Five free practice questions on air induction, exhaust and aftertreatment: content area E of the ASE A9 Light Vehicle Diesel Engines test, 12 of its 50 scored questions (24%). They are in the ASE format: four options, one best answer. Answer each one before opening the explanation. No account, no email.
01The questions
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Some light-duty diesel pickups provide exhaust braking with their variable-geometry turbocharger (VGT) instead of a separate exhaust brake valve. With fuel cut during the braking, how does the ECM use the VGT to slow the vehicle?
- It opens the vanes to drop the boost
- It closes the vanes to raise back pressure
- It closes the vanes to raise the boost
- It opens the vanes wide to raise back pressure
Show the answer
B. Correct. Nearly closed vanes restrict the exhaust leaving the cylinders, so each piston must push against high pressure on its exhaust stroke, and that work slows the vehicle. The ECM sets vane position to keep exhaust pressure within the manufacturer's limit.
Why not the others
A. Opening the vanes lets the exhaust pass freely, so the pistons do little work on the exhaust stroke and there is almost no braking. Braking comes from closing the vanes to build exhaust pressure.
C. Boost pushes down on the pistons during the intake stroke, which helps the engine turn rather than slowing it. The braking comes from exhaust pressure the pistons must push against.
D. Open vanes give the exhaust the widest path through the turbine, which lowers back pressure. Raising back pressure for braking takes closed vanes.
ID A9-E-008 · Task A9-E-3 (ASE task list)
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Some light-duty diesels have two intake ports per cylinder, and an ECM-controlled flap closes one port of each pair at low engine speed and light load. Why is that port closed?
- To lengthen the runner for low-speed torque
- To restrict airflow so idle speed drops
- To raise air swirl for better mixing
- To build intake pressure for the turbocharger
Show the answer
C. Correct. At low speed the air enters slowly, so forcing it through one port makes it spin in the cylinder; the swirl mixes the injected fuel with the air for cleaner, more complete burning. At higher speed and load the flaps open to let the engine breathe freely.
Why not the others
A. Long-runner tuning is a variable-length manifold strategy that uses intake pressure waves; closing one port of a pair is done to change how the air moves in the cylinder. It raises swirl for mixing.
B. A diesel's speed is set by the fuel injected, not by restricting air; closing a port is not how idle is controlled. The flap is there to create swirl.
D. Closing a port downstream of the compressor does not help the turbocharger build boost. Its purpose is the swirl that improves fuel and air mixing at low speed.
ID A9-E-013 · Task A9-E-4 (ASE task list)
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A pickup diesel's exhaust runs through a diesel oxidation catalyst (DOC) and then a diesel particulate filter (DPF), with an exhaust gas temperature sensor between the two. During an active regeneration, what does that sensor's reading tell the ECM?
- How much soot remains in the filter
- How much NOx the filter is trapping
- How much heat the DOC is adding
- How fast the turbocharger is spinning
Show the answer
C. Correct. Compared with the temperature ahead of the DOC, it shows whether the post-injected fuel is burning in the catalyst, and it is the temperature entering the DPF. The ECM trims the extra fuel to hold that inlet temperature at its regeneration target.
Why not the others
A. Remaining soot is judged from the filter's differential pressure and the ECM's soot model. The sensor between the DOC and DPF reports the temperature the DOC delivers to the filter.
B. A DPF traps soot particles, not NOx, and NOx is read by NOx sensors. The sensor between the DOC and DPF reports the heat the DOC is producing.
D. Turbocharger speed is not read from a temperature downstream of the DOC. That sensor shows how much the DOC is heating the exhaust going into the DPF.
ID A9-E-018 · Task A9-E-5 (ASE task list)
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A pickup's V8 diesel is hard to start cold. The manufacturer says each glow plug draws 11 to 13 amps after 10 seconds of glow. With an inductive ammeter clamped around the single feed that supplies all eight glow plugs, the total reads 84 amps at that point. What should the technician conclude?
- In range; diagnose another system
- Too high; check for a shorted glow plug
- Too low; replace all eight glow plugs
- Too low; check each plug's current
Show the answer
D. Correct. Eight plugs in specification draw 8 x 11 = 88 to 8 x 13 = 104 amps together, so 84 amps means at least one plug is drawing too little or none. The total cannot show which, so each plug's current or resistance is checked next.
