ASE A9 Air Induction and Exhaust Systems — Questions and Answers
Question 1: On a diesel engine equipped with a diesel particulate filter (DPF), which input does the ECM primarily use to estimate how much soot has accumulated in the filter?
- Oxygen sensor voltage in the tailpipe
- Differential pressure across the DPF inlet and outlet (Correct answer)
- Turbocharger shaft speed
- Intake air temperature at the air cleaner
Correct answer: Differential pressure across the DPF inlet and outlet
As soot loads the filter, exhaust flow is restricted, so the pressure difference between the DPF inlet and outlet rises. The ECM uses this differential pressure, along with modeled soot calculations, to decide when a regeneration is needed.
A DPF traps soot in its ceramic walls. As the trapped soot increases, the filter becomes more restrictive. A differential pressure sensor, plumbed to the inlet and outlet sides, reports this to the ECM. The ECM combines the reading with exhaust flow, temperature, and a soot-load model to calculate loading. When the calculated load passes a threshold, it requests a passive or active regeneration. A plugged or disconnected sensor tube can cause false soot-load readings and unnecessary regens.
Question 2: Diesel exhaust fluid (DEF) used in SCR systems is made of which mixture?
- 50% urea and 50% ethanol
- 10% ammonia and 90% distilled water
- 32.5% ammonium nitrate and 67.5% coolant
- 32.5% urea and 67.5% deionized water (Correct answer)
Correct answer: 32.5% urea and 67.5% deionized water
DEF is a solution of about 32.5% high-purity urea in 67.5% deionized water. This concentration gives the lowest freezing point for a urea-water mixture.
The SCR system injects DEF into the hot exhaust stream upstream of the SCR catalyst. Heat converts the urea to ammonia, and the ammonia reacts with NOx on the catalyst to form nitrogen and water. The 32.5% concentration is standardized because it gives the lowest freezing point for the solution. Contamination with water, fuel, coolant, or other fluids can damage the SCR catalyst and dosing components, so DEF quality is a key diagnostic check.
Question 3: Approximately at what temperature does standard DEF begin to freeze?
- 12°F (-11°C) (Correct answer)
- 32°F (0°C)
- -40°F (-40°C)
- -20°F (-29°C)
Correct answer: 12°F (-11°C)
A 32.5% urea solution freezes at about 12°F (-11°C). SCR systems therefore use heated DEF tanks and lines to thaw it in cold weather.
DEF freezes at about 12°F (-11°C), so vehicles in cold climates can have frozen DEF in the tank and lines. Frozen DEF does not damage the fluid, and it can expand, so tanks are designed to allow for that. Systems use engine coolant or electric heaters to thaw the tank, pump, and lines. DEF dosing is typically delayed until the fluid is liquid and the system is ready.
Question 4: A turbocharger with variable geometry vanes has vanes stuck in the fully open position. Which complaint is most likely?
- Excessive boost and turbo overspeed at idle
- Boost that cannot be reduced, causing wastegate rattle
- Low boost and sluggish response at low engine speeds (Correct answer)
- Increased exhaust back pressure with high boost at low rpm
Correct answer: Low boost and sluggish response at low engine speeds
Open vanes make the turbine nozzle larger, which lowers exhaust gas velocity across the turbine. The result is low boost and poor response at low rpm.
A VGT narrows the nozzle at low engine speed to speed exhaust gas up and spin the turbine harder, and opens it at high speed to avoid overboost. If the vanes stay open, the turbine behaves like a large, slow-to-spool unit. The driver sees poor low-speed torque, turbo lag, and often black smoke from a lack of air. Technicians check for carbon buildup, actuator operation, and position feedback with a scan tool before replacing the turbocharger.
Question 5: A diesel engine is losing coolant with no external leaks, and white exhaust smoke is present after startup. The engine has a cooled EGR system. Which component is the most likely source?
- A leaking turbocharger oil drain
- A restricted DPF
- A leaking EGR cooler (Correct answer)
- A failed DEF pump
Correct answer: A leaking EGR cooler
An internal leak in the EGR cooler lets coolant enter the exhaust gas stream and then the intake. That produces coolant loss and white smoke.
EGR coolers use engine coolant to cool recirculated exhaust gas. Thermal cycling and soot deposits can crack the cooler core, so coolant leaks into the EGR path. The coolant then goes into the intake and cylinders, where it burns as white smoke, and can cause hydrolock in severe cases. Technicians check for coolant loss, coolant residue in the EGR piping, and pressure test the cooler.
