ASE A8 flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Wet compression test

A compression test repeated after adding a small amount of oil to the cylinder. A significant pressure increase indicates worn rings or cylinder walls; little change points to valve or head-gasket leakage.

Cylinder leakage test

A test that applies regulated shop air to a cylinder at TDC and listens for leakage. Hiss at the throttle body indicates intake valve leakage; at the tailpipe, exhaust valve leakage; at the oil filler, ring leakage.

Vacuum gauge idle reading

At sea level a healthy engine usually produces a steady 17 to 22 inHg at idle. A low steady reading suggests intake leakage or late valve timing; a fluctuating needle suggests valve or ignition problems.

Blue-gray exhaust smoke

Blue-gray smoke from the tailpipe indicates oil is being burned. Common sources are worn piston rings, valve guides or seals, and a PCV system that is drawing oil into the intake.

Power balance test

A diagnostic test that disables one cylinder at a time and measures rpm drop. A cylinder with little or no rpm drop is contributing less power and should be checked for spark, fuel, and compression.

Rod bearing knock

A low-pitched, heavy knock that often increases with load and engine speed. It originates from excessive clearance between the connecting rod bearing and crankshaft journal and can worsen rapidly.

Positive fuel trim

Positive short-term or long-term fuel trim means the PCM is adding fuel because the engine is running lean. Total trim beyond about plus or minus 25 percent commonly sets a fuel-trim code.

Upstream oxygen sensor

The upstream O2 sensor measures oxygen in exhaust before the catalytic converter. High voltage, about 0.8 to 0.9 volt, indicates a rich mixture; low voltage, about 0.1 to 0.2 volt, indicates lean.

Five-volt reference

A regulated five-volt supply from the PCM used by many sensors. A short to ground pulls the reference low; a short to voltage raises it. Check reference and ground before replacing a sensor.

CAN bus 60 ohms

With the ignition off, resistance between CAN high and CAN low is typically about 60 ohms because two 120-ohm terminating resistors are in parallel. An open or short changes this reading.

MAP sensor voltage

A typical MAP sensor outputs low voltage at idle when manifold vacuum is high and higher voltage near wide-open throttle when vacuum is low. Signal voltage should track vacuum and throttle position.

Voltage drop test

Measuring voltage across a conductor while current flows through it. Excessive drop indicates resistance; a common guideline is less than 0.1 volt per connection and less than 0.5 volt for a ground circuit.

Waste spark ignition

An ignition system in which one coil fires two spark plugs simultaneously. One plug fires center-to-ground and the paired plug fires ground-to-center; both plugs fire every 360 degrees of crankshaft rotation.

Spark plug heat range

The spark plug's ability to dissipate heat. A hotter plug runs at a higher tip temperature and is used for fouling resistance; a colder plug dissipates more heat and is used for high-load or high-compression engines.

Hall effect sensor

A digital ignition or position sensor that switches a reference voltage on and off as a shutter or reluctor passes. It produces a square wave that the PCM uses for timing and rpm.

Ignition coil secondary resistance

The resistance measured across the secondary winding of an ignition coil. It is typically thousands of ohms; an open, shorted, or out-of-spec winding can cause weak or no spark.

Knock sensor function

A piezoelectric sensor that detects engine detonation and sends a signal to the PCM. The PCM then retards ignition timing to reduce knock and protect the engine from damage.

Exhaust backpressure test

A test that measures pressure ahead of the catalytic converter, usually at the upstream oxygen sensor port. Excessive backpressure, often above about 1.5 to 2 psi at idle, indicates a restricted converter or exhaust.

Fuel injector balance test

A test that compares how much fuel each injector delivers, often by measuring pressure drop when each injector is pulsed. A weak or clogged injector causes a lean misfire in that cylinder.

Unmetered air leak

Air entering the engine after the mass airflow sensor is not measured by the PCM. This causes a lean mixture, positive fuel trims, and possibly a rough idle or lean-code.

Catalyst temperature test

Under normal operating conditions the catalytic converter outlet should be hotter than its inlet because oxidation releases heat. An outlet that is cooler or equal to the inlet suggests reduced catalyst efficiency.

Returnless fuel system

A fuel system that does not return fuel from the rail to the tank. Pressure is regulated at the tank module or by pump speed control, so pressure tests are made at the service port.

PCV system function

The positive crankcase ventilation system uses intake vacuum to pull crankcase vapors into the intake for burning. A stuck-closed valve can cause oil leaks; a stuck-open valve can cause a lean rough idle.

EVAP purge valve

An electrically controlled valve that opens to allow fuel vapor stored in the charcoal canister to be drawn into the intake. The PCM commands purge only when conditions allow closed-loop control.

EVAP small leak threshold

OBD II evaporative emission monitors typically detect 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 system purpose

Exhaust gas recirculation routes a small amount of exhaust into the intake to lower combustion temperature and reduce nitrogen oxide emissions. A stuck-open EGR valve can cause rough idle or stalling.

Catalytic converter function

The catalytic converter oxidizes hydrocarbons and carbon monoxide and reduces nitrogen oxides in exhaust. It stores and releases oxygen to help oxidize pollutants during rich and lean excursions.

Torque-to-yield bolt

A fastener tightened to a specified torque and then rotated an additional angle. It stretches into the yield range to provide clamp load and often must be replaced after removal.

Interference engine

An engine in which the pistons and valves occupy the same space at different times. If the timing belt or chain breaks, the pistons can strike open valves and cause major damage.

Cooling system bleed

Removing trapped air from the cooling system after refilling. Air pockets can cause overheating, poor heater performance, and false low coolant readings; bleed at the highest point or specified vent.

Engine mount test

With the engine running, load the engine in drive and reverse while observing mount movement. Excessive lift or clunk indicates a separated or worn engine mount.

Valve clearance too tight

Insufficient clearance can hold a valve slightly open, causing compression loss, misfire, and burned valves. Clearance is checked with the engine at the specified temperature and adjusted to specification.