ASE Automobile (A1–A8) — All Questions

A flooded engine that cranks but will not start after repeated attempts can often be cleared by:

• a.cranking with the throttle held fully closed while cycling the key on and off
• b.pumping the throttle pedal rapidly several times just before cranking
• c.spraying a little water into the throttle body to wash the excess fuel out
• d.holding the throttle wide open while cranking to enter clear-flood mode✓

Most PCMs enter a clear-flood mode that cuts injector pulse width when the throttle is held wide open during cranking, letting excess fuel clear. Pumping a fuel-injected throttle does not add fuel and can worsen flooding. Once it starts, the mixture returns to normal control.

A technician suspects no spark on a coil-on-plug engine. The best tool to confirm the presence of spark is:

• a.a fuel pressure gauge
• b.a vacuum gauge
• c.a cooling-system pressure tester on the radiator neck
• d.a spark tester connected in place of the plug✓

A spark tester provides a calibrated gap to verify that the coil produces a strong spark under compression-like load. Simply pulling a plug wire can give a misleading result and risks a shock. Once spark is confirmed or ruled out, the technician checks the coil, driver, and triggering sensors.

A knock (detonation) sensor is best described as a device that:

• a.counts the crankshaft revolutions used to drive the dashboard tachometer
• b.detects engine vibration frequencies caused by abnormal combustion✓
• c.reports intake manifold vacuum and engine load conditions to the PCM
• d.measures the temperature of the exhaust manifold near the exhaust port

The knock sensor is a piezoelectric device tuned to the vibration frequency of detonation; when it senses knock, the PCM retards timing to protect the engine. A faulty sensor or wiring can cause reduced power from unnecessary timing retard or unheard knock. Torque specification matters when installing it.

Black smoke from the tailpipe of a gasoline engine most directly indicates:

• a.coolant leaking past a head gasket into the cylinder
• b.an excessively rich air-fuel mixture✓
• c.oil being drawn past the rings
• d.normal water-vapor condensation on a cold morning start

Black smoke is unburned fuel from a rich mixture, caused by faults such as leaking injectors, high fuel pressure, or a false-cold coolant sensor. White smoke suggests coolant, and blue suggests oil. A gas analyzer confirms high CO and HC with a rich condition.

A vacuum leak at the intake manifold typically causes an idle that is:

• a.unaffected because idle is entirely computer-controlled
• b.high or rough because of the extra unmetered air✓
• c.low and smooth because the mixture becomes noticeably richer
• d.normal until the engine reaches full operating temperature

Unmetered air from a vacuum leak leans the idle mixture and can raise or destabilize idle speed, and fuel trims go positive to compensate. Smoke testing or spraying a small amount of throttle-body cleaner near suspect areas helps locate it. Idle strategy tries to compensate but often cannot fully mask the leak.

Low fuel pressure caused by a weak pump or clogged filter most commonly produces which driveability complaint?

• a.a high, surging idle speed
• b.knock and pinging only on cold starts
• c.excessive blue exhaust smoke that appears on deceleration
• d.hesitation, lack of power, and lean misfire under load✓

When demand is high, low fuel pressure cannot supply enough fuel, so the engine hesitates, lacks power, and may lean-misfire under load. A fuel pressure gauge and volume test confirm supply. A restricted filter or failing pump are common causes.

The crankshaft position sensor and camshaft position sensor together allow the PCM to detect:

• a.the temperature of the intake air charge entering the engine
• b.the amount of fuel remaining in the vehicle's fuel tank
• c.the oxygen-storage efficiency of the catalytic converter
• d.timing-chain wear through a cam-to-crank correlation error✓

By comparing crankshaft and camshaft signals, the PCM verifies valve timing; a stretched timing chain shifts the relationship and sets a correlation code. This can cause rough running and reduced power. Verifying mechanical timing may be required after a correlation DTC.

An engine that starts and runs briefly, then stalls, and will restart only after sitting, may be exhibiting a classic symptom of:

• a.a single fouled spark plug that is causing one steady misfire
• b.a slightly loose gas cap that has set a small EVAP system code
• c.a failing crankshaft position sensor that drops out when hot✓
• d.a worn serpentine drive belt that is slipping under electrical load

A crank sensor that fails when it heats up can cut the signal, stalling the engine until it cools enough to work again. Testing while heating the sensor or watching rpm data during the stall helps confirm it. Heat-related intermittent sensor faults are a common driveability puzzle.