Question 6: Where should a technician connect a manometer or restriction gauge to measure intake air restriction on a turbocharged diesel engine?
- In the intake piping after the air filter and before the turbocharger compressor inlet (Correct answer)
- In the charge air cooler outlet near the intake manifold
- At the exhaust manifold before the turbine
- At the DPF outlet downstream of the catalyst
Correct answer: In the intake piping after the air filter and before the turbocharger compressor inlet
Intake restriction is measured in the piping between the air filter and the compressor inlet, with the engine at high load or rated speed. This shows the total restriction of the filter and ducting.
Air filter housings, filters, and ducts all add restriction ahead of the turbocharger. A gauge measures the vacuum in that piping, typically at high airflow, where restriction is greatest. Excess restriction reduces boost and power and can cause black smoke and oil leaking past turbo seals. A restriction indicator on the housing is only a rough guide. Measuring with a gauge is the accurate method.
Question 7: Which tool is commonly used to verify that DEF has the correct urea concentration?
- A coolant hydrometer
- A conductivity probe for engine oil
- A multimeter set to AC volts
- A refractometer (Correct answer)
Correct answer: A refractometer
A refractometer measures how much light bends in the fluid, which shows urea concentration. Proper DEF reads about 32.5%.
A DEF refractometer uses a drop of fluid and a scale calibrated for urea. Diluted DEF, such as DEF mixed with water, reads low, and contamination can give unusual results. Poor DEF quality can trigger DEF quality or SCR efficiency codes, and can cause derates. A technician should check DEF quality before replacing costly SCR components.
Question 8: During an active DPF regeneration, what is the main purpose of the diesel oxidation catalyst (DOC) located upstream of the filter?
- To trap soot before it reaches the filter
- To oxidize injected fuel and raise exhaust temperature to burn soot in the DPF (Correct answer)
- To reduce NOx by using DEF as a reductant
- To measure oxygen content in the exhaust
Correct answer: To oxidize injected fuel and raise exhaust temperature to burn soot in the DPF
Fuel injected during active regeneration is oxidized in the DOC, releasing heat that raises exhaust temperature. That heat burns soot in the DPF.
For active regeneration, the ECM raises exhaust temperature by late post-injection or by a dedicated hydrocarbon doser. The unburned fuel reaches the DOC, where it reacts with oxygen and releases heat. This brings DPF inlet temperature to the level needed to oxidize soot, typically several hundred degrees Celsius. A faulty DOC, such as one that is contaminated, may fail to heat the exhaust enough, causing incomplete regens and high soot loading.
Question 9: Which finding most directly points to a failed compressor-side oil seal in a turbocharger?
- Oil in the charge air cooler and intake piping (Correct answer)
- Coolant in the intake piping
- Excessive DEF consumption
- Low differential pressure across the DPF
Correct answer: Oil in the charge air cooler and intake piping
A worn compressor-side seal or bearing allows lubricating oil to enter the intake air path. Oil is then found in the intake piping and charge air cooler.
The turbo shaft runs on oil-lubricated bearings with seals at both the compressor and turbine ends. If the compressor-side seal leaks, oil gets into the charge air system and can be seen in the intake tract and cooler. Other causes, such as restricted drains or high crankcase pressure, should be checked too, since they can push oil past good seals. Technicians also check shaft play and wheel damage before replacing the turbo.
Question 10: A diesel engine idles roughly and stalls at low speed, and the scan tool shows EGR valve commanded closed but the EGR flow is high. Which is the most likely cause?
- A plugged EGR cooler
- A stuck-open EGR valve (Correct answer)
- A restricted DPF
- A failed intake air heater
Correct answer: A stuck-open EGR valve
If the EGR valve sticks open, exhaust gas enters the intake when none is commanded. That displaces fresh air at idle, causing rough running and stalling.
At idle, the engine needs as much fresh air as possible, so EGR is normally closed. When the valve sticks open from carbon buildup, exhaust gas dilutes the intake charge, which causes rough idle, low power, and sometimes stalling. A plugged cooler would reduce EGR flow instead. The technician compares commanded and actual EGR position or flow, then removes and inspects the valve for carbon and sticking.
ASE A9 Air Induction and Exhaust Systems
On a diesel engine equipped with a diesel particulate filter (DPF), which input does the ECM primarily use to estimate how much soot has accumulated in the filter?
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