ASE A1 key terms

A diagnostic test that cranks the engine with the throttle open and measures peak pressure in each cylinder, then compares readings to specifications to find leaking valves, worn rings, or a failed head gasket.

A test in which compressed air is introduced into a cylinder at top dead center; the percentage of air escaping and where it escapes (intake, exhaust, or crankcase) pinpoint the faulty component.

A test that connects a gauge to the intake manifold and interprets a steady or fluctuating needle to identify intake leaks, exhaust restriction, or valve and ring sealing problems.

A spring-loaded valve that meters crankcase blowby gases back into the intake. Stuck open, it causes a lean rough idle; stuck closed, it causes oil leaks and sludge from pressure buildup.

The clearance between the rocker arm and valve stem tip, or between cam lobe and bucket, on a mechanical valvetrain. Too tight holds a valve open; too loose causes noise and lost lift.

A lifter that uses engine oil pressure to automatically maintain zero valve lash. A collapsed or bleeding lifter causes valvetrain ticking, reduced valve lift, and often a misfire.

A small seal that controls how much oil travels down the valve guide. Worn seals let oil into the combustion chamber, producing blue smoke at startup and increased oil consumption.

The distance from the spring seat to the retainer with the valve closed. Too much height reduces spring pressure, which can cause valve float and poor sealing at high rpm.

The gap between the valve stem and its guide. Too much clearance lets oil enter the port and allows the valve to wobble, which wears the seat and destroys sealing.

A measurement taken with a straightedge and feeler gauge to detect head warpage. Excessive warpage prevents the head gasket from sealing and must be corrected by resurfacing or replacement.

The concentricity of the valve seat relative to the valve guide. Too much runout keeps the valve from sealing fully, causing compression loss and burned valves.

The difference between the base circle and the highest point of a cam lobe. Worn lobes reduce valve lift and duration, cutting engine breathing and power.

The number that multiplies camshaft lobe lift to produce valve lift. An incorrect ratio changes both valve lift and valvetrain geometry, affecting airflow and wear.

A metal rod that transfers motion from the lifter to the rocker arm on some engines. A bent pushrod usually indicates valve-to-piston contact or over-revving.

The difference in bore diameter from the top to the bottom of ring travel. Excessive taper lets rings lose their seal, causing blowby, oil burning, and reduced power.

The difference between the largest and smallest bore diameter measured at one level. Too much out-of-round requires boring or honing before new rings can seal.

The gap between the ends of a ring installed squarely in its bore. Too little gap causes ring butting and breakage; too much allows blowby and oil consumption.

The clearance between a piston ring and its groove. Excessive clearance lets the ring flutter and pump oil, leading to smoking and poor compression.

The oil clearance between the crankshaft journal and the bearing insert, usually checked with plastigauge. Too little clearance causes seizure; too much causes low oil pressure and knock.

A crushable plastic thread placed between a journal and its bearing and then measured after torqueing. The flattened width indicates the oil clearance.

The axial movement of the crankshaft, measured with a dial indicator. Excessive play from a worn thrust bearing allows the crank to walk and damage the block.

An abrasive process that produces a fine crosshatch pattern in the bore so it retains oil and helps new rings seat after boring or rebuilding.

A fastener tightened to a specified torque and then rotated an additional angle. It stretches permanently during use and must be replaced rather than reused.

A sensor that converts engine oil pressure into an electrical signal for the dashboard gauge or warning lamp. A faulty unit can show false low or high pressure.

A spring-loaded bypass valve that opens and returns oil to the pan when pressure exceeds a set limit, protecting the engine and pump from excessive pressure.

An oil whose two-number SAE rating, such as 5W-30, means it flows easily when cold and stays thick enough at operating temperature to protect the engine.

A temperature-controlled valve that blocks coolant flow to the radiator until the engine warms up, then opens to regulate operating temperature. A stuck thermostat causes overheating or no heat.

A cap that seals the cooling system and raises the coolant boiling point by holding pressure. A weak cap lets coolant boil away, causing overheating and coolant loss.

Stray electrical current passing through coolant that erodes metal parts such as heater cores and radiators. It is detected by measuring voltage between the coolant and ground.

A plastic reservoir that catches coolant pushed out of the radiator as it expands hot and returns it to the system as the engine cools.

The spark timing at idle with any advance disconnected or disabled. If set incorrectly, the engine may be hard to start, ping under load, or lose power.

A rating of how quickly a spark plug sheds heat. A plug that runs too hot can cause preignition, while one that runs too cold fouls and misfires.

A transformer that steps battery voltage up to thousands of volts so a spark can jump the plug gap. A weak coil causes misfires, especially under load.

A test that fires each injector one at a time and measures the resulting fuel pressure drop, comparing the drops to find injectors that flow too much or too little.

Pressure that builds in the exhaust when flow is restricted. It is measured at the oxygen sensor port; high readings indicate a clogged converter or crushed pipe.

A test that measures the current the starter motor pulls while cranking. Excessive draw points to a dragging starter, bad cables or connections, or a seized engine.

A test that measures the voltage lost across a cable or connection while current flows. More than about half a volt usually means excessive resistance in that part of the circuit.

The small alternating current voltage that remains in an alternator's output after rectification. Excessive ripple indicates a failed diode and can cause dim lights or a discharged battery.

Current that keeps flowing after all accessories are off and doors are closed. It is measured with an ammeter to find the circuit draining the battery overnight.