Your FREE ASE A9 Flashcards 2026 – 200+ Cards

The diesel ignition principle: air is compressed until it is hot enough to ignite injected fuel on its own — no spark plug.

Gasoline uses a spark to ignite a fuel-air mix; a diesel compresses air until it self-ignites injected fuel.

An electric heating element that preheats the diesel combustion chamber to aid cold starts. A good plug reads near 1 ohm.

Measure resistance: a good glow plug reads about 0.8-1.2 ohms; an open (infinite) reading means a burned-out element.

Supplies battery current to all glow plugs on command. If every plug is good but none preheat, suspect the relay/module.

Classic symptom of inoperative glow plugs or the glow plug circuit; the engine no longer needs them once warm.

Modern light-duty diesels run roughly 15:1-17:1, much higher than gasoline, to create the heat for compression ignition.

Disable fuel injection first, then crank. Healthy light-duty diesel reads about 300-450 psi per cylinder.

A diesel will try to start once a cylinder builds compression, so injectors/fuel supply must be disabled so it only cranks.

Cylinders should read within about 10% of each other; one cylinder ~30% low signals a localized fault.

Repeat after adding a little oil. Reading rises = worn rings/bore; stays low = valves or head gasket leak.

A scan-tool test comparing each cylinder's contribution to crankshaft acceleration to isolate a weak/misfiring cylinder.

Incomplete combustion - too much fuel or too little air: restricted air filter, low boost/failing turbo, over-fueling or leaking injectors, faulty EGR.

Engine oil burning - worn rings/cylinders, worn valve guides/seals, or leaking turbo seals. Often worse on startup or deceleration.

Unburned fuel or coolant: cold-start vapor and bad glow plugs, or coolant entering a cylinder (head gasket, cracked head, EGR cooler).

A metallic rattle from the rapid combustion pressure rise; worsened by mistimed/over-advanced injection, faulty injectors, or low-cetane fuel.

A measure of how readily diesel fuel ignites under compression - the diesel counterpart to octane. Higher cetane ignites more easily.

Cetane rates how easily diesel ignites under compression; octane rates a gasoline's resistance to knock. Diesels want HIGH cetane.

A diesel has no throttle plate, so it always draws a full charge of air; power is controlled by how much fuel is injected.

Verify fuel rail pressure with a scan tool while cranking; low rail pressure points to fuel supply, the HP pump, or pressure control.

A bottom-end knock is a mechanical bearing noise that worsens under load; diesel knock is a combustion-timing rattle.

A hiss that changes with load plus a loss of boost and power - pressurized intake air escaping a cracked pipe or loose clamp.

Often a restricted fuel filter limiting delivery - combustion stays lean and clean, so little or no smoke appears.

A dynamic test that reveals restrictions affecting cylinder filling and valve sealing under operating conditions a static test may miss.

Use a scan-tool cylinder balance/contribution test to find the weak cylinder, then check injector, compression, and mechanical causes.

Confirm and reproduce the complaint - noise, smoke, hard start, low power - before testing, then gather data and inspect.

The highest point of piston travel; many tests and adjustments are referenced to TDC on the compression stroke.

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

On-board diagnostics and the diagnostic trouble codes it stores; the starting point for scan-tool diagnosis.

The lowest point of piston travel in the cylinder, opposite TDC.

Isolate by speed and load: a misfire, a failing damper, a motor mount, or an out-of-balance rotating assembly each have distinct patterns.

Reports coolant temperature to the ECM, affecting fueling, glow-plug strategy, EGR, and the gauge.

Confirm actual pressure with a mechanical gauge before condemning the pump - the sensor or wiring may be at fault.

Persistent thick white smoke with coolant loss on a warm diesel points to coolant entering a cylinder (head gasket, cracked head, EGR cooler).

High blow-by from worn rings/cylinders pressurizes the crankcase and pushes oil out of seals - a sign of internal wear.

