Five free practice questions on air induction, exhaust and aftertreatment: content area E of the ASE T2 Diesel Engines test, 6 of its 55 scored questions (10.9%). 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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A truck with a fixed-geometry, wastegated turbocharger has run well past its air filter element's restriction limit. Any of these can result EXCEPT:
- lower power under full load.
- higher exhaust temperature under load.
- oil drawn past the compressor-end seal.
- higher boost pressure at full load.
Show the answer
D. Correct. The compressor raises the pressure of the air it receives, so a restricted element that lowers the pressure at the compressor inlet lowers boost, not raises it. The wastegate only bleeds exhaust past the turbine to cap boost at its setting; it can never push boost above that, so high restriction never shows as higher boost.
Why not the others
A. A restricted element lowers the air pressure at the compressor inlet, so less air mass reaches the cylinders and the engine loses power under load. This can result from running past the restriction limit.
B. With less air mass for the fuel being burned, combustion runs richer and hotter, so exhaust temperature climbs under load. This is a known result of high inlet restriction.
C. The compressor-end seal relies on air pressure in the compressor housing being near or above the center-housing pressure. High inlet restriction drops the pressure behind the compressor wheel, which can pull oil past the seal into the intake.
ID T2-E-004 · Task T2-E-1 (ASE task list)
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A fixed-geometry turbocharger has a wastegate opened by a spring-loaded diaphragm actuator that boost pressure pushes against. The manufacturer gives the rod travel the actuator should show at a stated test pressure. How is the actuator checked?
- Apply vacuum with a hand pump and measure how far the rod retracts
- Apply regulated air at the test pressure and measure the rod travel
- Watch boost at low idle and check that it stays below the test pressure
- Disconnect the rod and check that the valve arm swings without binding
Show the answer
B. Correct. The actuator is a diaphragm working against a calibrated spring, so feeding it regulated air at the stated pressure and measuring the rod movement with a dial indicator shows whether the spring and diaphragm open the wastegate at the right point. A leaking diaphragm also shows here as pressure that will not hold.
Why not the others
A. This actuator is opened by boost pressure on its diaphragm, not by vacuum, so a vacuum pump works it in the wrong direction and says nothing about its opening point. It is tested with regulated air pressure at the stated figure.
C. A diesel makes almost no boost at low idle, so the wastegate never has a reason to open there and the reading proves nothing about the actuator. Its opening point is checked by applying the stated air pressure and measuring rod travel.
D. Freeing the arm checks the wastegate valve for binding, which is worth doing, but it does not show at what pressure the actuator moves. The actuator itself is checked with regulated air at the test pressure while the rod travel is measured.
ID T2-E-007 · Task T2-E-2 (ASE task list)
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An inline six-cylinder diesel has a three-piece exhaust manifold whose sections fit together at slip joints with seal rings. What do the slip joints allow?
- The manifold to flex as the engine rocks
- The manifold to grow in length as it heats
- Front and rear exhaust pulses to stay apart
- Exhaust pressure to bleed off as the DPF loads
Show the answer
B. Correct. A long manifold on an inline six grows noticeably in length from cold to full-load temperature, and a one-piece casting fixed at every port would crack or pull on its studs. The slip joints let the sections slide on their seal rings, so worn rings or a seized joint show up as leaks or cracked sections.
Why not the others
A. The manifold is bolted to the cylinder head, so it moves with the engine as it rocks on its mounts. Movement between the engine and the frame-mounted exhaust is taken up by the flexible pipe after the turbocharger; the slip joints are there for heat growth.
C. The slip joints connect the sections into one passage, so they do not keep any exhaust pulses apart; any pulse division is cast into the manifold itself. The joints let the manifold grow in length as it heats.
D. The slip joints are meant to seal, and exhaust escaping from one is a leak to repair, not a pressure relief. Their job is to let the manifold sections slide as the manifold grows with heat.
ID T2-E-011 · Task T2-E-5 (ASE task list)
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On a diesel whose aftertreatment runs diesel oxidation catalyst (DOC), diesel particulate filter (DPF), then selective catalytic reduction (SCR) catalyst, a hydrocarbon doser sprays fuel into the exhaust ahead of the DOC during active regeneration. What does the DOC do with that fuel?
