Five ASE A9 Light Vehicle Diesel Engines practice test questions, answers included.

1. Recall · General Diagnosis

   For the first minute after a cold start, a pickup's diesel puffs white smoke with a sharp smell that stings the eyes, and it clears as the engine warms. What is the smoke?

   1. Steam from coolant leaking into a cylinder
   2. Unburned fuel from the cold cylinders
   3. Oil burning past cold piston rings
   4. Water vapor condensed in the cold exhaust
   Show the answer

   B. Correct. In a cold engine the compressed air may not get hot enough to light all the injected fuel promptly, so some leaves as an unburned white mist with a sharp fuel smell that irritates the eyes. It fades as the cylinders warm; if it is heavy or lasts, glow plugs or the intake heater, compression and injection are checked.

   Why not the others

   A. Coolant entering a cylinder makes white smoke with a sweet smell that continues after warm-up, along with a falling coolant level. Sharp, eye-stinging smoke that clears as the engine warms is unburned fuel.

   C. Burning oil makes blue-gray smoke with a burned-oil smell, not white smoke that stings the eyes. The stinging white smoke here is fuel that did not ignite.

   D. Condensed water vapor is thin and odorless; it has no sharp smell and does not sting the eyes. That smell shows this smoke is unburned fuel.

   ID A9-A-002 · Task A9-A-10 (ASE task list)

2. Recall · Cylinder Head and Valve Train Diagnosis and Repair

   On a light-duty pickup's pushrod V8 diesel with hydraulic roller lifters, the manufacturer's procedure says that after the rocker arms are torqued, the crankshaft is not to be turned for a stated waiting time. What is the wait for?

   1. Letting the lifters fill with oil first
   2. Letting compressed lifters bleed down
   3. Letting rocker bolt thread-locker cure
   4. Letting the head gasket take its set
   Show the answer

   B. Correct. Torquing a rocker whose lifter is partway up its cam lobe forces the lifter's plunger down against the oil inside it, and until that oil leaks out the valve is held open farther than normal. Turning the engine before the lifters bleed down can let a piston strike an open valve, and a diesel's close piston-to-valve clearance leaves little margin.

   Why not the others

   A. The wait does the opposite: oil has to leak out of the lifters, not in. A full lifter compressed by the rocker holds its valve open too far until it bleeds down, and turning the crankshaft then can let a piston hit the valve.

   C. The wait is for the hydraulic lifters, not a thread compound. Lifters compressed by torquing the rockers hold their valves open until oil leaks out of them, and turning the engine before then risks valve-to-piston contact.

   D. The wait protects the valves, not the gasket. Compressed lifters hold their valves open until they bleed down, and turning the engine before then risks a piston striking an open valve.

   ID A9-B-002 · Task A9-B-9 (ASE task list)

3. EXCEPT · Lubrication and Cooling Systems Diagnosis and Repair

   A light-duty diesel's water pump is being replaced, and the old pump's impeller vanes are pitted and eaten away by cavitation. Any of these could have contributed to the damage EXCEPT:

   1. a pressure cap that opened well below its rating.
   2. a partly collapsed hose on the pump's inlet side.
   3. a thermostat stuck in the open position.
   4. coolant run close to its boiling point under load.
   Show the answer

   C. Correct. A thermostat stuck open keeps the coolant cooler than normal, which leaves more margin before vapor can form at the impeller. It causes slow warm-up and weak heat, not cavitation; low system pressure, an inlet restriction and coolant near boiling are what pit an impeller.

   Why not the others

   A. Cavitation happens where pressure at the impeller falls low enough for coolant to flash into vapor bubbles that collapse against the vanes. A cap that opens too soon lowers system pressure and with it the margin before the coolant flashes, so it could have contributed.

   B. A restriction on the pump's inlet side, such as a collapsing lower hose, lowers the pressure at the impeller eye, where vapor bubbles form most easily. That makes cavitation more likely, so it could have contributed.

   D. Coolant near its boiling point needs only a small pressure drop at the impeller to flash into vapor, so hard running close to boiling promotes cavitation and could have contributed.

   ID A9-D-020 · Task A9-D-11 (ASE task list)

4. Recall · Air Induction and Exhaust Systems Diagnosis and Repair

   Why does a turbocharged light-duty diesel pass its compressed intake air through a charge air cooler before it reaches the intake manifold?

   1. To pack more oxygen into each charge
   2. To raise the boost pressure in the manifold
   3. To warm the air for easier cold starts
   4. To trap oil vapor before it reaches the engine
   Show the answer

   A. Correct. Compressing air heats it, and hot air is less dense; cooling it after the compressor puts more oxygen into each cylinder for the fuel to burn. The cooler charge also lowers peak combustion temperature, which helps hold NOx down.

   Why not the others

   B. Cooling does not add pressure; air loses a little pressure flowing through the cooler. Its benefit is density: cooler air carries more oxygen at the same pressure.

   C. The cooler removes heat from the charge rather than adding it. Cold-start heat comes from glow plugs or an intake air heater; the cooler exists to make the charge denser.

   D. A charge air cooler is a heat exchanger, not a separator; oil mist from crankcase ventilation is removed by the ventilation system's own separator. The cooler makes the charge denser.

   ID A9-E-006 · Task A9-E-2 (ASE task list)

5. Recall · Fuel System Diagnosis and Repair

   A light-duty pickup's common rail high-pressure pump failed internally, and its outlet carried metal particles into the rail. Why could the engine's fuel filter not have stopped those particles before they reached the injectors?

   1. The pump sits downstream of the filter
   2. The filter's bypass opens at rail pressure
   3. Its media passes particles of that size
   4. Return fuel carries them around the filter
   Show the answer

   A. Correct. The filter cleans fuel on the low-pressure side before it enters the high-pressure pump, so anything the pump sheds goes straight from its outlet into the rail, the lines and the injectors. That is why an internal pump failure usually means cleaning or replacing the parts downstream of the pump as the manufacturer directs, not the pump alone.

   Why not the others

   B. The filter is on the low-pressure side and never sees rail pressure. The particles were made by the pump after the fuel had already passed through the filter, so no filter setting could have caught them.

   C. A filter can stop only what flows through it, so its rating does not matter here. These particles were created downstream of the filter, at the pump, and went straight to the rail.

   D. Return fuel goes back to the tank and has to pass through the filter again before it can reach the pump. The rail and injectors are fed directly from the pump's outlet, which is where the debris came from.

   ID A9-F-001 · Task A9-F-7 (ASE task list)

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