The casting that seals the cylinders and houses the valves, seats, guides, injectors, and (on OHC engines) the camshaft.

Seals combustion pressure between head and block and keeps coolant and oil passages separate; blown = white smoke, coolant loss, milky oil.

Causes head-gasket failure and compression loss; check flatness with a straightedge and feeler gauge and machine it flat if needed.

Torqued to a value then turned a set angle, stretching it for even clamping. TTY head bolts are replaced once removed.

Tighten in the manufacturer's crisscross pattern in stages so the head clamps evenly without warping.

Opens and closes to admit air and release exhaust; must seal tightly against its seat. A diesel intake valve is usually larger than the exhaust.

A hardened ring pressed into the head that the valve seals against; improves sealing and resists wear.

The bore that supports and aligns the valve stem; wear lets oil into the chamber and the valve seals poorly.

Keeps oil from running down the valve stem into the chamber or exhaust; failure causes blue smoke and oil consumption.

A classic source of blue smoke and oil consumption - oil seeps past the guide and burns.

The shaft whose lobes open the valves in time with the crankshaft; its position sets valve timing.

Closes the valve and keeps the follower on the cam lobe; a weak spring causes valve float and poor sealing.

The small valve-train clearance for thermal expansion. Too little burns valves and loses compression; too much taps and wears.

Measuring and setting the clearance between cam/rocker and valve to spec; many diesels need periodic mechanical adjustment.

Uses oil pressure to automatically maintain zero valve lash, eliminating periodic adjustment.

Transfers cam motion to the valve; pivots to open the valve as the lobe lifts it.

A valve that no longer seals (often from too-little lash or carbon); causes low compression in one cylinder.

The brief period when both intake and exhaust valves are open near TDC during the exhaust-intake transition.

Pistons can strike open valves if timing is lost - a snapped timing belt/chain bends valves. Keep timing correct.

Inspect for cracks with magnetic-particle, dye-penetrant, or pressure testing - common between valve seats on a diesel head.

Side-to-side guide wear that lets oil into the chamber, causing excessive oil consumption.

Ensures the valve fully closes and seals when hot, preventing burned valves and lost compression.

An overhead-valve design where the cam in the block actuates valves through lifters, pushrods, and rockers.

The camshaft sits in the cylinder head and acts more directly on the valves, often via rockers or buckets.

At high RPM the valve fails to follow the cam because of weak springs - it causes misfire and can bend valves.

Machining the head's deck flat to restore sealing; excessive removal can change compression and valvetrain geometry.

Secures the injector in the head; a leaking injector sleeve/cup can let coolant or combustion gases migrate.

Glow plugs thread into the cylinder head, reaching into each combustion chamber to preheat the air.

Usually heavy cast iron for strength, durability, and heat dissipation under high diesel cylinder pressures; some modern blocks use aluminum.

A replaceable bore insert so worn cylinders can be re-serviced. Wet liners contact coolant directly; dry liners press into the block.

A wet liner is sealed against the coolant jacket and contacts coolant directly; a dry liner is pressed into a bore and does not.

How far the liner stands above the deck; must be within spec for correct head-gasket sealing.

Machining to enlarge/restore the cylinder bore to a specified size, often to fit oversize pistons or a new liner.

Machining the bore to a smooth crosshatch finish that helps the rings seat and retains an oil film.

The angled honing marks in the cylinder wall that hold oil and help rings seat during break-in.

Machining the block's top (deck) surface flat and smooth so the head gasket seals correctly.

Machining the main bearing bores back into true alignment so the crankshaft turns freely and bearings wear evenly.

Converts the pistons' reciprocating motion into rotation; rides in the main bearings.

Supports the crankshaft in the block; clearance is checked with Plastigage or a micrometer.

Connects a connecting rod to the crank journal; excess clearance causes a bottom-end knock and low oil pressure.

A calibrated plastic strip crushed between bearing and journal to measure oil clearance; don't rotate the crank while it's installed.

