ASE L2 cheat sheet

Diesel compression ignition ignites fuel by the heat of compressed air; compression ratios commonly range from about 14:1 to 23:1.

A mechanical compression test requires normal cranking speed, disabled fuel delivery, and comparison of all cylinders.

Relative compression using cranking current can identify a weak cylinder; confirm with a mechanical gauge before teardown.

White smoke at cold start that clears can indicate low compression, retarded injection timing, or faulty glow plugs; blue smoke indicates oil; black smoke indicates overfueling or air restriction.

Excessive crankcase blowby that increases with load indicates ring or cylinder wear, piston damage, or a failed head gasket or head; a wet compression test helps distinguish ring wear from valve leakage.

Diesel dilution of engine oil raises oil level and lowers viscosity; sources include leaking injector nozzles, excessive DPF regeneration, and incomplete combustion.

Combustion gases in coolant-bubbles, a positive hydrocarbon test, or coolant pushed out-point to a head gasket, cracked head, or liner seal failure.

Cylinder balance or power contribution tests compare each cylinder's speed contribution; a weak cylinder may show low exhaust port temperature and a misfire code.

Injection timing too retarded typically causes white smoke, hard starting, and low power; too advanced causes knock and high peak cylinder pressure.

The ECM calculates injection quantity from rail pressure, throttle or APP, MAP or MAF, intake air temperature, and engine speed; injection timing from CKP and CMP plus coolant and fuel temperature.

Loss of CKP signal usually causes no injection and no start because the ECM cannot establish engine speed or position; CMP is used for cylinder identification and sync.

A shorted sensor on a shared 5V reference can pull down the reference and disable multiple sensors; measure reference voltage with the sensors connected.

CAN bus termination is normally two 120-ohm resistors, giving about 60 ohms across CAN high and CAN low with power off.

Injector trim or IMA codes compensate for manufacturing differences; after injector replacement the new code must be programmed or imbalance and misfire can result.

Accelerator pedal position sensors are redundant and must agree within a plausibility window; disagreement triggers limp or derate.

Voltage-drop testing under load finds high-resistance grounds and power feeds that a static voltmeter may not reveal.

Bidirectional scan-tool controls-injector cutout, VGT sweep, EGR command, glow plug test-confirm actuator and harness operation.

Freeze frame records conditions at fault set; pending, stored, and active status guide intermittent diagnosis.

Low turbo boost can come from intake restriction, charge-air leaks, exhaust restriction, stuck VGT vanes or wastegate, or MAP sensor error; compare actual to desired boost.

A charge-air cooler or pipe leak causes low boost, black smoke, and power loss; pressure-test the intake tract.

VGT vanes stuck open cause low boost and lag; stuck closed cause overboost and possible derate; command an actuator sweep and compare.

A wastegate actuator stuck open limits boost; stuck closed allows overboost; check actuator travel and control pressure.

Turbo bearing failure signs include oil in charge pipes, excessive shaft radial or axial play, wheel contact, and blue smoke; verify oil supply and drain restriction.

Exhaust manifold leaks show soot tracks and can whistle; they reduce turbo drive pressure and cause low boost.

Exhaust backpressure must be measured with a gauge and compared with OEM specification; high backpressure from a clogged DPF or crushed pipe reduces power.

Air filter restriction and intake pipe collapse reduce boost and raise smoke; a restriction indicator or vacuum gauge confirms the condition.

Common-rail injection pressure can exceed 20,000 psi; high-pressure lines must not be cracked open while the engine is running.

Low-pressure fuel supply faults-clogged filter, weak lift pump, air leak-cause low power, hard starting, and rail-pressure codes.

An injector leak-off or return-flow test compares return volume; excessive return indicates worn injector internals and can cause hard starting or rough idle.

Rail pressure commanded versus actual: actual low with high regulator duty can indicate supply restriction, leaking injector, or pump wear.

Pilot injection reduces combustion noise and NOx; missing pilot injection can cause diesel knock and rough idle.

Fuel contamination by water, gasoline, or microbes changes lubricity and combustion and can damage high-pressure components.

Injector nozzle coking or erosion causes imbalance, smoke, and power loss; cutout testing and trim data help isolate the faulty cylinder.

Air in fuel is visible as bubbles in a clear return hose and causes rough running or stalling; check filter seals and supply lines.

Injection quantity is primarily controlled by rail pressure and injector open time; injection timing is controlled by the ECM pulse.

The DOC oxidizes CO and HC and converts NO to NO2, which supports passive DPF regeneration.

DPF differential pressure rises with soot load; low differential pressure can indicate a missing sensor, disconnected hose, or cracked DPF.

Active DPF regeneration raises exhaust temperature by post-injection or HC dosing; frequent regens can indicate excessive soot from intake leaks, injector problems, or short trips.

DPF ash is noncombustible material from oil additives and wear metals; it permanently raises backpressure and requires cleaning or replacement.

SCR reduces NOx using DEF, a urea-water solution typically containing 32.5 percent urea; NOx sensors monitor before and after the SCR catalyst.

DEF quality or concentration faults, frozen DEF, or a failed dosing pump can trigger SCR efficiency codes and torque derate.

A leaking EGR cooler can cause coolant loss, white smoke, and misfire; pressure-test the cooler and check for coolant in the intake.

An EGR valve stuck open causes rough idle and low power; stuck closed causes high NOx and possible derate.

Ammonia slip from over-dosing DEF is detected by downstream NOx or ammonia sensing and can set SCR efficiency codes.