ASE A9 General Diagnosis — Questions and Answers

Question 1: A customer says their diesel pickup "runs weak and smokes." What should the technician do FIRST?

Interview the customer and verify the complaint by operating the vehicle under the same conditions (Correct answer)
Replace the fuel filter and air filter as a precaution
Remove the injectors for flow testing
Perform a cylinder leakage test on all cylinders

Correct answer: Interview the customer and verify the complaint by operating the vehicle under the same conditions

Diagnosis starts by getting the details of the complaint and verifying it. Otherwise, the technician may end up repairing a condition that is not the actual problem.

A vague complaint like "weak and smokes" can mean many things: low power, black or white smoke, or an issue only under load or when cold. The technician should interview the customer about when it occurs, then try to duplicate the symptom, ideally on a road test or under the reported conditions. After that, check for stored codes and do a visual inspection. Replacing parts or doing invasive tests before verifying the complaint wastes time and often misses the real fault.

Question 2: When a scan tool retrieves a stored DTC, what does the associated freeze frame data show?

The total number of times the code has set in the vehicle's life
Engine operating conditions recorded at the moment the fault was stored (Correct answer)
The factory-specified repair procedure for the code
Live readings that update continuously as the engine runs

Correct answer: Engine operating conditions recorded at the moment the fault was stored

Freeze frame captures a snapshot of key parameters, such as rpm, load, and coolant temperature, at the time the fault was logged.

Freeze frame data helps the technician recreate the conditions under which a fault occurred. It typically includes engine speed, calculated load, coolant temperature, and vehicle speed. Using these values, the technician can duplicate the circumstances during a road test or while watching live data. It is different from live data, which changes in real time, and it does not give repair instructions.

Question 3: A diesel engine emits white smoke with a sweet odor, and the coolant reservoir level keeps dropping with no external leaks. Which is the MOST likely cause?

A clogged air cleaner element
Excessive injection pump timing advance
Worn turbocharger oil seals
Coolant entering the combustion chamber through a failed head gasket or cracked head (Correct answer)

Correct answer: Coolant entering the combustion chamber through a failed head gasket or cracked head

Coolant vaporizing in the cylinders produces white smoke with a sweet smell and unexplained coolant loss.

A restricted air cleaner causes black smoke, and worn turbo seals cause blue smoke from oil. Coolant burned in the cylinders produces white, sweet-smelling steam from the exhaust. A failed head gasket, cracked cylinder head, or cracked liner can all let coolant into the combustion chamber. A combustion leak test or pressure test can confirm it.

Question 4: A technician performs a crankcase pressure (blow-by) test and finds a reading well above specification at a steady rpm. What does this most likely indicate?

A clogged fuel return line
A weak lift pump
Excessive combustion gas passing worn piston rings and cylinder walls (Correct answer)
A stuck-open thermostat

Correct answer: Excessive combustion gas passing worn piston rings and cylinder walls

High crankcase pressure means combustion gases are leaking past the rings into the crankcase, which points to worn rings or cylinders.

Blow-by is measured with a gauge or manometer at the crankcase breather or dipstick tube. A small amount is normal, but readings above spec suggest worn rings, scored cylinder walls, or broken rings. Turbocharger seal problems or a restricted breather can also affect readings, so check those too. Excessive blow-by commonly comes with oil consumption, low power, and smoke.

Question 5: A diesel engine cranks at normal speed but will not start right after the fuel filter was replaced. The new filter was installed dry. What is the MOST likely cause?

Air in the fuel system because it was not primed (Correct answer)
A discharged battery
A failed starter solenoid
Low engine oil pressure

Correct answer: Air in the fuel system because it was not primed

A dry-installed filter leaves air in the fuel system, which prevents the injection system from building pressure and delivering fuel.

Diesel injection systems need a solid, air-free supply of fuel. Air is compressible, so it prevents pressure from building at the injection pump or high-pressure rail. After a filter service, prime the system with the hand primer, then bleed it per the manufacturer's procedure. A battery or starter fault would show up as slow or no cranking, which is not the case here.

Question 6: A technician suspects a failed head gasket and uses a combustion leak (block) tester on the coolant. What result confirms exhaust gas in the cooling system?

The fluid stays blue
The fluid turns yellow (Correct answer)
The fluid turns clear
The fluid turns dark red

Correct answer: The fluid turns yellow

The blue test fluid changes to yellow when combustion gases (CO2) are present in the coolant vapor.