Takes enormous combustion load; often has a bowl in the crown that shapes the combustion chamber for direct injection.

Seal compression and control oil; worn rings cause blue smoke and low compression.

The gap between ring ends in the bore; too little lets the ends butt and scuff the cylinder when hot.

Difference in bore diameter top to bottom from wear; excessive taper calls for reboring or a new liner.

A bore worn unevenly so it is no longer a perfect circle; like taper, a reason to rebore or re-liner.

Stores rotational energy and connects the engine to the transmission/clutch to transfer power.

Allows for casting/coolant passages and some block expansion; a failed plug leaks coolant.

Measures cylinder-wall thickness and detects internal cracks/voids in an engine block.

A crack-detection method (Magnaflux) for ferrous parts like a cast-iron block or crankshaft.

A crack-detection method using penetrating dye and developer, usable on non-ferrous parts like aluminum.

Links the piston to the crankshaft journal; checked for straightness, bend, and twist during a rebuild.

The polished bearing surface on the crank; worn or out-of-round journals are reground to an undersize.

Dampens crankshaft torsional vibration; a failing damper causes vibration and can throw off timing marks.

A dial bore gauge measures bore diameter, taper, and out-of-round precisely during machining.

Secures the crankshaft mains; some diesels use a bedplate or girdle for extra bottom-end rigidity.

Circulates pressurized oil through the bearings, lifters, and galleries; wear or a clogged pickup causes low oil pressure.

Causes: low oil level, too-thin or fuel-diluted oil, a worn pump, a clogged pickup, or worn bearings.

Limits maximum oil pressure; stuck open lowers pressure, stuck closed can spike it.

Removes contaminants and soot from the engine oil to keep it clean.

Monitors oil pressure and triggers a warning light or gauge reading if pressure drops too low.

Carries extra detergent/dispersant additives to hold soot in suspension; use the specified diesel-rated oil, not a generic gasoline grade.

Resistance to flow; a multigrade like 15W-40 flows when cold (the W) and protects when hot. Use the specified grade.

Fuel leaking into the oil thins it and lowers pressure; a known cause of low oil pressure and accelerated wear on diesels.

A heat exchanger that controls oil temperature; a clogged cooler raises oil temp and can reduce film strength.

Stores the engine's oil supply and holds the pickup; the sump for the lubrication system.

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 too cool - poor economy, weak heat, and poor emissions/aftertreatment performance.

Belt- or gear-driven pump that circulates coolant through the engine and radiator; leaks at the weep hole when failing.

Sheds engine heat to the air; airflow and the fan cool the coolant before it returns to the engine.

Seals the system and raises the coolant boiling point; also lets coolant return from the overflow tank.

Usually an ethylene glycol-based mixture for heat transfer and corrosion protection; some diesels need additives to prevent liner cavitation.

Cools recirculated exhaust gas before the intake; a leaking EGR cooler can put coolant into the intake/exhaust (white smoke, coolant loss).

Vibration-induced pitting of wet liners; the correct coolant/additive (SCA) prevents it - a diesel-specific cooling concern.

An additive that protects wet cylinder liners from cavitation erosion; checked and maintained per the maintenance schedule.

Some diesels add a coolant filter that also doses SCA to protect the cooling system and liners.

Pulls air through the radiator at low speed; a viscous or electric clutch engages the fan when coolant temp rises.

Tools to test coolant freeze point and concentration for proper protection.

Apply pressure to find external leaks and check the cap; combustion-gas testing finds a head-gasket leak into the coolant.

Lab testing of used oil detects wear metals, fuel/coolant dilution, and soot loading to catch problems early.

Full-flow filters all oil to the bearings; a bypass filter cleans a portion very finely - some diesels use both.

An exhaust-driven compressor that pressurizes (boosts) intake air so more air enters the cylinders for more power.

Exhaust gas spins the turbine, which drives the compressor wheel to force pressurized air into the engine.

A turbo with movable vanes the ECM adjusts to control boost across the RPM range, improving response and cutting lag.