- Converts it into ammonia for the SCR catalyst
- Passes it through to burn on the soot in the DPF
- Stores it until the DPF is hot enough to burn it
- Burns it to heat the exhaust entering the DPF
Show the answer
D. Correct. The DOC's catalyst oxidizes the dosed fuel without a flame, and the heat released raises the temperature of the exhaust entering the DPF to the point where the trapped soot burns. That is why the DOC must be above its light-off temperature before dosing starts.
Why not the others
A. Ammonia for the SCR catalyst comes from diesel exhaust fluid sprayed by a separate doser after the DPF, not from fuel. The DOC oxidizes the dosed fuel to make heat for regenerating the DPF.
B. Raw fuel reaching the DPF could overheat the filter, and the soot does not need fuel to burn: it burns in the exhaust's leftover oxygen once it is hot enough. The DOC burns the dosed fuel itself, and that heat is what carries into the DPF.
C. A DOC is not a storage device for fuel; it oxidizes hydrocarbons as they pass over the catalyst. That oxidation is the heat source that brings the DPF up to regeneration temperature.
ID T2-E-014 · Task T2-E-7 (ASE task list)
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On a diesel with a selective catalytic reduction (SCR) system, diesel exhaust fluid (DEF) is sprayed into the exhaust ahead of the SCR catalyst. What does the ammonia from the DEF do in the catalyst?
- Reacts with NOx to form nitrogen and water
- Burns the soot that slips past the DPF
- Turns HC and CO into carbon dioxide and water
- Cools the exhaust to protect the SCR catalyst
Show the answer
A. Correct. Heat breaks the urea in DEF down into ammonia, and over the SCR catalyst the ammonia reacts with oxides of nitrogen (NOx) to leave nitrogen and water vapor. The ECM doses DEF to match the NOx coming from the engine, using NOx sensors to check the result.
Why not the others
B. Soot is trapped and burned in the DPF; ammonia does not oxidize soot. In the SCR catalyst, the ammonia reacts with oxides of nitrogen to form nitrogen and water.
C. Oxidizing hydrocarbons and carbon monoxide is the job of the diesel oxidation catalyst at the front of the system. The ammonia in the SCR catalyst reduces oxides of nitrogen to nitrogen and water.
D. The small amount of fluid sprayed does cool the exhaust slightly, but that is not its purpose. The ammonia it releases is the reducing agent that turns oxides of nitrogen into nitrogen and water.
ID T2-E-015 · Task T2-E-7 (ASE task list)
02Answer key
| Question | Answer |
|---|---|
| Q1 | D |
| Q2 | B |
| Q3 | B |
| Q4 | D |
| Q5 | A |
03What area E covers
This content area covers the air going into a truck diesel and the exhaust leaving it: air cleaners and inlet restriction, fixed and variable-geometry turbochargers and wastegates, intake manifold sensors, charge air coolers, exhaust manifolds and piping, grid heaters and glow plugs, EGR, and the aftertreatment system with its DOC, DPF, DEF and SCR parts and regeneration.
The ASE test specification calls this content area “Air Induction and Exhaust Systems Diagnosis and Repair”. It carries 6 of the 55 scored questions, 10.9% of the test.
- Service air cleaners and measure inlet restriction
- Test fixed and variable-geometry turbochargers and wastegates
- Check intake manifold sensors and charge air coolers
- Inspect exhaust manifolds, piping and slip joints
- Test grid heaters and glow plug systems
- Diagnose DOC, DPF, DEF and SCR components
- Check how DPF regeneration is working
- Diagnose EGR valves, coolers and sensors
04What the questions turn on
Soot burns, ash stays
A complete regeneration burns off soot, but ash from oil additives stays in the filter, so high differential pressure after a good regeneration points to ash. A cracked substrate lets soot through and lowers the reading instead of raising it.
No leak, still failing
A charge air cooler that passes its leak test can still fail. Pressure drop over the limit means a restriction inside. A small temperature drop with normal pressure drop points to fins blocked on the outside by bugs or dirt.
Right volume, weak fluid
SCR turns NOx into nitrogen and water using ammonia from DEF. When a dosing test delivers the specified amount but conversion stays low, the fluid may carry too little urea, as DEF diluted with water does, so its concentration is checked.
What a test cannot show
A test light at a glow plug shows only that voltage reaches it; an ohmmeter with the feed wire off finds an open element. A block test finding combustion gas in the coolant cannot tell a leaking EGR cooler from a head gasket.
05The other content areas
The T2 test has eight content areas. Each has its own page of free questions:
The ASE T2 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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