ASE A8 emissions controls practice questions.

Five free practice questions on emissions controls: content area D of the ASE A8 Engine Performance test, 8 of its 50 scored questions (16%). They are in the ASE format: four options, one best answer. Answer each one before opening the explanation. No account, no email.

01The questions

1. EXCEPT · Emissions Control Systems Diagnosis and Repair

   Any of these could restrict crankcase ventilation flow EXCEPT:

   1. a breather hose cracked along its length.
   2. condensation frozen solid inside the PCV hose.
   3. a PCV hose collapsed by engine heat and age.
   4. sludge built up inside a fixed PCV orifice.
   Show the answer

   A. Correct. A crack in the fresh-air breather hose adds an opening instead of closing one, so crankcase vapor still flows through the PCV valve to the intake. The crack can let in dirt that bypasses the air filter and, on a MAF engine, air the sensor does not measure, so the hose is replaced.

   Why not the others

   B. This could restrict flow. Blowby carries water vapor, and in freezing weather, especially on short trips, it can condense and freeze in a PCV hose, blocking the passage and letting crankcase pressure build.

   C. This could restrict flow. An aged or heat-softened PCV hose can collapse under manifold vacuum and close off the path to the intake; PCV hoses are replaced with oil-resistant hose of the right type.

   D. This could restrict flow. Engines with a fixed orifice instead of a valve meter crankcase flow through a small calibrated hole, and sludge or varnish can narrow or close it, so the orifice is cleaned or replaced at service.

2. Diagnosis · Emissions Control Systems Diagnosis and Repair

   A warm engine with an electric EGR valve idles rough and stalls when the vehicle coasts to a stop, but runs normally at cruise. Unplugging the valve (its spring closes it when it is free to move) does not change the idle, and a smoke test finds no intake leaks. Which of these could be the cause?

   1. A PCM output stuck commanding the EGR valve open
   2. Carbon holding the EGR valve pintle off its seat
   3. Carbon restricting the EGR passages to the intake
   4. An intake manifold gasket leaking at one runner
   Show the answer

   B. Correct. With carbon holding the pintle open, exhaust leaks into the intake at idle, when airflow is lowest and EGR should be off, so the engine idles rough and stalls. Unplugging the valve cannot close it because the pintle is jammed mechanically; the valve and seat are cleaned or the valve is replaced.

   Why not the others

   A. A stuck command would hold the valve open electrically, but unplugging the connector removes that command and would let the spring close the valve and smooth the idle. Because the idle did not change, the valve is held open mechanically.

   C. Restricted passages reduce EGR flow when the valve opens; they cannot add exhaust at idle, when the valve should be closed. A rough idle from the EGR system means too much exhaust is getting in, not too little.

   D. A leaking intake gasket would cause a rough idle, but the smoke test found no intake leaks, so unmetered air is ruled out. An idle that stays rough with the valve unplugged points to a valve held open mechanically.

3. Recall · Emissions Control Systems Diagnosis and Repair

   On an engine that pumps secondary air into the exhaust ports, ahead of the upstream HO2S, during cold-start warm-up, what should that sensor show while the air is flowing, once the heated sensor is active?

   1. The same voltage it reads after the air shuts off
   2. A high voltage, showing a rich exhaust
   3. Steady switching between rich and lean
   4. A low voltage, showing a lean exhaust
   Show the answer

   D. Correct. The pumped air adds oxygen to the exhaust ahead of the sensor, so even though a cold engine runs rich, the sensor sees excess oxygen and reads low. Many PCMs use that lean swing to confirm the air is flowing; a reading that stays high while the pump runs points to a flow problem.

   Why not the others

   A. If the sensor read the same with and without the pump running, no air would be reaching the exhaust ahead of it. A working system makes it read low and lean while the air flows.

   B. A high voltage means little oxygen in the exhaust. With secondary air flowing ahead of the sensor, the extra oxygen should drive it low; a high reading while the pump runs suggests the air is not getting through.

   C. Normal rich-lean switching is closed-loop fuel control, which comes after warm-up. While secondary air is flowing, the added oxygen should hold the sensor low and lean.

4. Diagnosis · Emissions Control Systems Diagnosis and Repair

   A vehicle is hard to start and runs rough for several seconds right after the tank is filled. It starts and idles normally at all other times, including after sitting hot for the same length of time without refueling. Which of these could be the cause?