Intake-air pressure produced by the turbo above atmospheric; a charge-air leak hisses under load and reduces power.

A valve that bleeds exhaust around the turbine to limit and control maximum boost pressure.

Cools the compressed, heated intake air so it is denser - more oxygen per cylinder for more complete combustion.

Draws in and filters outside air; a restricted air filter richens the mixture and can cause black smoke and low power.

Distributes the cool, dense, boosted air to the cylinders for combustion.

Warms incoming air to ease cold starting on some diesels, complementing or instead of glow plugs.

Reports intake/boost pressure to the ECM for fueling and boost control.

Measures intake air mass so the ECM can control fueling and EGR accurately.

Collects exhaust from the cylinders and directs it to the turbocharger turbine.

Reduces exhaust noise in the system; not an emissions device.

Routes metered, often cooled exhaust gas back into the intake to lower combustion temperature and reduce NOx.

Controls how much exhaust is recirculated; stuck open causes rough idle/stalling, stuck closed raises NOx and can cause knock.

Oxidizes carbon monoxide and unburned hydrocarbons into CO2 and water, and helps generate heat to regenerate the DPF.

Traps soot (particulate matter) from the exhaust and periodically regenerates by burning it off at high temperature.

Burning accumulated soot out of the DPF; passive, active (system adds heat), or a forced/service regen done with a scan tool.

Passive regen happens at normal high-load exhaust temps; active regen adds fuel/heat to raise temperature and burn soot.

A technician-initiated regeneration using a scan tool to clean a loaded DPF that hasn't regenerated on its own.

Frequent short, cool trips can prevent DPF regeneration and clog the filter, eventually needing a forced regen.

Uses ammonia (from injected DEF) to convert oxides of nitrogen (NOx) into harmless nitrogen and water.

A 32.5% urea and water solution sprayed ahead of the SCR, where it breaks down into the ammonia used to reduce NOx.

Exhaust flow: DOC (CO/HC) -> DPF (soot, regenerates) -> SCR + DEF (NOx).

Oxides of nitrogen, formed at high combustion temperatures; reduced by EGR (cooling) and SCR (DEF/ammonia).

Carbon particles from incomplete combustion; trapped by the DPF and the key reason for diesel particulate control.

Aftertreatment catalysts (DOC/SCR) that reduce harmful exhaust emissions; diesels use a multi-stage train, not a single 3-way cat.

A crack or loose clamp in the intercooler plumbing; hisses under load, drops boost, and reduces power.

Leaking turbo oil seals let oil into the intake or exhaust, causing blue smoke.

Some diesels use an intake throttle or swirl valves to aid EGR flow, regeneration, and emissions control.

A restricted exhaust or plugged DPF raises back-pressure, choking the engine and cutting power at higher RPM.

Routes blow-by gases from the crankcase, often through a separator, back to the intake or atmosphere per the design.

Monitors exhaust temperature to protect the turbo and to manage DPF regeneration.

Measures the pressure difference across the DPF to estimate soot load and trigger regeneration.

The delay before the turbo spools up and makes boost; a VGT reduces lag by changing vane angle at low speed.

How full of trapped particulate the DPF is; high load triggers regeneration or, if ignored, can plug the filter.

Excess unreacted ammonia passing through the SCR; controlled by precise DEF dosing and sometimes a cleanup catalyst.

DEF is 32.5% urea; off-spec or contaminated DEF triggers faults, and DEF freezes around -11 C (12 F) but thaws without harm.

Separates oil from blow-by before returning vapors to the intake, reducing oil carryover and intake fouling.

Indicates air-filter restriction; high restriction reduces airflow, richening the mixture and causing black smoke.

The fuel used in diesel engines; it self-ignites under high compression rather than from a spark.

A high-pressure injection system with a shared rail (accumulator) holding fuel for all injectors, with ECM-controlled timing/quantity.