Technician A says a mass airflow sensor that under-reports airflow will make the mixture lean. Technician B says cleaning a contaminated MAF sensor with an approved cleaner can restore accurate readings. Who is correct?

• a.Both A and B✓
• b.Neither A nor B
• c.Technician B only
• d.Technician A only

A MAF that reads low tells the PCM less air is entering than truly is, so it under-fuels and leans the mixture, and dirt on the sensing element is a frequent cause that MAF-specific cleaner can correct. Both technicians are correct. Never touch or scrub the delicate sensing wire.

A gasoline engine that fails an emissions test for high hydrocarbons (HC) is most likely experiencing:

• a.a misfire leaving unburned fuel in the exhaust✓
• b.high combustion temperatures that form oxides of nitrogen
• c.a lean mixture that burns cleanly
• d.completely normal operation

Hydrocarbons are unburned fuel, so high HC points to a misfire from ignition, fuel, or compression faults, or a failing catalyst. High CO indicates a rich mixture, while high NOx indicates high temperature. Finding and fixing the misfire lowers HC.

The purpose of the secondary air injection system on some vehicles is to:

• a.recirculate exhaust gas back into the intake to help lower NOx
• b.add extra fuel into the exhaust to raise catalyst temperature at idle
• c.add air to the exhaust so the catalyst warms and oxidizes emissions faster✓
• d.cool the exhaust manifold and downpipe during periods of heavy load

Secondary air injection pumps fresh air into the exhaust during cold start so extra oxygen helps oxidize HC and CO and warms the catalyst quickly. A stuck valve or failed pump can set related codes. It operates only briefly after start-up.

A cylinder with very low compression will most likely cause:

• a.an abnormally high charging-system voltage from the alternator
• b.a false lean trouble code set on the opposite cylinder bank
• c.a misfire and rough running on that cylinder✓
• d.a rich mixture on all cylinders

Low compression means that cylinder cannot make normal power, producing a misfire and rough idle. Causes include burned valves, worn rings, or a blown head gasket, confirmed with a compression or leak-down test. The misfire may set a cylinder-specific code.

Ethanol content in gasoline matters to the fuel system because ethanol:

• a.removes any need for oxygen-sensor feedback in the fuel-control system
• b.cannot be detected or compensated for in any way by the PCM
• c.changes the stoichiometric ratio, so flex-fuel systems adjust fueling✓
• d.raises the fuel pressure inside a sealed tank entirely on its own

Ethanol requires a richer air-fuel ratio than straight gasoline, so flex-fuel vehicles use a fuel composition sensor or fuel-trim learning to add fuel as ethanol content rises. Incorrect fuel can cause lean or rich operation. Fuel-system materials are chosen to resist ethanol.

A P0442 (small EVAP leak) code is frequently caused by:

• a.a worn crankshaft position sensor dropping its signal
• b.a loose, damaged, or missing fuel cap✓
• c.low fuel pressure at the injector rail
• d.a failed catalytic converter

A small EVAP leak commonly traces to a fuel cap that is loose, cracked, or has a bad seal, letting vapor escape. The system checks itself by pressurizing or applying vacuum and watching for decay. Tightening or replacing the cap often clears it after a drive cycle.

An injector balance test is used to:

• a.check the oxygen-storage efficiency of the catalytic converter
• b.compare fuel delivery among injectors by measuring pressure drop✓
• c.measure the electrical resistance of the ignition coil primary winding
• d.verify the internal calibration of the mass airflow sensor element

Pulsing each injector for the same time and comparing the fuel-rail pressure drop, or the rpm change from cylinder power balance, reveals a weak or leaking injector. A cylinder that differs sharply signals a fueling problem. This isolates a mechanical injector fault from other misfire causes.

