Your FREE ASE A1 Flashcards 2026 – 200+ Cards

Intake, compression, power, exhaust — completed in two crankshaft revolutions per cylinder. Memory aid: Suck, Squeeze, Bang, Blow.

Measures cranking pressure per cylinder to judge how well rings, valves, and the head gasket seal. Readings should be within ~10% of each other.

Compression test repeated after adding oil to the cylinder. Reading rises = worn rings; stays low = valves or head gasket leak.

Applies compressed air to a cylinder at TDC and reads the percent leaking. Locates the leak by where air escapes (valves, rings, or head gasket).

Coolant entering the combustion chamber — usually a blown head gasket or cracked head. (Thin cold-start vapor is normal.)

Oil burning in the cylinder — worn piston rings, worn valve guides, or bad valve stem seals.

A rich air-fuel mixture — too much fuel or too little air (clogged air filter, leaking injector, faulty sensor).

Uncontrolled, spontaneous combustion AFTER the spark. Caused by low octane, too much advance, lean mixture, carbon, overheating, or high compression.

The mixture ignites BEFORE the spark, from a hot spot such as a glowing carbon deposit or hot spark plug.

Reads manifold vacuum at idle. Steady low = vacuum leak; fluctuating = burned valve; slowly dropping = restricted exhaust.

Disables one cylinder at a time and watches RPM. Little or no RPM drop = that cylinder is weak or dead.

The highest point of piston travel in the cylinder. Many tests and adjustments are made at TDC on the compression stroke.

The lowest point of piston travel in the cylinder, opposite TDC.

Combustion gases that slip past the piston rings into the crankcase. Excessive blow-by indicates worn rings or cylinders.

Positive Crankcase Ventilation — draws blow-by gases from the crankcase back into the intake to be burned, relieving pressure and cutting emissions.

Leans the idle and causes a vacuum-leak-like rough idle.

Builds crankcase pressure that pushes oil out through seals and gaskets.

A cylinder failing to fire properly — from ignition, fuel, or compression loss. Causes rough idle, poor power, and a flashing or steady check-engine light.

An active misfire severe enough to damage the catalytic converter. Stop and diagnose the misfire.

A stored emissions-related fault that is not an immediate threat; retrieve the code and diagnose.

A deep, rhythmic hammering that worsens under load — typically worn rod or main bearings.

A light tapping that rises with RPM — usually a worn lifter or excessive valve lash.

Measures oxygen in the exhaust and tells the computer if the mixture is rich or lean for closed-loop fuel control.

Detects detonation so the engine computer can retard ignition timing to protect the engine.

Reports coolant temperature to the computer, affecting fuel mixture, timing, and the temperature gauge.

Tells the computer how far the throttle is open so it can adjust fuel and timing.

Reports camshaft (valve) timing to the computer for ignition and injection control.

Reports crankshaft speed and position; the primary input for ignition timing and engine RPM.

A snapshot of sensor readings captured when a trouble code set, used to recreate the conditions of a fault.

On-Board Diagnostics II — the standardized system that monitors emissions components and stores diagnostic trouble codes (DTCs).

A code stored by the computer identifying a detected fault (e.g., P0300 = random misfire).

The signature ASE format: two statements; choose whether A only, B only, both, or neither is correct. Judge each statement separately.

Provides rotational inertia for the crankshaft and carries the starter ring gear; the flexplate is used with automatic transmissions.

Set by the manufacturer; #1 is defined per engine layout. Firing order and cylinder ID are needed to diagnose a specific misfire.

The sequence in which the cylinders fire, designed to balance the engine and smooth power delivery.

Too much air or too little fuel. Can cause misfire, hesitation, knock, and higher combustion temperatures.

Too much fuel or too little air. Causes black smoke, fouled plugs, poor economy, and high emissions.

A system that changes valve opening timing (and sometimes lift) for better power, economy, and emissions across the RPM range.

Coolant mixing with oil — typically from a blown head gasket, cracked head/block, or a leaking oil cooler.

Usually a catalytic converter issue, often from a rich mixture overloading the converter.

The pressure below atmospheric in the intake manifold; a healthy engine reads a steady ~17–21 in. Hg at idle (varies by engine and altitude).

No-crank = the starter won't turn the engine (electrical/starter). No-start = it cranks but won't run (spark, fuel, or compression).

The three things every engine needs to run; a no-start diagnosis checks all three.

A device that reads DTCs, live data, and freeze-frame from the OBD-II system to guide diagnosis.

Pressurizes the system with a hand pump to find external coolant leaks and check the cap and head-gasket sealing.

Uses a chemical that changes color if combustion gases are present in the coolant — confirms a head-gasket or cracked-head leak.

Oil burning shows blue smoke and fouled plugs. Past rings = worse under load/acceleration; past valve seals = puff on startup or deceleration.