   1. A leaking EVAP service port valve
   2. A fuel injector leaking when closed
   3. A canister vent valve stuck open
   4. A purge valve that is stuck open
   Show the answer

   D. Correct. Refueling loads the canister with displaced vapor. A purge valve stuck open lets intake vacuum draw that heavy vapor into the engine as soon as it starts, so the mixture goes very rich until the canister clears. The valve is replaced and the lines checked for charcoal that may have jammed it.

   Why not the others

   A. A leaking service port is a small leak that can set an EVAP leak DTC, but it lets vapor out to the air, not into the engine. Only a path from the loaded canister into the intake explains the rich start.

   B. A leaking injector wets the cylinder during any hot soak, so the engine would also be hard to start after sitting hot without refueling, which it is not. The problem follows refueling, which is when the canister is loaded with vapor.

   C. A vent valve stuck open lets outside air into the canister's fresh-air side and makes the system fail a leak test; it adds no fuel to the engine. Rough starting after refueling needs a path from the canister into the intake.

5. EXCEPT · Emissions Control Systems Diagnosis and Repair

   Any of these could set a DTC for an EVAP large leak EXCEPT:

   1. a purge hose disconnected at the canister.
   2. a vapor line cracked between the tank and canister.
   3. a filler neck rusted through above the fuel level.
   4. a canister vent valve stuck closed.
   Show the answer

   D. Correct. A vent valve stuck closed seals the canister's fresh-air side, so the system holds test vacuum or pressure just as a tight system would; it is not a leak. It blocks the fresh-air path instead, which shows up as a vent restriction fault rather than a leak.

   Why not the others

   A. This could set it. A hose off at the canister leaves an open path to the atmosphere, so the sealed system cannot hold the test vacuum or pressure.

   B. This could set it. A cracked line between the tank and canister opens the vapor side to the air, and a large enough crack lets the test vacuum or pressure escape quickly.

   C. This could set it. A filler neck rusted through above the fuel level opens the tank's vapor space to the air, so the system cannot hold the test vacuum or pressure.

02Answer key

03What area D covers

This content area covers the systems that control emissions apart from fuel and spark: positive crankcase ventilation, exhaust gas recirculation and its sensors and coolers, secondary air injection, catalytic converters and the monitor that judges them, and the evaporative emissions system with its canister, purge and vent valves, fuel tank pressure sensor and leak testing.

The ASE test specification calls this content area “Emissions Control Systems Diagnosis and Repair”. It carries 8 of the 50 scored questions, 16% of the test.

• Diagnose and service PCV valves, separators and hoses
• Diagnose EGR valves, passages, coolers and sensors
• Interpret EGR and secondary air scan data and DTCs
• Test secondary air pumps, valves and circuits
• Diagnose catalytic converter DTCs and root causes
• Test EVAP canisters, purge and vent valves
• Smoke test and leak test the EVAP system

04What the questions turn on

EGR at the wrong time

Exhaust admitted at idle makes the engine stumble, so an EGR valve held open by carbon gives a rough idle and stalls. Blocked EGR passages do the opposite: no recirculation under load, a hotter burn and pinging on light acceleration.

How the converter is judged

A working converter stores oxygen, so the downstream sensor signal stays fairly steady. When it starts switching almost as often as the upstream sensor, the PCM reads lost efficiency. An exhaust leak ahead of the downstream sensor can produce the same signal.

Seal the system before smoke testing

For an EVAP smoke test, the normally open vent valve is commanded closed and the purge valve stays closed, so the system can hold test pressure. Smoke streaming from the vent filter with the vent valve commanded shut means that valve is not sealing.

PCV flow and oil

A PCV valve stuck closed traps blowby, so crankcase pressure pushes oily vapor back into the air filter housing and out past the gaskets. A valve that flows more than specified pulls extra oil mist into the intake to be burned.

05The other content areas

The A8 test has five content areas. Each has its own page of free questions:

The ASE A8 Engine Performance practice test page describes the full question bank. How these questions are written and checked: how our questions are made. Found a mistake? Tell us and we will fix it.

ToolboxPrep is an independent study site. It is not affiliated with or endorsed by ASE (the National Institute for Automotive Service Excellence). Our questions are original practice questions written to the published ASE test specifications; they are not actual test questions.