ASE L2 key terms

A method of igniting diesel fuel by spraying it into air that compression has heated to a very high temperature, so the engine needs no spark plug to start combustion.

The short period between the start of fuel injection and the beginning of combustion, during which injected fuel mixes with hot compressed air and prepares to burn.

A sharp metallic knocking sound caused by a large amount of fuel burning at once after a long ignition delay, producing an abrupt pressure rise in the cylinder.

Combustion gases and pressure that escape past the piston rings and cylinder walls into the crankcase, often measured to judge the condition of rings and cylinders.

A cranking test that measures the peak pressure each cylinder can build, used to find leaking valves, worn rings, damaged pistons, or a failed head gasket.

A test that supplies shop air to a cylinder held at top dead center and measures how much escapes, with the location of the escaping air identifying the faulty part.

A running test that cuts fuel to one cylinder at a time and measures the resulting change in engine speed or power to identify a weak or dead cylinder.

The specified clearance in the valvetrain between the rocker arm and the valve stem, which must be set correctly so valves open fully and close completely when the engine is hot.

An electrically heated element in the combustion chamber that warms the air for cold starting and helps keep combustion smooth just after start-up on engines so equipped.

A system that vents pressure and oil fumes from the crankcase back into the intake, and when restricted or failed can cause oil leaks and damaged seals.

The computer that reads sensor inputs, controls fuel delivery, timing, turbo boost, and emissions devices, and stores fault codes to aid diagnosis.

A regulated low voltage, commonly 5 volts, supplied by the ECM to sensors so each sensor can return a signal that varies with what it measures.

A sensor, often built as two opposing potentiometers, that reports pedal travel to the ECM so it can calculate requested fueling and detect a faulty signal.

The CAN-based communication bus that lets the engine ECM share data with other vehicle modules, and faults on it can stop communication and set codes.

A stored code that identifies a circuit or system fault the ECM has detected, usually paired with a description and sometimes a count of how often it occurred.

The correlation between cam and crank position signals that lets the ECM know which cylinder is on its compression stroke so injection timing can be correct.

A measurement of the voltage lost across a connection, cable, or ground while current is flowing, used to find resistance that a simple voltage reading would miss.

An exhaust-driven compressor that forces extra air into the engine, raising power and reducing smoke by increasing the mass of air available for combustion.

A valve that routes exhaust gas around the turbine to limit boost pressure, protecting the engine and turbo from over-speed and over-boost.

A turbo whose turbine vanes change angle to control exhaust speed and boost across the whole rpm range, and which can also drive EGR flow and exhaust braking.

A heat exchanger that cools compressed air from the turbo before it reaches the intake, increasing air density; leaks cause low boost, high intake temperature, and black smoke.

A sensor that reports intake manifold pressure to the ECM, which uses the value to meter fuel and to check turbocharger and EGR performance.

A gauge that shows how much vacuum the intake creates, warning when the filter is clogged enough to limit airflow, raise exhaust smoke, and cut power.

The pressure built up in the exhaust system ahead of the turbo or aftertreatment, which when too high reduces power and can damage engine components.

A fuel system that stores fuel at very high pressure in a shared rail and uses the ECM to time and meter injection electronically at each injector.

The point, relative to piston position, at which the ECM starts fuel delivery; being too advanced or too retarded changes power, knock, emissions, and starting.

A solenoid on the high-pressure fuel pump that regulates how much fuel enters the pumping element, letting the ECM control rail pressure.

A code marked on each injector and programmed into the ECM so it can adjust fueling for that particular injector's measured flow characteristics.

A test that measures the fuel returning from each injector over a set time; excessive return from one injector points to internal wear or a stuck internal part.

A unit injector that uses high-pressure engine oil, switched by a solenoid valve, to pressurize and fire the fuel, so oil pressure and oil condition affect fueling.

An injector that combines the pump and nozzle in one housing driven by the engine's camshaft or rocker, removing the need for external high-pressure lines.

A probe in the fuel filter and separator that warns the driver when water has collected, protecting the high-pressure pump and injectors from corrosion and wear.

A heat exchanger that lowers the temperature of exhaust gas recirculated into the intake; a leaking cooler can push coolant into the intake and cause white smoke.

A filter in the exhaust that traps soot from combustion, and the collected soot must be burned off periodically to keep back pressure within limits.

A process in which the ECM raises exhaust temperature, usually through extra fueling or a burner, to burn soot out of the DPF while the vehicle runs or is parked.

A urea-and-water solution injected into the exhaust ahead of the SCR catalyst so the catalyst can convert nitrogen oxides into nitrogen and water.

An aftertreatment system that uses a catalyst and injected DEF to turn nitrogen oxides in the exhaust into harmless nitrogen and water vapor.

A sensor in the exhaust that measures nitrogen oxide levels before and after the catalyst, letting the ECM check SCR efficiency and DEF dosing.

A flow-through catalyst that oxidizes hydrocarbons and carbon monoxide and, by generating heat, helps raise exhaust temperature for DPF regeneration.

Unreacted ammonia passing through the SCR catalyst into the tailpipe, usually from too much DEF dosing, which a downstream sensor or the ECM's model can detect.