General Diagnosis Flashcards

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

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.

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

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.

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?

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 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?

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.

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?

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.

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?

Answer: The fluid turns yellow

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

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?

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.

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?

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.

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?

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.

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

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.

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

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.