Spark plug gap is important because a gap that is too wide will:

• a.raise the effective compression pressure inside that cylinder
• b.fire the mixture far too easily and rapidly foul the plug with raw fuel
• c.require more voltage to fire and can cause misfires under load✓
• d.lower peak combustion temperature and reduce the formation of NOx

A wider gap needs higher secondary voltage to jump; if the coil cannot supply it under load, the plug misfires. Too small a gap gives a weak spark that may not reliably ignite the mixture. Setting the specified gap ensures consistent ignition.

Technician A says an engine that runs well cold but stumbles once warm may have a coolant temperature sensor or fuel-delivery fault. Technician B says scan-tool live data helps confirm which sensor reading is out of range. Who is correct?

• a.Both A and B✓
• b.Technician A only
• c.Neither A nor B
• d.Technician B only

A warm stumble can come from a sensor whose reading shifts with temperature or from fuel delivery that falls off when hot, and comparing live data to known-good values pinpoints the offending signal. Both technicians are correct. Freeze-frame and fuel-trim data guide the diagnosis.

The oxygen sensor heater circuit is important because it:

• a.brings the sensor to operating temperature so closed loop begins sooner✓
• b.measures the exhaust backpressure present at the sensor
• c.produces the sensor's output voltage signal all by itself
• d.cools the sensor to protect it from exhaust heat damage

A built-in heater lets the oxygen sensor reach its roughly 600-degree operating temperature soon after start-up, so the engine enters closed loop and reduces emissions faster. A failed heater sets a heater-circuit code and delays closed loop. The heater is a separate circuit from the sensing element.

A vehicle sets a P0128 (coolant temperature below thermostat regulating temperature) code most often because:

• a.the radiator pressure cap is holding far too much system pressure
• b.the water pump is circulating the coolant far too quickly to warm it
• c.the electric cooling fan never turns on at any coolant temperature
• d.the thermostat is stuck open and the engine never fully warms✓

P0128 means the engine did not reach expected temperature in the expected time, and a thermostat stuck open is the usual cause, letting coolant flow before warm-up is complete. This wastes fuel and delays full closed-loop efficiency. Verifying warm-up with a scan tool confirms the fault.

Mode 6 data on an OBD-II scan tool is useful because it shows:

• a.a live oscilloscope trace of the current through each ignition coil
• b.the vehicle's complete repair and maintenance service history
• c.the exact street location where the diagnostic code first set
• d.the results and limits of the PCM's non-continuous self-tests✓

Mode 6 reports the pass/fail test values and thresholds behind the readiness monitors, such as catalyst and EVAP tests, so a technician can spot a component trending toward failure before a code sets. Reading it alongside freeze frame aids diagnosis. Test IDs must be matched to the vehicle's definitions.

A dirty or sticking throttle body can cause a driveability complaint of:

• a.a steady high-speed vibration that is felt through the driver's seat
• b.an excessively high charging voltage seen at highway cruising speed
• c.unstable idle, stalling, or hesitation from restricted airflow✓
• d.engine knock that appears only under very heavy acceleration

Carbon buildup on the throttle bore and plate disturbs the small airflow that controls idle, causing rough idle, stalling, or tip-in hesitation. Cleaning the throttle body and, on some vehicles, performing an idle relearn restores normal operation. Electronic throttles may need a relearn after cleaning.

When diagnosing a misfire, performing a cylinder power balance test helps by:

• a.identifying which cylinder contributes least to engine output✓
• b.testing the state of charge and condition of the vehicle's battery
• c.measuring the precise octane rating of the fuel in the tank
• d.checking the front-wheel alignment angles such as camber and toe

A power balance test disables cylinders one at a time (or uses scan-tool misfire counts) to find the weak cylinder by its small effect on rpm. The cylinder that changes engine speed the least is the suspect. Follow-up compression, injector, and ignition tests find the root cause.

A restricted or plugged fuel injector on one cylinder, compared with a leaking injector, is more likely to cause:

• a.an overcharging condition in the vehicle's charging system
• b.black exhaust smoke coming from all of the cylinders under load
• c.a lean misfire and hesitation from that one cylinder✓
• d.a rich condition and hard starting that affects the whole engine

A restricted injector delivers too little fuel to its cylinder, leaning it out and causing a misfire or hesitation, while a leaking injector adds fuel and riches the mixture. Comparing symptoms helps direct the diagnosis. An injector balance or flow test confirms which injector is at fault.