Why not the others
A. This comes from averaging 84 / 8 = 10.5 amps and rounding it up to 11. Eight good plugs draw at least 8 x 11 = 88 amps, so 84 amps is short, and each plug's current must be checked.
B. This compares 84 amps with one plug's 11 to 13 amps, but the clamp is on the feed to all eight. Against 8 x 11 = 88 to 104 amps, 84 amps is low.
C. The total is low, but 84 amps is only about one plug's worth below the 8 x 12 = 96 amps eight mid-range plugs would draw, so most plugs are working. Measuring each plug finds the failed one or ones instead of replacing good parts.
ID A9-E-025 · Task A9-E-7 (ASE task list)
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The manufacturer of a light-duty diesel with a diesel particulate filter (DPF) specifies an oil category with a limited sulfated ash content. Why does the DPF make low ash content important?
- Ash particles in the exhaust plug the DEF doser
- Low-ash oil resists thinning from regeneration fuel
- Ash-forming additives raise the engine's soot output
- Ash from burned oil stays in the filter for good
Show the answer
D. Correct. Oil that reaches the combustion chambers and burns, past the rings, valve guides or turbocharger seals, leaves metallic ash from its additives, and ash does not burn away in regeneration. It builds up in the filter until the filter is cleaned or replaced, so lower-ash oil stretches that interval.
Why not the others
A. The DEF doser sprays liquid into the exhaust and is clogged by dried urea, not oil ash. Ash matters because it builds up in the DPF for good.
B. Fuel dilution from post-injection is a real concern during regeneration, but ash content does not prevent it. The ash limit exists because ash collects in the filter permanently.
C. Soot comes from incomplete fuel combustion; ash is the unburnable residue of oil additives. The concern is ash that the DPF traps and cannot burn off.
ID A9-E-030 · Task A9-E-8 (ASE task list)
02Answer key
| Question | Answer |
|---|---|
| Q1 | B |
| Q2 | C |
| Q3 | C |
| Q4 | D |
| Q5 | D |
03What area E covers
This content area covers how a light-duty diesel breathes in and out: air filters and restriction indicators, boost tests and charge air coolers, fixed and variable-geometry turbochargers, intake manifolds, swirl flaps and throttle valves, exhaust back pressure and temperature sensors, glow plugs and intake heaters, EGR valves and coolers, crankcase ventilation, and the DOC, DPF, SCR and DEF parts with their regeneration.
The ASE test specification calls this content area “Air Induction and Exhaust Systems Diagnosis and Repair”. It carries 12 of the 50 scored questions, 24% of the test.
- Service air filters and check inlet restriction
- Test boost and inspect charge air coolers
- Diagnose turbochargers, VGT actuators and wastegates
- Check intake manifolds, swirl flaps and throttle valves
- Test exhaust back pressure and temperature sensors
- Diagnose glow plugs and intake air heaters
- Test DOC, DPF, SCR and DEF parts and regeneration
- Diagnose EGR and crankcase ventilation components
04What the questions turn on
Why regeneration stops
The ECM controls regeneration heat from its exhaust temperature sensors, so a sensor circuit fault can inhibit regeneration while soot builds, and that fault is repaired first. Ash from burned oil never burns off, so lower-ash oil stretches the time before the filter needs service.
Read a DEF dosing test
A dosing test into a graduated container separates supply faults from doser faults. Low pressure with low delivery places the fault on the supply side, such as a restricted pump filter. Correct delivery with dripping after shutoff points to a doser that does not seat.
Glow plug current and voltage
Total glow plug current is compared with the plug count times the per-plug range. A low total shows at least one plug draws too little, not which one. Voltage at the module output but none at the far end of that wire means an open.
EGR flow is calculated
Many ECMs calculate EGR flow as total charge minus the fresh air the MAF sensor reports, so a MAF reading low makes EGR flow look high. An EGR valve stuck open at full load displaces fresh air: black smoke, low power and less NOx.
05The other content areas
The A9 test has six content areas. Each has its own page of free questions:
The ASE A9 Light Vehicle Diesel Engines 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.
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