat and damage the catalyst.

Misfire with flashing MIL: raw fuel entering the exhaust can overheat the catalyst; a flashing MIL indicates a catalyst-damaging misfire.

High NOx causes include high combustion temperature, lean mixture, excessive spark advance, inoperative EGR, and overheating; high CO indicates rich; high HC indicates misfire or unburned fuel.

Secondary air: an air pump injects air into the exhaust to oxidize HC and CO during warm-up; check the pump, check valve, and passages for blockage or leakage.

Torque-to-yield bolts: tighten to a specified torque and then rotate an additional angle; they stretch into yield to provide clamp load and often must be replaced after removal.

Timing belt or chain: align crankshaft and camshaft marks before tensioning; an interference engine can suffer valve damage if the belt or chain breaks.

Cooling system: pressure test for leaks; bleed air at the highest point; verify thermostat opening temperature and coolant mixture.

Oil priming: after a rebuild, prime the oil system before starting; disable ignition and fuel and crank until oil pressure builds.

Serpentine belt: route per diagram; check tensioner, alignment, and pulley condition; chirp or squeal often indicates misalignment or a worn belt.

Engine mounts: load the engine in drive and reverse while observing mount movement; excessive lift or clunk indicates a worn or separated mount.

Head gasket: compression or leakdown testing, bubbles in coolant, and oil/coolant mixing are indicators; check head and block flatness and resurface per specification.

Valve adjustment: check at the specified temperature; too tight can cause compression loss, misfire, and burned valves; too loose causes noise and reduced lift.