ASE T1 Gasoline Engines (Medium/Heavy Truck) key terms

A diagnostic procedure that cranks the engine and measures peak cylinder pressure; a dry test is run as-is and a wet test adds oil to the cylinder to see whether low pressure is caused by rings or valves.

A test that pressurizes a cylinder with the piston at top dead center and measures the percentage of air escaping, using the sound of the leak to locate bad rings, intake valves, exhaust valves, or head gasket.

Combustion gases that escape past the piston rings into the crankcase during engine operation; excessive blowby shows up as heavy crankcase pressure and indicates worn rings or cylinder walls.

The point in the crankshaft rotation where the piston is at its highest position in the cylinder; used as the reference for valve timing, injection timing, and ignition timing adjustments.

The measured clearance between the tip of the valve stem and the rocker arm, cam follower, or pushrod on a mechanically adjusted valve train; correct clearance keeps valves fully closing and seating.

The gradual sinking of a valve face into its seat over time, which reduces valve lash and can cause a valve to hang open; common on engines with hard seats and unleaded fuel.

The machined ring in the cylinder head that the valve face contacts to seal the combustion chamber and conduct heat away from the valve; may be integral or a pressed-in insert.

The bore in the cylinder head that keeps the valve stem moving in a straight line and carries heat from the stem to the head; worn guides cause oil consumption and poor valve sealing.

A small seal at the top of the valve guide that meters oil to the valve stem; a failed seal lets oil run down the stem and burn, producing blue exhaust smoke after idling.

A lever that transfers cam or pushrod motion to the valve stem; its ratio multiplies lift, and its adjustment screw or shim sets valve lash on many heavy-duty engines.

A hollow or solid rod that carries motion from the camshaft lifter up to the rocker arm in an overhead-valve engine; it may also carry oil to the rocker assembly.

The rotating shaft with lobes that opens and closes the valves and, on some engines, drives the fuel injection pump; its lobes and bearing journals wear and affect timing and lift.

Loss of the original lift profile on a cam lobe, usually from poor lubrication or wrong break-in oil; it reduces valve lift and can cause misfire, low power, or a dead cylinder.

A replaceable cylinder sleeve pressed or sealed into the block; a wet liner contacts coolant directly and seals with O-rings at the bottom, while a dry liner is surrounded by block metal.

The difference in cylinder bore diameter from top to bottom, caused by wear from ring pressure; it is measured with a bore gauge and compared to the manufacturer's limit.

The clearance between the two ends of a piston ring when it is fitted squarely in its cylinder; too little gap causes ring breakage as the ring expands with heat.

The oil-filled gap between the crankshaft main journal and its bearing shell, checked with plastigage or a micrometer and bore gauge; it sets oil pressure and load capacity.

The small axial movement of the crankshaft allowed by its thrust bearing; too little clearance can seize the engine, while too much lets the crank walk and damage bearings.

A main bearing with flanges or separate thrust washers that limit front-to-back crankshaft movement and absorb the force of the clutch or torque converter pushing on the crank.

A crushable plastic thread placed across a bearing journal and tightened to torque; the width of the flattened thread is compared to a scale to read bearing oil clearance.

A spring-loaded valve in the lubrication system that opens to bypass oil back to the pan or pump inlet when pressure exceeds a set value, protecting seals and the oil filter.

The gear, rotor, or vane device that draws oil from the pan and pushes it under pressure through the galleries, filter, and cooler to the engine bearings.

The resistance to oil flow through the engine galleries, read by a gauge in psi or kPa; low hot-idle pressure points to worn bearings, a weak pump, or a stuck relief valve.

The rate at which an engine uses lubricating oil, measured against fuel used or miles driven; excess use comes from worn rings, valve guides, seals, or turbocharger seals.

The system that pulls blowby gases and vapors out of the crankcase and routes them into the intake to be burned, preventing pressure buildup and sludge; it may be open or closed.

A temperature-controlled valve that stays closed to speed warm-up and opens to send coolant to the radiator; a stuck-open unit causes slow warm-up and low heater output.

The radiator or surge tank cap that holds a set pressure on the cooling system, raising the coolant boiling point; a weak cap lets the system boil over and lose coolant.

A thermistor that changes resistance with coolant temperature and reports it to the engine control module; a faulty one can cause rich running, poor cold starts, or fan errors.

Air or combustion gas mixed into the coolant, which reduces heat transfer and causes overheating, coolant loss, and a surging temperature gauge; often traced to a bad head gasket.

The sealing layer between the cylinder head and block that keeps combustion gases, coolant, and oil in their proper passages; failure can cause overheating, white smoke, or coolant in the oil.

The pressure above atmospheric that a turbocharger forces into the intake, measured at the manifold; low boost causes power loss and black smoke, high boost can damage the engine.

A valve that bypasses exhaust gas around the turbine to limit boost pressure; a stuck-closed wastegate can overboost the engine while a stuck-open one causes low power.

A heat exchanger that cools compressed intake air after the turbocharger, making it denser and increasing power; a leak in it causes low boost and can let dirt into the engine.

The resistance to exhaust flow downstream of the engine, measured at a port; high back pressure from a clogged converter or muffler reduces power and raises combustion temperature.

A buildup of soot in the exhaust filter that raises back pressure; the engine control system monitors the pressure drop across it and starts a regeneration cycle when the limit is reached.

A test that fires each injector in turn and measures the resulting change in engine speed or cylinder contribution, identifying weak or clogged injectors without disassembly.

A device that holds fuel pressure at a set value relative to intake manifold pressure, returning excess fuel to the tank; a failed unit causes rich or lean running and hard starts.

The length of time, in milliseconds, that the control module holds a fuel injector open on each firing; it is the main way the computer changes the air-fuel mixture.

The percentage of fuel the control module adds or subtracts from its base calculation to hold the target air-fuel ratio, shown as short-term and long-term values on a scan tool.

A sensor in the exhaust that produces a voltage or current signal proportional to oxygen content, letting the control module judge whether the mixture is rich or lean.

A transformer that steps battery voltage up to the high voltage needed to jump the spark plug gap; coil-on-plug designs mount one coil directly above each plug.

The moment, in crankshaft degrees before or after top dead center, when the spark occurs; incorrect timing causes hard starting, knock, power loss, or overheating.

The air space between the center and ground electrodes of a spark plug; it must match the engine specification because too wide a gap misfires under load and too narrow a gap weakens the spark.

A piezoelectric sensor bolted to the block that detects the vibration of detonation and signals the control module to retard ignition timing and protect the engine.

The controller area network used on heavy-duty trucks for communication among the engine control module, transmission, brakes, and dash; faults set communication codes and can derate the engine.

The computer that reads sensor inputs, controls fuel and ignition or injection timing, and runs self-diagnostics; it stores fault codes and can command a power derate.

A stored code set by the control module when a monitored circuit or system falls outside expected limits; the code narrows the fault but does not by itself name the failed part.

A measurement of the voltage lost across a connection or cable while current flows; more than about 0.5 volt on a heavy cable indicates excessive resistance such as corrosion or a loose terminal.

Marks on the crankshaft pulley, flywheel, or gears and their matching pointers that show piston position and allow the camshaft and injection pump to be set to the correct phase.

A head or main bolt tightened past its elastic limit in stages to a specified torque plus angle; it must be replaced rather than reused because it has already stretched permanently.

The specified order and stages in which fasteners such as head bolts are tightened, usually from the center outward, to seat the gasket evenly and avoid warping the head.