A block tester draws air from the radiator or expansion tank through a chemical test fluid. Combustion gases react with the fluid, changing it from blue to yellow. This indicates a leak between the combustion chamber and cooling system. Results are often confirmed with a cooling system pressure test or by checking for oil contamination.

Question 7: A diesel engine produces black smoke and low power under load. A scan tool shows boost pressure well below specification. Which component should be checked FIRST?

The engine oil dipstick
The charge air (intake) system for leaks and restrictions (Correct answer)
The starter motor ground
The coolant thermostat

Correct answer: The charge air (intake) system for leaks and restrictions

Low boost reduces airflow, so fuel is not fully burned, causing black smoke and low power. Charge air leaks and restrictions are common causes.

Check the air filter, intake ducts, charge air cooler, clamps, and hoses for restrictions or leaks, and perform a boost leak test. Also examine the turbocharger and its control (wastegate or VGT actuator) if the intake is sound. Fixing the cause of low boost restores the air/fuel ratio. Cooling and starting system parts do not cause low boost with black smoke.

Question 8: A vibration is felt at a certain engine speed. It occurs at the same rpm whether the vehicle is parked in neutral or moving in different gears. What does this suggest?

A worn tire
An out-of-balance driveshaft
An engine-related cause such as a misfire, damper, or flywheel problem (Correct answer)
A bent wheel

Correct answer: An engine-related cause such as a misfire, damper, or flywheel problem

If the vibration follows engine rpm instead of vehicle speed, the cause is in the engine or its accessories.

Tires, wheels, and driveshaft imbalance produce vibrations that follow vehicle speed. A vibration that tracks engine rpm points to the engine: a misfire, a damaged harmonic balancer, a flywheel or flexplate problem, loose accessory pulleys, or worn mounts. Running the engine in neutral at various rpm helps separate the two groups. A cylinder balance test can further narrow down a misfire.

Question 9: During a cylinder balance (power contribution) test, one cylinder shows almost no drop in engine speed when it is cut out. What does this indicate?

That cylinder is contributing little or no power (Correct answer)
That cylinder is producing the most power
The turbocharger is overboosting
The fuel filter is plugged

Correct answer: That cylinder is contributing little or no power

Cutting out a working cylinder causes a noticeable rpm drop; little or no change means that cylinder was already not contributing.

The scan tool can disable injectors one at a time while the engine idles. A strong cylinder causes a clear rpm drop when cut out. A weak cylinder causes little change because it was producing little power anyway. Causes include a bad injector, low compression, or a mechanical problem, so follow up with compression, leakage, or injector tests.

Question 10: A diesel engine cranks very slowly and will not start. The batteries are fully charged. What should the technician check next?

Fuel injector return flow
Voltage drop in the starter circuit cables and connections (Correct answer)
Turbocharger shaft play
Coolant concentration

Correct answer: Voltage drop in the starter circuit cables and connections

With charged batteries, slow cranking is often caused by high resistance in the starting circuit, which a voltage drop test finds.

The starter draws very high current, so even small amounts of resistance in cables, grounds, or connections cause a significant voltage drop and slow cranking. Measure the voltage drop on the positive side and the ground side while cranking. Excess drop points to the corroded or loose connection. Also consider mechanical drag from the starter or engine, but check the circuit first.

Question 11: A diesel engine emits blue-gray smoke, especially after idling, and oil consumption has increased. Which condition is the MOST likely cause?

A clogged fuel filter
Air in the fuel lines
Low fuel pressure from a weak lift pump
Engine oil entering the combustion chamber past worn valve guides, seals, or piston rings (Correct answer)

Correct answer: Engine oil entering the combustion chamber past worn valve guides, seals, or piston rings

Blue smoke is burned lubricating oil, which enters the cylinders through worn valve stem seals, guides, rings, or a leaking turbo seal.

Black smoke indicates too much fuel or too little air, and white smoke indicates unburned fuel or coolant. Blue-gray smoke indicates oil is being burned. Oil can reach the combustion chamber through worn valve guides and seals, worn rings or cylinders, or through the turbocharger if the seals fail. Crankcase pressure and turbo checks help pin down the source.

ASE A9 General Diagnosis

A customer says their diesel pickup "runs weak and smokes." What should the technician do FIRST?

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