Separates pressure generation (the pump) from injection timing (the ECM-fired injectors), enabling precise multi-event injection.

A cam-driven pump that raises fuel to extreme pressure (often 20,000-30,000+ psi) for the common rail and injectors.

A low-pressure pump that moves fuel from the tank through the filters to the high-pressure pump.

An ECM-controlled (solenoid or piezo) valve that sprays a finely atomized charge into the chamber, often several events per stroke.

Atomizes and delivers fuel into the combustion chamber; nozzle condition affects spray pattern and combustion quality.

A small early injection that begins combustion gently before the main injection, softening the pressure rise to cut knock and noise.

Modern injectors can fire pilot, main, and post injections per stroke for power, economy, low noise, and emissions/regeneration.

Fuel is injected directly into the combustion chamber; standard on modern diesels for precise control and efficiency.

Older diesels mechanically injected fuel (inline/distributor pumps) instead of electronic common rail.

An indirect-injection design where fuel is injected and starts to ignite in a small chamber before entering the main chamber.

Measures common-rail pressure and reports it to the ECM for closed-loop pressure control.

Maintains the commanded rail pressure by controlling pump output or bleeding excess fuel back.

Limits maximum fuel pressure to protect the system from over-pressure.

Controls the quantity of fuel the high-pressure pump delivers to the rail.

Removes particulate from the fuel; injection runs at fine tolerances, so even small dirt causes rapid wear or failure.

Removes water from diesel fuel; water corrodes and destroys high-pressure pumps and injectors and must be drained on schedule.

Many diesels use a primary (coarse) filter near the tank and a secondary (fine) filter before the injection system.

Limits fuel delivery, causing low power - often with little or no smoke because the mixture stays lean.

Returns excess fuel and injector leak-off to the tank, helping cool the system and maintain pressure.

Heats the fuel to prevent waxing or gelling in cold temperatures, keeping fuel flowing.

Diesel paraffin solidifying in cold weather, restricting flow; prevented by fuel heaters, anti-gel additives, or winter-blend fuel.

Measures fuel returning from each injector; uneven return points to a worn or leaking injector.

When fuel is injected relative to TDC; too advanced/retarded affects power, emissions, and diesel knock.

Low-cetane fuel ignites less readily, worsening cold-start trouble, white smoke, and knock.

The engine control module that commands injection timing, quantity, rail pressure, and emissions functions.

On a crank-no-start, verifying rail pressure during cranking is the most direct first step before replacing parts.

Solenoid injectors use an electromagnet to open; piezo injectors use a fast piezo stack for quicker, more precise actuation.

A glow plug heats the chamber for cold starts; the injector delivers fuel. Both matter to cold-start, but they are different components.

Air ingestion (loose connection, bad seal, empty filter) causes hard starting, rough running, and stalling; the system must be bled/primed.

Stores the diesel fuel supply; the lift pump draws from it and the return line sends excess fuel back.

The shaped recess in a direct-injection diesel piston that mixes the injected fuel spray with the swirling compressed air.

ECM-learned correction values for each injector's flow; injector calibration codes (IQA/trim) must be programmed when replacing an injector.

New injectors carry calibration codes that must be entered into the ECM so fueling stays accurate cylinder to cylinder.

A late injection used mainly to raise exhaust temperature for active DPF regeneration.

The primary fuel event that produces engine power, following any pilot injection.

Excess high-pressure fuel and injector leak-off spill back through the return circuit to the tank.

Older mechanical pumps (inline plunger-per-cylinder or rotary distributor) replaced by common rail on modern light-duty diesels.

A fuel additive that raises ignition quality to reduce knock and improve cold starting.

Low-sulfur fuel required for modern diesels so DPF/SCR aftertreatment works and isn't poisoned.

Diesel blended with fatty-acid esters; higher blends can affect filters, seals, and cold-flow, so follow the manufacturer's limit.

Removing air after filter service or a run-dry, using a primer pump or key cycles so the engine will start and run smoothly.