ASE S2 Diesel Engines (School Bus) key terms

A diagnostic procedure that cranks the engine and measures peak cylinder pressure, first dry and then with oil added, to reveal worn rings versus leaking valves or head gaskets.

A procedure that pressurizes each cylinder with regulated air at top dead center and measures the percentage lost, while listening at the intake, exhaust, crankcase, and radiator to locate the leak.

Combustion gases that escape past the piston rings into the crankcase, measured at the breather or oil fill to judge ring and cylinder wear.

The point where a piston reaches its highest travel in the bore, used as the reference for valve timing, injection timing, and leak-down testing.

Measuring the head deck with a straightedge and feeler gauge, or with a precision plate, to detect warpage before resurfacing or replacement.

Sinking of a valve seat into the cylinder head from wear or repeated refacing, which reduces valve stem protrusion and can require seat replacement.

The specified gap between the rocker arm or cam follower and the valve stem tip that allows for thermal expansion and controls valve opening duration.

A pivoting link on some diesel heads that lets one pushrod or rocker operate two valves, which must be set so both valves open equally.

Loss of sealing between the cylinder head and block, shown by coolant loss, white exhaust smoke, bubbling in the radiator, or combustion gas in the coolant.

A replaceable sleeve pressed into the block bore; wet liners contact coolant directly and need seals, while dry liners do not.

The clearance between the ends of a ring installed in its bore, which must be measured in the cylinder to confirm correct fit and sealing.

A spring-loaded valve in the lubrication circuit that opens to limit maximum oil pressure and protect seals and bearings at high engine speed.

The component that draws oil from the pan and forces it under pressure through the galleries and filter to the bearings and upper engine.

The manufacturer's required pressure range at a stated speed and temperature, used to judge whether wear, a bad gauge, or a faulty pump is causing low readings.

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

Supplemental coolant additives that protect wet-sleeve cylinder liners from cavitation pitting and control corrosion and scale in the cooling system.

A radiator cap that holds system pressure to raise the coolant boiling point and uses a relief valve plus a vacuum valve to manage expansion and contraction.

The pressure above atmospheric that the turbocharger delivers to the intake manifold, monitored to verify turbo performance and detect leaks or restrictions.

A valve that bypasses exhaust gas around the turbine to limit boost pressure and protect the engine from overboost.

A turbo whose turbine inlet vanes change angle to control exhaust energy and boost across the speed range, often also used to assist engine braking.

A heat exchanger that cools compressed intake air after the turbocharger, increasing air density and lowering combustion temperatures and NOx.

Resistance to exhaust flow measured upstream of the aftertreatment, used to find plugged filters, collapsed pipes, or a restricted diesel particulate filter.

A trap in the exhaust that collects soot and periodically burns it off during regeneration, monitored by differential pressure sensors.

The process of burning accumulated soot from a particulate filter, either automatically during normal hot operation or by raising exhaust temperature on command.

A catalytic converter that oxidizes hydrocarbons and carbon monoxide and generates heat for filter regeneration and downstream aftertreatment.

An aftertreatment system that injects diesel exhaust fluid into the exhaust to convert nitrogen oxides into nitrogen and water over a catalyst.

A urea-water solution stored in a separate tank and injected into the exhaust for SCR, with quality and level monitored by the engine controller.

An exhaust sensor that measures nitrogen oxide concentration before and after the SCR catalyst so the controller can verify conversion efficiency and dosing.

A heat exchanger that lowers the temperature of recirculated exhaust gas before it enters the intake, reducing combustion temperature and NOx formation.

Routing a portion of exhaust back into the intake to dilute the charge and lower peak combustion temperature, cutting nitrogen oxide emissions.

A calibration code printed on each injector that must be programmed into the engine controller so fueling is matched to that injector's actual flow.

The high-pressure fuel stored in a shared rail and delivered to all injectors, measured by a rail pressure sensor and controlled by a pump regulator.

Measuring the fuel that returns from each injector to compare leakage rates and identify a worn or stuck injector causing misfire or hard starting.

The crank angle at which fuel delivery begins, which affects starting, power, economy, and emissions and must match the manufacturer's specification.

Setting the slave piston clearance on a compression-release engine brake so the exhaust valve opens at the correct point without contacting the piston.

An engine brake that opens the exhaust valve near top dead center of compression, releasing stored energy so the engine retards the vehicle.

The voltage lost across cables, connections, and the solenoid while cranking, tested to find high resistance that causes slow cranking.

The amperage the starter pulls while turning the engine, measured with a clamp meter to judge starter condition and mechanical resistance.

The percentage of a battery's capacity currently available, estimated from open-circuit voltage or a conductance test before diagnosing starting or charging complaints.

The regulated voltage the alternator maintains at the battery while running, checked to confirm the charging system is working within specification.

Numeric codes stored by the engine controller when a monitored circuit or system fails a self-test, used as a starting point for diagnosis.

The controller area network used on heavy vehicles so the engine, transmission, and body controllers can share data and diagnostic information.

The regulated supply the controller provides to sensors, checked when readings are skewed or codes indicate an open or shorted circuit.