ASE T2 Diesel Engines (Medium/Heavy Truck) key terms

The measured gap between the valve stem tip and the rocker arm or cam follower when the valve is fully closed. It allows for thermal expansion and is set to the engine manufacturer's specification during adjustment.

A pivoting lever that transfers motion from the camshaft or pushrod to the valve stem. Its adjusting screw or pushrod seat sets valve lash on many medium and heavy diesel engines.

A slender metal rod that transmits motion from a camshaft follower up to the rocker arm in an overhead-valve engine. It may be hollow to carry oil to the rocker assembly.

The eccentric raised portion of a camshaft that opens a valve as it rotates. Lobe lift, profile, and base circle wear directly affect valve timing and lift.

The machined sealing surface in the cylinder head that the valve face contacts when closed. Seat width and concentricity affect compression sealing and heat transfer from the valve.

Sinking of the valve face into the cylinder head seat over time due to wear or repeated refacing. Excessive recession reduces valve lash and can cause loss of compression or valve-to-piston contact.

A bore or replaceable sleeve in the cylinder head that keeps the valve stem moving in a straight line. Worn guides cause oil consumption, poor seating, and uneven valve wear.

A small seal at the top of the valve guide that limits oil from running down the valve stem into the port. Failed seals produce oil smoke at startup.

A tube pressed into the cylinder head that isolates a unit injector from coolant and combustion gases. Leaking sleeves cause coolant in the fuel or fuel in the coolant.

The casting that caps the cylinders and houses valves, injectors, and often camshafts. It must seal combustion pressure while passing coolant and oil to the valve train.

The sealing layer between cylinder head and block that contains combustion pressure and separates coolant and oil passages. Failures cause compression loss, coolant loss, or fluid cross-contamination.

A replaceable sleeve pressed or cast into the engine block that forms the cylinder bore. Liner protrusion, roundness, and crosshatch determine ring sealing and head gasket life.

The amount a cylinder liner stands above the block deck surface. Incorrect protrusion prevents the head gasket from sealing combustion pressure and must be measured during assembly.

A split metal ring on the piston that seals combustion gases, controls oil, and transfers heat to the cylinder wall. Ring end gap and side clearance are critical wear checks.

Combustion gases that escape past the piston rings into the crankcase. Excessive blowby indicates worn rings, liners, or damaged pistons and shows up as heavy crankcase venting.

A bearing that supports the crankshaft in the block. Oil clearance, wear pattern, and crush fit determine whether the bearing can carry load and maintain oil pressure.

The bearing shell between the connecting rod and crankshaft journal. It converts combustion force into rotation and is checked for clearance, wear, and scoring during overhaul.

The small axial clearance between the crankshaft thrust surfaces and the block or bearing. Excessive end play causes main bearing thrust wear and can damage the transmission and flywheel.

The component that pressurizes engine oil and delivers it to bearings and the valve train. Relief valves in the pump limit maximum pressure and protect the system when oil is cold.

The force of oil delivered by the pump against system restriction, read at the gallery or filter housing. Low pressure at operating speed indicates pump wear, leaks, or excessive bearing clearance.

A valve that opens when the filter is restricted or oil is cold, allowing unfiltered oil to reach the engine. It protects against oil starvation but lets debris pass if it stays open.

A canister or cartridge that removes particles from engine oil before it reaches the bearings. Correct micron rating and bypass function are essential to protect the lubrication system.

A device that vents crankcase pressure to the atmosphere or intake while separating oil mist. A clogged breather raises crankcase pressure and forces oil past seals and gaskets.

A temperature-controlled valve that regulates coolant flow to the radiator to maintain operating temperature. A stuck-open thermostat causes overcooling; a stuck-closed one causes overheating.

A thermistor that reports coolant temperature to the engine control module. Its signal drives fuel timing, fan control, and overheating protection.

A heat exchanger that transfers heat from engine coolant to air. Internal or external blockage reduces cooling capacity and can cause overheating under load.

An exhaust-driven compressor that forces more air into the engine. Boost pressure, shaft play, and oil supply condition are key diagnosis points.

The pressure above atmospheric that the turbocharger delivers to the intake manifold. Low boost reduces power; high boost can set overboost codes or damage the engine.

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

A heat exchanger that cools compressed intake air before it enters the engine. Leaks or blockage reduce air density and power and can set boost or temperature codes.

An exhaust aftertreatment device that traps soot and is regenerated by heat. High backpressure or frequent regeneration indicates excessive soot loading or a failed regeneration.

A catalytic converter that oxidizes hydrocarbons and carbon monoxide and generates heat for DPF regeneration. Degradation raises regeneration temperatures and soot output.

The pressure that exhaust gas must overcome to leave the engine. Excessive backpressure from a clogged DPF or damaged pipe reduces power and raises exhaust temperature.

A fuel injector that combines the high-pressure pump and nozzle in one unit driven by the camshaft or rocker. It requires precise timing and lash adjustment for correct injection.

A manifold that stores pressurized fuel and distributes it to the injectors. Rail pressure is controlled by the ECM and is a primary input for injection quantity.

The point in the piston cycle at which fuel is injected. Advanced timing raises cylinder pressure and NOx; retarded timing reduces power and raises exhaust temperature.

An electric motor that turns the engine crankshaft through a drive gear until it starts. Low cranking speed points to battery, cable, or starter faults.

A belt-driven generator that charges the battery and supplies electrical loads while the engine runs. Diode or regulator failure causes low charging voltage or battery drain.

The percentage of a battery's capacity currently stored, estimated from open-circuit voltage or a conductance test. Low charge causes slow cranking and false electrical faults.

A measurement of voltage lost across a cable or connection while current flows. Excess drop identifies high-resistance cables, grounds, or connections in starting and charging circuits.

The computer that controls fuel delivery, timing, and emissions devices using sensor inputs. It stores fault codes and protects the engine through derate strategies.

The CAN bus protocol used on heavy trucks for communication between the ECM and other modules. Faults cause missing data, warning lamps, and communication codes.

A regulated low voltage supplied by the ECM to power sensors. A shorted or open reference circuit affects multiple sensors at once and sets related codes.

A retarder that opens exhaust valves near top dead center to release compressed air and absorb energy. It slows the vehicle without using service brakes on long downgrades.

A valve that restricts exhaust flow to create backpressure and slow the engine. It provides less retarding force than a compression brake but is simpler and lighter.

An electrically controlled valve that supplies oil pressure to actuate the engine brake mechanism. A failed solenoid disables braking on the affected cylinders.