Coolant loss with no visible leak points internally: head gasket, cracked head/block, or a leaking intake gasket.

Combustion in the intake or exhaust outside the cylinder — from incorrect timing, a lean mixture, or a leaking valve.

Unstable idle from a vacuum leak, misfire, low compression, dirty injectors, or a faulty idle-control system.

The casting that seals the top of the cylinders and houses the valves, seats, guides, and (on OHC engines) the camshaft.

The seal between the cylinder head and block that contains combustion pressure and keeps coolant and oil passages separate.

Causes white smoke, milky oil, overheating, coolant loss, or combustion gases in the coolant.

A non-flat head deck that prevents proper gasket sealing; must be machined flat before reassembly. Check with a straightedge and feeler gauge.

Tightened to a torque value then turned a set angle, stretching into its yield range. TTY bolts are replaced once removed.

A crisscross pattern done in stages so the head clamps evenly without warping. Wrong sequence or over-torque causes gasket failure.

Opens and closes the intake or exhaust port; must seal tightly against its seat to hold compression.

The machined ring the valve face seals against. Worn or recessed seats cause compression loss and poor sealing.

The bore that supports and aligns the valve stem. Wear lets oil into the chamber and the valve seals poorly.

Keeps oil from running down the valve stem into the combustion chamber. Failure causes blue smoke, often a puff on startup.

A valve that no longer seals because heat and leakage eroded its face — causes low compression and a misfire in that cylinder.

The small valve-train gap allowing for thermal expansion. Too little burns valves; too much causes ticking and wear.

Holds the valve slightly open, causing burned valves and lost compression.

Causes a ticking noise and accelerated valve-train wear.

Uses engine oil pressure to maintain zero valve lash automatically, so no periodic adjustment is needed.

A solid valve-train follower that requires periodic manual lash adjustment to spec.

The lobed shaft that opens the valves in time with the crankshaft; its position relative to the crank sets valve timing.

A pivoting lever that transfers camshaft (or pushrod) motion to open the valve.

A rod that transfers cam-lobe motion from a lifter in the block up to the rocker arm in the head.

OHC places the camshaft(s) in the head, driven by a belt/chain; OHV (pushrod) keeps the cam in the block.

Closes the valve and keeps the follower on the cam. A weak or broken spring causes valve float and poor sealing.

A tool that safely compresses the valve spring to remove and install valves and keepers.

Small wedge-shaped locks that retain the spring retainer on the valve stem.

The cap held by the keepers that holds the valve spring on the stem.

A toothed belt that keeps the camshaft(s) timed to the crankshaft. On an interference engine, a broken belt can bend valves.

A metal chain linking crank and cam timing; longer-lived than a belt but can stretch or jump teeth when worn.

An engine in which open valves occupy piston space. A broken timing belt/chain lets pistons strike valves and bend them.

At high RPM the valve spring can't close the valve fast enough, so it loses contact with the cam — causing misfire and power loss.

The machined angle on the valve that seals against the seat (commonly 45°); checked and ground during a valve job.

Machining valves and seats to restore a tight seal; includes checking guides, springs, and seals.

A seat cut with three angles to control seat width and improve sealing and flow.

Excess clearance lets oil into the chamber and the valve wobble — causing oil consumption and poor sealing.

The main casting that holds the cylinders, crankshaft, and supporting structure of the engine.

The machined cylinder in which the piston travels. Wear creates taper and out-of-round conditions.

The difference in cylinder diameter between the top and bottom of ring travel due to wear. Excess taper calls for reboring.

A cylinder worn unevenly so it is no longer a perfect circle; like taper, a reason to rebore and fit oversize pistons.

The unworn lip at the top of the bore above ring travel. It must be reamed off before removing pistons.

The tool used to cut away the cylinder ridge before pushing pistons out, preventing ring and piston damage.

Machining a worn cylinder to a larger, true diameter so an oversize piston can be fitted.

Finishing the bore with a crosshatch pattern that helps new rings seat and retain oil.

Converts the pistons' up-and-down motion into rotation. Rides on main bearings in the block.

Links a piston to the crankshaft journal; its big end rides on a rod bearing.

The component that moves in the cylinder, transmitting combustion force through the rod to the crankshaft.

Compression rings seal combustion pressure; the oil-control ring wipes oil from the cylinder wall. Worn rings cause blue smoke and low compression.

The clearance between ring ends in the bore. Too little gap lets ends butt and scuff the cylinder when hot; checked with a feeler gauge.

Connects the piston to the small end of the connecting rod, allowing the rod to pivot.

Supports the crankshaft in the block. Excess clearance causes a deep knock and low oil pressure.

Rides between the connecting rod big end and the crank journal. Wear causes a knock that worsens under load.

A calibrated plastic strip crushed between a bearing and journal to measure oil clearance by comparing the flattened width to a chart.

