ASE L1 cheat sheet

A four-stroke cycle spans 720 degrees of crankshaft rotation; the camshaft turns at exactly half crankshaft speed, so a 2:1 crank-to-cam ratio must be confirmed before any timing diagnosis.

Compression ratio = (swept volume + clearance volume) / clearance volume; a 10:1 engine squeezes the charge to one-tenth of its original volume.

Volumetric efficiency = actual air ingested / theoretical displacement volume; a naturally aspirated engine peaks near 80–90 percent, while boost pushes it above 100 percent.

Healthy idle vacuum at sea level is roughly 17–22 in-Hg with a steady needle; a snap-throttle test should spike toward 25 in-Hg and settle back to the idle value.

Wide-open-throttle vacuum near 0–3 in-Hg confirms the intake path is unrestricted; a reading that stays high indicates a plugged exhaust or a throttle that is not opening fully.

Cylinder leakage below about 10 percent is acceptable; hissing at the throttle body points to an intake valve, at the tailpipe to an exhaust valve, at the oil filler to rings, and bubbles in the radiator to a head gasket.

Power balance rule: disable one cylinder at a time and watch rpm drop; the cylinder with the smallest drop is the weak one, and equal drops across all cylinders indicates a system-wide problem.

Fuel trim sign convention: positive trim means the PCM is adding fuel to correct a lean condition; negative trim means it is removing fuel to correct a rich condition; total trim beyond about 25 percent sets a code.

Valve lash out of adjustment shows as low and erratic vacuum plus reduced compression; verify with a compression and leakage test before removing the cylinder head.

OBD II DTC format is five characters: P for powertrain, then a digit where 0 means SAE-defined and 1 means manufacturer-defined, then a subsystem digit (1/2 fuel and air metering, 3 ignition or misfire, 4 auxiliary emissions, 7 transmission), then two fault digits.

Continuous monitors are misfire, fuel system, and comprehensive components; all other monitors are non-continuous and only run after their enable criteria (temperature, time, speed, fuel level) are met.

Freeze frame is captured the instant a fault first sets and is erased together with the codes, long-term fuel trim, and readiness status whenever codes are cleared.

Short-term fuel trim is instantaneous and normally fluctuates within about plus or minus 10 percent; long-term fuel trim is the learned average, and a total correction beyond about 25 percent sets P0171 or P0172.

Sensors share a regulated 5-volt reference; a single internally shorted sensor can drag the entire reference low, so a reference fault usually produces codes for several unrelated sensors at once.

Oxygen sensor heater codes (P0135 family) set when heater current is out of range; ohms-check the heater element cold and compare to the other bank before condemning the sensor.

Misfire classification: Type A can damage the catalyst, sets a code after about 200 crankshaft revolutions and flashes the MIL; Type B sets after about 1,000 revolutions and illuminates the MIL under normal two-trip logic.

Actuator testing with a scan tool confirms PCM output and wiring integrity; a commanded actuator that does not respond but ohms correctly points to a driver circuit or PCM fault, not the component.

Required firing voltage rises with wider plug gaps, higher cylinder pressure, leaner mixtures, and increased secondary resistance; a lower-than-normal firing kV suggests a shorted plug, carbon-tracked cap, or grounded secondary.

Spark burn time is typically 1.5–2.0 milliseconds; shorter burn times accompany lean mixtures or high resistance, longer burn times accompany rich mixtures and fouled plugs.

Typical coil resistance is 0.4–2.0 ohms on the primary and 6,000–30,000 ohms on the secondary; always compare against the manufacturer specification because coil-on-plug values differ widely.

Waste-spark systems fire two plugs in series, one on compression and its mate on exhaust, so one plug in each pair wears with reverse polarity; replace both plugs in a pair together.

Dwell is the time the primary circuit is grounded to build magnetic flux; too little dwell weakens the spark, while excessive dwell saturates and overheats the coil.

Base ignition timing is checked with the timing-set connector or SPOUT circuit disconnected at a specified rpm; a typical specification is 10 degrees BTDC, with the PCM controlling advance once reconnected.

