Your FREE ASE T2 Flashcards 2026 – 200+ Cards

How a diesel ignites fuel: air is compressed enough to self-heat to about 540°C (1000°F), then injected fuel ignites from that heat — no spark plug.

Intake (air only), compression, power (compression ignition), exhaust — two crankshaft revolutions per cylinder.

A cold-start aid that heats the chamber or intake air to help a cold diesel start and cut white smoke. Not an igniter — combustion is still by compression.

Incomplete combustion from too much fuel or too little air — restricted air filter, low boost, or over-fueling injectors.

Oil burning in the cylinder — worn rings or liners, worn valve guides/seals, or a leaking turbo seal.

Unburned fuel from low cylinder temperature (cold engine, weak glow plugs, low compression) or coolant entering the chamber.

Disable each injector one at a time and watch RPM/sound. Little or no change = that cylinder is weak (injector, compression, or electrical).

Check that cylinder's injector wiring harness before tearing down — rule out an electrical fault first.

Measures mechanical sealing of rings, valves, and liners; separates a fuel/injector problem from a mechanical one on a weak cylinder.

Confirm by inspecting crankshaft and camshaft timing marks — the most direct check of injection timing.

Damps crankshaft torsional vibration. A damaged balancer can cause noise and vibration only at high RPM.

The first instrument on a modern diesel — pull fault codes and freeze-frame data, and command components to confirm operation.

A snapshot of sensor values captured when a fault code set, used to recreate the conditions of the concern.

Increased fuel consumption and decreased power — not enough air for proper combustion, often with black smoke.

Check the fuel injectors next — they directly affect the engine's ability to make power.

Fuel vaporizes from residual heat before reaching the engine, causing hard starting and erratic idle after sitting hot.

Verify → gather codes/data → test to isolate → interpret → repair → confirm. Diagnose the cause, not just the symptom.

Confirm and reproduce the customer complaint before testing — never start replacing parts off an assumption.

A deep, rhythmic knock that worsens under load points to the bottom end — rod or main bearings.

A lighter tick that rises with RPM is usually excessive valve lash or a worn lifter/follower.

Incomplete combustion because cylinder temperatures are too low to fully burn the fuel.

Oil burning in the engine — typically worn piston rings or cylinder liners.

Most likely air trapped in the fuel system — it can't hold the pressure/flow needed to keep running.

The symptom is a clue, not the diagnosis. T2 questions reward picking the correct next diagnostic step.

Glow plugs are for cold starts; they do not cause black smoke or low power during normal running.

Typically about 16:1 to 22:1 — far higher than gasoline — to self-heat the air enough for compression ignition.

Weak or dead glow plugs cause hard cold starts and white smoke until the engine warms.

Applies compressed air to a cylinder at TDC and reads where it escapes (valves, rings, or head gasket).

Uniformly low = worn rings/liners or timing; one low cylinder = a leaking valve, head gasket, or that hole's rings.

Judge each technician's statement separately as true or false, then pick A only, B only, both, or neither.

Seals the top of the cylinders and houses the valves, seats, guides, and (on OHC engines) the camshaft.

Causes head-gasket failure, coolant/oil leaks, overheating, and compression loss. Must be machined flat.

Use a straightedge and feeler gauge across the deck; high spots or warp call for resurfacing.

Alters combustion-chamber volume and valve-to-piston clearance, causing low compression on all cylinders.

Often repaired by installing valve-seat inserts rather than replacing the entire cylinder head.

Can cause intermittent misfiring and loss of compression as the crack intermittently breaks the chamber seal.

Lets oil into the combustion chamber, increasing oil consumption and causing blue smoke.

Clearance in the valve train for thermal expansion. Too little burns valves; too much ticks and wears.

A valve may not fully close, risking valve-to-piston interference and burned valves with lost compression.

Cause a loud ticking that increases with RPM because they can't maintain proper clearance.

Valve float at high RPM — the valve doesn't follow the cam profile, stressing the pushrod.

Usually caused by poor or contaminated oil; the lobes rely on a clean oil film against the lifters/followers.

From inadequate lubrication or debris; the friction can stress and even snap the camshaft.

Incorrect cam timing can cause hot-start difficulty (clearances tighten cold, expand hot) and compression issues.

Often camshaft-lobe deterioration if the cam wasn't replaced or was improperly installed/lubricated.

Caused by poor sealing or too-little lash; produces low compression and a misfire in one cylinder.

A hardened ring pressed into the head to restore a worn or damaged valve seat without replacing the head.