Never rotate the crankshaft while Plastigage is installed — it ruins the reading.

The small gap that holds an oil film between bearing and journal. Too tight starves oil; too loose knocks and drops pressure.

The polished bearing surface on the crank (main or rod). Worn or out-of-round journals require regrinding to undersize.

The crank's permitted fore-aft movement, controlled by the thrust bearing; measured with a dial indicator.

Machining the main bearing bores back into true alignment so the crank is properly supported and lubricated.

The block's top surface that meets the head. It must be flat and clean for the head gasket to seal.

Reveals surface cracks in ferrous parts (block, crank) by applying a magnetic field and iron particles.

Finds surface cracks in non-ferrous parts (aluminum heads) using penetrating dye and developer.

A replaceable bore insert pressed into the block; guides the piston and can be replaced instead of reboring.

Mounted on the crank snout to absorb torsional vibration; a separated balancer ring can throw off timing marks.

Plain (sleeve) bearings of soft metal that conform to the journal and hold an oil film under pressure.

Oversize pistons fit a rebored cylinder; undersize bearings fit a reground (smaller) crank journal.

Tightens fasteners to a specified value so clamping force is correct and even; essential for bearing caps and head bolts.

Used together to measure bore and journal diameters precisely when checking wear and clearance.

The honing finish on the cylinder wall (about 45°) that holds oil and helps new rings seat.

Caps are installed in their original position and order and torqued in sequence to keep the crank aligned.

Circulates pressurized oil through the bearings, lifters, and galleries. A worn pump or pickup can cause low oil pressure.

Low oil level, too-thin oil, a worn oil pump, a clogged pickup screen, or worn bearings.

Removes contaminants from circulating oil. Its bypass valve still feeds oil if the filter clogs, protecting the engine.

Oil's resistance to flow. A 5W-30 flows like a 5-weight when cold (the W) and protects like a 30-weight at operating temperature.

Behaves like a thin oil for cold starts and a thicker oil when hot, thanks to viscosity-index improvers.

Limits maximum oil pressure by bleeding off excess, protecting the system from over-pressure on cold starts.

Drilled passages in the block and head that carry pressurized oil to the bearings and valve train.

Transfers heat from the oil to coolant or air; an internal failure can mix oil and coolant.

A temperature-controlled valve that blocks coolant flow to the radiator until the engine warms, then opens to regulate temperature.

Causes the engine to overheat — no coolant flow to the radiator.

Keeps the engine running too cool, hurting fuel economy, emissions, and heater output.

Sheds engine heat to the air as coolant flows through its core; the fan adds airflow at low speeds.

Seals the system and raises the coolant boiling point (about 5–6°F per psi); its vacuum valve lets coolant return from the overflow tank.

The belt- or chain-driven pump that circulates coolant through the engine and radiator. A leak often appears at the weep hole.

An ethylene- or propylene-glycol mix with water that resists freezing and boiling and protects against corrosion. Use the specified type.

Pulls air through the radiator at low vehicle speeds. Electric fans run on demand; a fan clutch engages when hot.

Holds expanding coolant and returns it as the engine cools, keeping the system full and air-free.

A small radiator in the dash that uses engine coolant to heat the cabin; a leak fogs the windshield with sweet-smelling steam.

Low coolant, stuck thermostat, failed water pump, bad fan, plugged radiator, weak cap, or a blown head gasket.

Mixing incompatible coolants can gel or cause corrosion and water-pump damage; always use the specified coolant.

Removing trapped air after a coolant refill so the system fills completely and doesn't form hot spots.

Warns of dangerously low oil pressure; stop the engine to avoid bearing damage and diagnose the cause.

A single belt that drives the water pump, alternator, and accessories. A loose or worn belt squeals and can cause overheating.

Maintains proper tension on the serpentine belt automatically; a worn tensioner causes belt noise and slip.

Shows engine temperature; a sudden rise warns of overheating before damage occurs.

Replacing oil and filter at the manufacturer's specified mileage or time to remove contaminants and maintain lubrication.

Thickened, degraded oil deposits from heat, moisture, and neglected changes; can block galleries and the pickup.

Sends actual oil pressure to the gauge or warning light; a faulty unit can give a false low reading.

The target coolant temperature range (commonly ~195–220°F) the thermostat maintains for efficiency and emissions.

Carry coolant between engine and radiator; cracked, swollen, or soft hoses can burst and cause overheating.

Coolant additives prevent rust and electrolysis; depleted additives let corrosion damage the radiator and pump.

Each psi of system pressure raises coolant's boiling point about 3°C (5–6°F), letting the engine run hotter without boiling.

Directs airflow through the radiator core for the fan; a missing shroud reduces cooling at idle.

A 50/50 antifreeze-water mix typically protects to about -34°F and raises the boiling point with the pressure cap.