Misfire detection compares crankshaft velocity variation from the CKP sensor firing event to event; the CMP sensor identifies which cylinder is at fault, so a missing CMP signal produces a random-cylinder misfire code.

Stoichiometric ratio is about 14.7:1 by mass for gasoline and about 14.1:1 for E10; alcohol content shifts the ratio and therefore shifts normal fuel trim readings.

Port fuel injection typically runs 35–60 psi at the rail; pressure must be checked with the key on engine off, at idle with the vacuum line connected and disconnected, and again after shutoff for leak-down.

Injector coil resistance is commonly 12–16 ohms on saturated drivers and 2–3 ohms on peak-and-hold drivers; an injector balance test compares the pressure drop created by an equal pulse on each injector.

Idle injector pulse width is normally about 2–4 milliseconds; unusually long pulse widths at idle indicate a lean condition or a false sensor input, not necessarily a weak pump.

Speed-density systems compute airflow from MAP, intake air temperature, and rpm; mass-air systems measure airflow directly in grams per second, so a contaminated MAF element causes lean or rich errors without any pressure symptom.

A vacuum leak leans the mixture most at idle and the effect diminishes as rpm and airflow rise, producing positive fuel trim that improves with engine speed plus a rough or rolling idle.

Air filter restriction is measured as pressure drop in inches of water; a reading above the manufacturer's limit under load indicates a clogged element.

Gasoline direct injection holds roughly 500 to 2,000 psi or more at the rail from a cam-driven high-pressure pump; a weak low-pressure supply causes high-pressure codes and lean misfire under load.

Electronic throttle bodies require a relearn after cleaning or replacement; a carboned throttle plate produces a low, unstable idle and stored throttle performance codes.

Gas interpretation rule: high CO means rich, high HC means misfire or unburned fuel, high NOx means excessive combustion temperature, high O2 means lean or misfire, and low CO2 means incomplete combustion.

Catalyst monitor logic compares upstream and downstream oxygen sensor activity; if the rear sensor switches nearly as much as the front sensor, oxygen storage is lost and P0420 or P0430 sets.

EGR dilutes the incoming charge and lowers peak combustion temperature to suppress NOx; EGR does not flow at idle or wide-open throttle and flows during light-load cruise.

EVAP leak detection targets leaks as small as 0.020 inch; a smoke or nitrogen test with the canister vent commanded closed locates leaks that a pressure test alone cannot pinpoint.

The purge solenoid is normally commanded only in closed loop; a stuck-open purge valve causes a rich idle and rough running, while a stuck-closed valve causes fuel odor and an EVAP code.

A functioning PCV system shows slight vacuum at the dipstick tube or oil filler cap; a plugged PCV valve pressurizes the crankcase and causes oil leaks and sludge.

Secondary air injection adds air to the exhaust manifold on cold start to oxidize HC and CO, then routes air to the air cleaner once the system enters closed loop to protect the catalyst.

Misfire and rich running destroy catalysts by overheating them; always repair the root cause before replacing a converter, or the new unit will fail the same way.

On OBD II inspections a commanded malfunction indicator lamp is an automatic failure regardless of tailpipe readings, because a confirmed fault is stored in the PCM.

Readiness rule: most programs allow one non-continuous monitor to remain incomplete, and the EVAP monitor is often exempt; a battery disconnect or code clear resets all monitors to not ready.

Two-speed idle testing samples the tailpipe at curb idle and at roughly 2,500 rpm; an HC failure that clears at higher rpm points to a misfire or rich condition at low airflow.

Loaded-mode testing (ASM 25/25 or 50/15) adds load to expose NOx and CO failures that do not appear at an unloaded idle.

High NOx on a loaded test traces to excessive combustion temperature: lean mixture, over-advanced timing, inoperative EGR, or a cooling system that is not maintaining temperature.

High CO indicates a rich air-fuel ratio from a leaking injector, high fuel pressure, a biased coolant sensor, or a saturated EVAP purge.

Preconditioning before a retest means a fully warmed engine plus a complete drive cycle so monitors can set; clearing codes and driving straight to the test lane usually produces a rejected test.

A gas cap pressure or vacuum integrity failure is a common EVAP-related I/M failure; verify the seal, the filler neck, and the cap before replacing any component.