Many diesels use torque-then-angle (torque-to-yield) head bolts tightened in sequence; replace once removed if specified.

When valves can't follow the cam profile at high RPM; can bend pushrods and cause valve-to-piston contact.

Holds the cylinders (often replaceable liners), crankshaft, pistons, and bearings.

A replaceable cylinder wear surface (wet or dry) so the bore can be renewed without scrapping the block.

A liner in direct contact with coolant, sealed by O-rings at the bottom. A failed seal lets coolant into the crankcase.

How far a liner stands above the deck; out of spec causes head-gasket sealing problems and premature gasket failure.

Prevents the head gasket from sealing properly, leading to premature head-gasket failure.

Difference in cylinder diameter top vs. bottom of ring travel due to wear; excessive taper calls for reboring/re-sleeving.

A bore worn unevenly so it's no longer a true circle; often from uneven thermal expansion or overheating.

Require reboring the cylinders to the oversize spec for correct piston-to-cylinder clearance.

Removes high spots so the deck is flat and seals the head gasket; checked with a straightedge and feeler gauge.

Can let coolant into the oil passages even with a good head gasket; high-pressure blocks are repaired by welding with pre/post-heat.

A plastic strip crushed between a bearing and journal to measure oil clearance by comparing the flattened width to a chart.

Causes a deep bottom-end knock under load and low oil pressure (worst warm at idle).

Uneven load on the bearings causes excessive or repeated main-bearing wear even with a good crankshaft.

Controls crankshaft axial (end-to-end) movement; incorrect installation can cause premature main-bearing failure.

If the seal and crank surface are good, suspect a misaligned bell housing placing uneven pressure on the seal.

Seal combustion and control oil. Worn rings/liners cause blue smoke and low compression.

Next step: measure cylinder-wall clearances to verify the bores are in spec and the rings are seating.

Removing the unworn lip at the top of the bore before pushing pistons out, so rings and pistons aren't damaged.

Magnetic-particle (Magnaflux) or dye-penetrant inspection find cracks in the block, head, or crankshaft.

Combustion gases slipping past the rings into the crankcase; excessive blow-by indicates worn rings or liners.

A smooth, shiny bore surface that prevents new rings from seating, causing oil use and blow-by.

Cross-hatching the cylinder/liner surface so new rings seat and retain oil; done after boring or before a re-ring.

Circulates pressurized oil through the bearings and galleries. A worn pump or failing gears can lower pressure or shed metal flakes.

Most likely excessive main bearing clearance after a rebuild — oil escapes the bearings faster at low pump speed.

Points to aerated oil (air bubbles) or air on the pump's suction side; the pump can't maintain steady pressure.

Oil mixed with air bubbles, which the pump can't pressurize consistently — causing erratic oil-pressure readings.

Coolant contamination — think head gasket, cracked oil cooler, cracked block/head, or a failed liner seal.

A heat exchanger that uses coolant to cool engine oil. A cracked oil cooler mixes coolant and oil.

Allows coolant to enter the oil (or oil into coolant), causing contamination and bearing wear — a commonly missed cause.

Increase engine speed and observe the pressure at which the valve opens to confirm it limits pressure correctly.

First check that the new oil filter is installed correctly — improper installation can restrict oil flow.

Failing oil-pump gears can shed metal flakes found in the oil at a change.

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

Causes overheating — no coolant flow reaches the radiator.

Keeps the engine running too cool, hurting economy and heater output and slowing warm-up.

Suspect airflow — radiator fins blocked by debris reduce cooling efficiency.

Makes the cooling fan run at full speed all the time regardless of engine temperature.

Belt- or gear-driven pump that circulates coolant through the engine and radiator. A weep-hole leak signals a failing seal.

Verify pulley-system alignment next — misalignment causes noise and wear on belts and pulleys.

Indicates contamination (rust/scale) and the need to flush the cooling system.

Can corrode aluminum cooling-system parts; coolant must be maintained at the correct chemistry.

A clogged coolant passage near that cylinder can show up as an unexpected temperature drop during the test.

Seals the system and raises the coolant boiling point; also lets coolant return from the overflow as the engine cools.

Head gasket, cracked oil cooler, cracked block/head, or a failed wet-liner seal.

Vents blow-by gases from the crankcase; a blocked or failed system can build pressure and push out oil.

Heavy-diesel coolant uses supplemental coolant additives to protect wet liners from cavitation pitting.

Vibration-formed bubbles erode wet liners; controlled by correct coolant additives and a filled system.

Exhaust-driven; its turbine spins a compressor to force more air into the cylinders (boost) for more power.