Measures oil level (and reveals coolant contamination/milkiness); check on level ground with the engine off and cooled.

The proportion of air to fuel by weight; the ideal (stoichiometric) ratio for gasoline is about 14.7:1.

The ideal 14.7:1 air-fuel ratio for complete combustion of gasoline; the computer trims fueling toward it.

An electrically controlled valve that sprays a metered amount of fuel into the intake or cylinder. Clogged or leaking injectors upset the mixture.

Maintains correct fuel-rail pressure so injectors deliver the right amount of fuel.

Delivers fuel from the tank to the injectors at the required pressure and volume. A weak pump leans the mixture under load.

Measures the amount of air entering the engine so the computer can match fuel delivery.

Measures intake manifold pressure (engine load) so the computer can calculate fuel and timing.

Cleans incoming air. A clogged filter restricts airflow, richening the mixture and reducing power.

Controls airflow into the intake based on accelerator input; carbon buildup can cause rough idle.

Distributes the air (or air-fuel mixture) to the cylinders. A leaking gasket causes a lean condition and rough idle.

Unmetered air entering after the MAF leans the mixture, causing high idle, rough idle, and lean codes.

Steps up battery voltage to the tens of thousands of volts needed to jump the spark-plug gap.

Delivers the spark that ignites the mixture. Reading plug condition (fouled, oily, white) helps diagnose combustion problems.

The set distance the spark jumps; too wide or too narrow a gap causes weak spark and misfires.

Routes high voltage to each spark plug in firing order and controls spark timing on older ignition systems.

Uses individual coils (often one per plug) controlled by the computer instead of a distributor.

When the spark fires relative to piston position. Too advanced causes knock; too retarded loses power and runs hot.

Converts CO, hydrocarbons, and NOx into less harmful CO₂, water, nitrogen, and oxygen. Needs correct air-fuel ratio to work.

Chokes the exhaust, causing power loss especially at higher RPM and high back-pressure.

Exhaust Gas Recirculation — admits inert exhaust into the intake to lower combustion temperature and reduce NOx.

Causes rough idle and stalling because exhaust gas dilutes the mixture at idle.

Raises NOx and can cause spark knock (detonation) under load.

Before the converter; its rich/lean signal is the main input for closed-loop fuel control.

After the converter; monitors catalytic-converter efficiency by comparing its signal to the upstream sensor.

The computer uses O₂ sensor feedback to continuously trim fuel toward the ideal ratio once the engine is warm.

On a cold start the computer ignores the O₂ sensor and uses programmed values until the engine and sensor warm up.

How much the computer adds or subtracts fuel to keep the mixture correct; large trims indicate a fuel or air problem.

Collects exhaust from the cylinders into the exhaust system; cracks or leaks cause ticking noise and false O₂ readings.

Reduces exhaust noise. A rusted or internally collapsed muffler can restrict flow.

Resistance to exhaust flow; high back-pressure (plugged converter/muffler) reduces power and can overheat the engine.

Meters air around a closed throttle to set idle speed; a faulty IAC causes high, low, or unstable idle.

Captures fuel-tank vapors and routes them to be burned; a loose gas cap can set an EVAP leak code.

A fuel's resistance to knock. Higher octane resists detonation; using too low an octane can cause spark knock.

Removes dirt and rust from fuel before the injectors; a clogged filter starves the engine, especially under load.

Extra fuel added at startup (now controlled by the computer/sensors) to run a richer mixture until the engine warms.

Generates electrical power and charges the battery while the engine runs; a failed alternator drains the battery.

Stores electrical energy to crank the starter and power systems when the engine is off.

Cranks the engine for starting; a no-crank with good battery points to the starter, solenoid, or wiring.

Controls coil firing (when not handled directly by the engine computer) for proper spark timing.

A plug coated with carbon (rich), oil (worn rings/seals), or fuel (flooding) that misfires; reading the plug guides diagnosis.

A misfire from too little fuel — often a vacuum leak, weak fuel supply, or dirty injectors.

A misfire from too much fuel fouling the plug — leaking injector, high fuel pressure, or a faulty sensor.

The computer that uses sensor inputs to control fuel, spark, and emissions for the engine.

A faulty ECT sensor can make the computer over- or under-fuel, causing rich/lean running and poor cold-start behavior.

Lets outside air reach the sensor, making the mixture read falsely lean and causing the computer to over-fuel.

A stuck-open PCV valve acts like a vacuum leak, leaning and roughening the idle.

Often an intake or vacuum leak drawing unmetered air; can lean the mixture and roughen idle.

Detonation heard under acceleration; suspect low octane, over-advanced timing, lean mixture, or carbon buildup.

Deposits in the chamber and on valves that raise compression, cause hot spots, and can lead to knock or sticking valves.

Compares each injector's contribution (pressure drop or power) to find a clogged or weak injector causing a misfire.