ASE H2 Diesel Engines (Transit Bus) key terms

A diagnostic procedure that measures cylinder pressure while cranking; a dry test shows sealing of rings and valves, and adding oil for a wet test helps isolate worn piston rings from valve or head gasket leaks.

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

Combustion gases that escape past the piston rings into the crankcase during engine operation; excessive blowby raises crankcase pressure and often indicates worn rings, scored liners, or damaged pistons.

The exact point in the piston stroke where the piston reaches its highest travel in the cylinder; used as a reference for valve timing, injection timing, and leakage testing.

The casting bolted to the top of the engine block that closes the cylinders and houses the valves, injector sleeves, and coolant and oil passages; it must seal combustion pressure and fluids.

A sealing layer between the cylinder head and block that contains combustion pressure and separates coolant and oil passages; failure can cause coolant loss, overheating, or white exhaust smoke.

The clearance between the valve tip and the rocker arm or cam follower, set to allow for heat expansion; incorrect lash causes noise, lost lift, or burned valves.

A lever that transfers motion from the camshaft or pushrod to the valve stem; on many diesel engines it also carries an adjusting screw used to set valve lash.

A small bar placed over the stems of two adjacent valves so a single rocker arm can open both; its setting must be adjusted so both valves open equally.

The hardened ring in the cylinder head that the valve face contacts to seal combustion pressure; worn or recessed seats cause compression loss and require grinding or replacement.

The gradual sinking of a valve into its seat from wear or pounding, which reduces valve lash and can hold a valve open; it is checked during overhead set procedures.

A replaceable sleeve pressed or cast into the engine block that forms the cylinder bore; liners may be wet, contacting coolant directly, or dry, surrounded by block material.

A split metal ring fitted in a groove on the piston that seals combustion gases, controls oil on the cylinder wall, and transfers heat to the liner; rings include compression and oil types.

A maintenance procedure for adjusting valve lash and injector settings on a diesel engine, typically done with the engine at a specified temperature and each cylinder positioned at its firing point.

An instrument that displays oil pressure in the lubrication system; low readings can indicate a worn pump, restricted pickup, diluted oil, or excessive bearing clearance.

A gear, rotor, or piston-type device driven by the engine that moves oil from the sump through the filter and galleries to lubricate bearings and cool components.

A spring-loaded valve that opens to bypass excess oil back to the sump when system pressure exceeds a set value, protecting seals and the filter from overpressure.

A temperature-sensitive valve that stays closed to speed engine warm-up and opens at a rated temperature to allow coolant flow through the radiator for cooling.

Supplemental coolant additives that protect wet-sleeve diesel liners from cavitation erosion and control scale and corrosion; concentration must be tested and maintained.

A heat exchanger that transfers heat from engine coolant to the air; it relies on airflow and coolant flow to maintain operating temperature.

An exhaust-driven compressor that forces more air into the engine to increase power; its turbine wheel is spun by exhaust gas and its compressor wheel pressurizes intake air.

A valve that bypasses exhaust gas around the turbine to limit boost pressure; it may be controlled by a spring actuator or by the engine control module.

A heat exchanger, often called an intercooler, that cools compressed air from the turbocharger before it enters the engine, increasing air density and lowering combustion temperatures.

A sensor that measures intake manifold pressure and reports it to the engine control module so fueling and turbo control can be adjusted for the current operating condition.

A system that routes a portion of exhaust gas back into the intake to lower combustion temperature and reduce nitrogen oxide emissions; it uses a valve and often a cooler.

An exhaust aftertreatment device that traps soot particles; it is cleaned by regeneration, which burns the collected soot at high temperature, sometimes aided by fuel dosing.

The process of raising exhaust temperature to burn soot trapped in a diesel particulate filter; it may be passive, active, or forced by a service tool.

A sensor in the exhaust stream that measures nitrogen oxide concentration and provides feedback for aftertreatment control, such as selective catalytic reduction dosing.

A urea solution injected into the exhaust before a selective catalytic reduction catalyst to convert nitrogen oxides into nitrogen and water; its level and quality are monitored by the engine controls.

A retarder that opens exhaust valves near top dead center to release compressed air, creating braking force; it is used to slow the vehicle and reduce service brake wear.

A valve that restricts exhaust flow to create backpressure and slow the engine; it provides auxiliary braking on grades and is often used with a compression release brake.

A common name for a compression-release engine brake that uses hydraulic pressure to open exhaust valves at the top of the compression stroke, absorbing energy and slowing the vehicle.

A sensor that detects the rotational position and speed of the crankshaft and sends the signal to the engine control module for injection timing and rpm calculation.

A sensor that identifies the camshaft position so the engine control module can determine which cylinder is on its compression stroke and synchronize injection and valve events.

The computer that monitors sensors and controls fuel injection, turbo boost, EGR, and aftertreatment; it stores diagnostic trouble codes and can be reprogrammed.

A code stored by the engine control module when a monitored circuit or system fails a test; it guides diagnosis but does not by itself identify the failed part.

The standardized diagnostic socket that allows a scan tool to communicate with the engine control module and other modules for reading codes and live data.

A device that atomizes and delivers fuel into the combustion chamber at the correct time and quantity, often controlled electronically by the engine control module.

A pump that pressurizes diesel fuel to very high pressure for injection; it is driven by the engine and its output is regulated by the engine control module.

A test that measures fuel returning from each injector to compare internal leakage; excessive return from one injector indicates wear and can cause rough running or hard starting.

An electrically heated element in the combustion chamber that warms the air for cold starting; it stays on briefly after start to reduce white smoke.

A test that applies a heavy current draw to a battery while monitoring voltage to determine whether it can deliver cranking power; low voltage under load indicates a weak battery.

An electric motor that cranks the engine through a drive gear engaging the flywheel ring gear; it draws high current and depends on good battery and cable connections.

A belt-driven generator that produces alternating current, rectifies it to direct current, and charges the battery while supplying electrical loads during engine operation.

A safety procedure that isolates energy sources and applies locks and tags before service, preventing accidental start-up or release of stored energy during repair.

Protective gear such as safety glasses, gloves, hearing protection, and steel-toe boots worn to reduce injury risk while working on engines and equipment.

General practices that keep a service area safe, including clean floors, proper lifting, correct tool use, ventilation, and safe handling of fuels and chemicals.