ASE L1 flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Volumetric efficiency

The ratio of air actually drawn into a cylinder to that cylinder's theoretical displacement volume. Naturally aspirated engines typically peak near 80 to 90 percent; forced induction pushes the number above 100 percent.

Cylinder leakage test

Shop air is applied to a cylinder held at top dead center on compression and the percentage escaping is measured. Leakage under about 10 percent is acceptable; the escaping air's location identifies the failed seal.

Power balance test

Each cylinder is disabled in turn while rpm drop is recorded. A cylinder that produces little or no rpm drop is contributing less power and is the weak or dead cylinder.

Compression ratio formula

Compression ratio equals swept volume plus clearance volume, divided by clearance volume. A 10:1 ratio means the charge is squeezed into one-tenth of the volume it occupied at bottom dead center.

Intake manifold vacuum reading

At sea level a healthy engine idles at roughly 17 to 22 inches of mercury with a steady needle. A snap throttle should spike near 25 inches and then settle back to the idle value.

OBD II DTC structure

Five characters: P for powertrain, then a digit where 0 is SAE-defined and 1 is manufacturer-defined, then a subsystem digit, then two fault digits identifying the specific circuit or failure.

Readiness monitors

Self-tests the PCM must complete before an inspection. Misfire, fuel system, and comprehensive component monitors run continuously; non-continuous monitors require specific temperature, speed, time, and fuel level conditions.

Short term fuel trim

An instantaneous PCM correction expressed as a percentage. Positive values mean the PCM is adding fuel to correct a lean condition; values beyond about plus or minus 10 percent indicate a fault.

Long term fuel trim

A learned correction stored in keep-alive memory and carried across drive cycles. It shifts the midpoint around which short term trim works and resets whenever codes are cleared or the battery is disconnected.

Freeze frame data

A snapshot of sensor values and operating conditions captured the moment a fault first matures. It is used to duplicate the failure and is erased whenever diagnostic codes are cleared.

Type A versus Type B misfire

Type A misfire can damage the catalyst, sets a code after about 200 crankshaft revolutions, and flashes the warning lamp. Type B is less severe and sets after about 1,000 revolutions.

Spark firing voltage

The kilovolts required to jump the plug gap. It rises with wider gaps, higher compression, leaner mixtures, and higher secondary resistance; an unusually low reading suggests a shorted or fouled plug.

Spark burn time

How long the spark sustains its arc, typically 1.5 to 2.0 milliseconds. Short burn times point to a lean mixture or high resistance, while long burn times suggest a rich mixture.

Waste spark ignition

One coil fires two plugs in series, one on its compression stroke and its mate on exhaust. The pair operates with opposite polarity, so both plugs must be serviced together.

Ignition coil dwell

The time the primary circuit is grounded so the coil can build magnetic flux. Too little dwell weakens the spark; excessive dwell saturates the coil and causes overheating or driver failure.

Misfire detection method

The PCM monitors crankshaft speed fluctuations from the crankshaft position sensor. Each firing cylinder should accelerate the crank equally, so a weak cylinder produces a measurable slowdown that is counted as a misfire.

Stoichiometric air-fuel ratio

For gasoline the chemically correct ratio is about 14.7 pounds of air to one pound of fuel. E10 blends run nearer 14.1 to 1 because ethanol carries oxygen in its molecule.

Injector balance test

Each injector is pulsed for an identical time and the resulting drop in fuel pressure is recorded. An injector producing a smaller drop flows less fuel and must be cleaned or replaced.

Speed density fuel control

A system that calculates airflow from manifold absolute pressure, intake air temperature, and engine rpm instead of measuring it directly. It has no mass airflow sensor to become contaminated.

Mass airflow sensor output

Reported in grams per second. At idle a healthy engine typically draws roughly its displacement in liters per second, and the value climbs sharply during acceleration and under load.

Vacuum leak symptom pattern

Unmetered air leans the mixture most at idle, producing positive fuel trim that improves as rpm rises, along with a rough or rolling idle and possibly a lean bank code.

Gasoline direct injection pressure

Direct injection holds fuel at roughly 500 to 2,000 psi or higher at the rail, generated by a cam-driven high-pressure pump, compared with 35 to 60 psi in a port injection system.

Catalyst efficiency test

The downstream oxygen sensor should remain relatively flat while the upstream sensor switches rapidly. If the rear sensor mirrors the front sensor's activity, the converter has lost oxygen storage capacity.

EGR valve purpose

Recirculated exhaust dilutes the incoming charge and lowers peak combustion temperature, which suppresses oxides of nitrogen. EGR normally does not flow at idle or at wide-open throttle.

Positive crankcase ventilation

Crankcase blowby vapors are routed into the intake to be burned rather than vented to atmosphere. A working system shows slight vacuum at the dipstick tube or oil filler cap.

EVAP purge operation

Stored fuel vapor is drawn from the canister into the intake during closed loop cruise. The PCM commands purge solenoid duty cycle and watches fuel trim to confirm that flow actually occurred.

Five gas analysis

Exhaust is measured for hydrocarbons, carbon monoxide, carbon dioxide, oxygen, and oxides of nitrogen. High carbon dioxide with low oxygen indicates complete combustion; the other gases reveal the specific fault.

Readiness monitor incomplete

An inspection can be rejected or failed when too many monitors have not run. Most programs allow one incomplete non-continuous monitor, and a battery disconnect erases all learned readiness status.

MIL commanded during inspection

Under OBD II inspection a commanded check engine light is an automatic failure regardless of tailpipe readings, because the PCM is holding a confirmed fault for an emissions-related system.

Two speed idle test

Tailpipe emissions are sampled at curb idle and again at about 2,500 rpm. High hydrocarbons that clear at the higher speed usually indicate a low-airflow misfire or rich condition.

High NOx on loaded test

Oxides of nitrogen rise with combustion temperature. A loaded-mode failure typically traces to a lean mixture, over-advanced spark timing, an inoperative EGR system, or an engine running hotter than designed.

Drive cycle to set monitors

A generic cycle warms the engine to operating temperature, cruises at steady speed, then includes acceleration, deceleration, and idle periods so the non-continuous monitors can complete their enable criteria.