Above-atmospheric intake pressure made by the turbo. Low boost causes low power and black smoke.

Cools the hot, compressed air from the turbo so it's denser, improving combustion and lowering emissions.

Usually a failed turbocharger oil seal letting engine oil into the induction system.

Loses turbocharger boost pressure, reducing engine power.

A valve that limits maximum boost by bypassing exhaust around the turbine; a faulty actuator causes under- or over-boost.

A faulty turbocharger wastegate actuator can prevent correct boost, lowering power.

Pressure built up in the exhaust; excessive (plugged DPF, restriction) reduces power and raises temperatures.

Can decrease back pressure and lose low-end torque if not properly tuned.

Often a crack in the exhaust manifold or pipe letting exhaust gases escape.

Routes inert exhaust into the intake to lower combustion temperature and reduce NOx.

Reduces EGR flow, which raises NOx emissions.

Use a scan tool to command the EGR valve while monitoring engine RPM/data — the most accurate functional check.

First aftertreatment device; oxidizes carbon monoxide and hydrocarbons and helps raise temperature for DPF regen.

Traps soot from the exhaust; the soot is burned off during regeneration. A plugged DPF restricts exhaust and cuts power.

Burning off soot collected in the DPF, passively while driving or actively by raising exhaust temperature.

Sprays DEF into the exhaust to convert NOx into harmless nitrogen and water over a catalyst.

A urea-and-water solution used by SCR to reduce NOx. Running out can force a reduced-power (derate) mode.

DOC → DPF → SCR (DEF): oxidize, then trap soot, then kill NOx.

Often an air-induction issue (restriction or leak) causing incomplete combustion from insufficient air.

Low power and poor acceleration with black smoke from inadequate air supply.

Can over-estimate air density and cause over-fueling, hurting performance and emissions.

The pressurized intake air supplied by the turbo and cooled by the CAC before it enters the cylinders.

On some diesels controls intake airflow (for EGR/regen/shutdown); an intermittent power loss can trace to it.

Inspect compressor/turbine shaft for excessive play and the wheels for damage or oil leakage.

Reads the pressure drop across the DPF to estimate soot load and trigger regeneration.

Measures NOx before/after SCR so the ECM can meter DEF and verify aftertreatment efficiency.

A technician-commanded DPF regen used when soot load is high and passive/active regen hasn't cleaned the filter.

Injects the correct amount of DEF into the SCR system based on NOx and temperature; clogging or freezing causes faults.

A reduced-power protection mode triggered by faults such as low DEF, high DPF soot, or aftertreatment problems.

The delay before the turbo builds boost; excessive lag can come from leaks, a worn turbo, or a sticking VGT.

A turbo with movable vanes the ECM adjusts for boost across the RPM range; sticking vanes cause low power or codes.

Causes a whistle/tick and a boost or back-pressure leak; can lower turbo performance.

An injector with its own high-pressure pump per cylinder, cam-driven and (on electronic units) solenoid-fired.

Misfires only its own cylinder, since each injector serves one cylinder.

Hydraulically actuated, electronically controlled unit injection — uses high-pressure engine oil to actuate the injectors.

Low or aerated engine oil can cause hard starting and rough running because oil actuates the injectors.

One high-pressure pump charges a shared rail; the ECM commands solenoid/piezo injectors to fire.

Fuel pressure in the common rail, controlled by the ECM. A drop under demand points to a failing high-pressure pump.

Most likely a failing high-pressure fuel pump that can't keep up with increased demand.

A compressible bubble that absorbs injection pressure — causing hard starting, starting-then-stalling, and rough running.

Air trapped in the fuel system; prime and bleed the air out before running.

Removing trapped air (often with a manual primer pump) after fuel-side work so a solid fuel column reaches the injectors.

After a high-pressure common-rail repair, bleed air from the rail with the manual primer before cranking to prevent air lock.

Lowers high-side fuel pressure and reduces a cylinder's output; can indicate an internal injector leak.

Causes a rich condition, incomplete combustion, blue smoke, and a misfire on that one cylinder.

Typically a sticking nozzle needle interrupting proper fuel flow.

A defective fuel injector seal can let fuel into the chamber and past the rings into the crankcase, contaminating the oil.

Fuel drained back toward the tank — a bad check valve failed to hold prime.

Suspect an open injector coil, seized plunger, or blown fuse — not a clogged filter (which would affect all injectors).

Monitor live data for injector performance variance to pinpoint the failing injector.

Often an inoperative fuel cooler that should reduce fuel temperature as it circulates.

Check for air in the system, incorrect fuel timing, and clogged filters — high fuel pressure is not a hard-start cause.

Check the injector wiring harness for that cylinder first before invasive teardown.

Often excessive fuel return from an internal injector leak.

Air in the fuel system can cause combustion-timing issues that produce excessive noise and vibration.

Removes water and contaminants; primary and secondary filters protect injection components. Clogged filters cause hard starting and low power.

Corrodes and damages injection components; a fuel/water separator drains collected water.

Low-pressure pump that moves fuel from the tank through the filters to the high-pressure pump or injectors.

Generates the very high pressure needed for diesel injection; failure shows as low rail pressure and poor power.

The pressure at which a mechanical injector nozzle opens to spray; incorrect pressure affects atomization and combustion.

Breaking fuel into a fine mist for complete combustion; poor atomization (worn nozzle) causes smoke and lost power.

When fuel is injected relative to piston position; too advanced/retarded causes knock, smoke, and power loss.

The computer that controls injection quantity and timing, rail pressure, EGR, and aftertreatment from sensor inputs.

Can raise system pressure or fuel temperature and disturb injector operation.

A measure of diesel fuel's ignition quality; low cetane can cause hard starting, noise, and white smoke.

Common-rail injectors use fast solenoid or piezo actuators commanded by the ECM for multiple injection events per cycle.

A common-rail injector can fire pilot, main, and post injections per cycle to cut noise, emissions, and aid regen.

Controls supply or rail pressure; a fault can cause low power, hard starting, or pressure that won't hold.

The set pressure at which a mechanical injector nozzle opens; tested on a pop tester for spray pattern and pressure.

Limits fueling until boost rises so the engine doesn't pour black smoke on acceleration.

A bench tool that pressurizes a mechanical injector to check its opening (crack) pressure and spray pattern.

Can cause a diesel knock, higher cylinder pressure, and possible engine damage.

Lowers fuel temperature in the return circuit; an inoperative cooler raises fuel temperature and can hurt performance.

Cranks the high-compression diesel to start. Slow cranking with a good battery points to circuit resistance.

High resistance in the starter circuit — corroded/loose cables or poor grounds cause a voltage drop.

Measures lost voltage across a cable/connection under load to find high resistance the meter alone won't show.

Indicates poor ground connections adding resistance to the starting circuit.

Often a defective starter solenoid that can't pass current to the motor.

A sticking pinion gear stays meshed with the flywheel after the engine starts.

Both a weak battery (low cranking speed) and high circuit resistance can be the cause.

Belt-driven generator that recharges the batteries and powers the electrical system while the engine runs.

Most likely loose drive-belt tension — the alternator can't spin fast enough at idle to make voltage.

Verify the wiring connections at the alternator and the warning-light circuit before re-replacing parts.

Controls alternator output voltage; a fault causes under- or over-charging.

Reduce output and add AC ripple; cause low charging voltage and possible electrical noise.

Measure charging voltage with the engine running and load applied to verify the system is keeping up.

In the starter, lets the pinion freewheel once the engine starts so the starter isn't back-driven.

A current draw with everything off that can drain batteries overnight; found with a current test on the circuits.

A battery can be fully charged yet still fail a load test if its capacity is gone — test both before condemning.

Heavy trucks use multiple batteries in parallel for cranking; a weak cell drags down the whole bank.

Uses gear reduction for high cranking torque on big diesels; high resistance still causes slow cranking.

A 'Jake brake' that opens the exhaust valves near TDC with fuel cut, releasing compression energy so the engine absorbs power.

Reduces the load on the service (wheel) brakes on long downgrades, preventing brake fade.

It turns the engine into an air compressor that throws away its compressed-air energy, so the engine resists turning.

Check for supply voltage at the engine brake switch and solenoids first when the brake fails to activate.

Points to a worn/sticking mechanism, a defective solenoid on one cylinder, or wrong slave-piston (control-valve) clearance.

Check the engine brake control-valve (slave-piston) clearance first.

Can indicate a broken engine-brake actuator spring.

Controls oil flow to the slave piston that opens the exhaust valve; a defective solenoid disables that cylinder's braking.

Typically requires the throttle closed, the clutch engaged, and a minimum RPM; fuel is cut when it activates.

The hydraulic piston in the engine brake that pushes the exhaust valve open near TDC to release compression.

The engine brake slows the truck by absorbing engine power; it supplements, not replaces, the service (wheel) brakes.

The loud 'bark' is the released compressed air escaping the exhaust; excessive noise can indicate a fault or missing muffling.