Mixed Deck — All ASE A9 Topics Flashcards

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.

Why should a high-pressure fuel line normally be replaced instead of reused or reshaped when it has been removed from a common rail diesel?

Answer: The sealing end can deform and the line can be damaged, which may lead to leaks or fatigue cracks at high pressure

Sealing surfaces deform when tightened and lines can be bent or stressed, so reuse risks leaks or cracks at very high pressure. Many manufacturers specify replacement.

Technicians find pitting and erosion on the outside of removed wet cylinder liners, mostly on the thrust side. What is the best way to prevent this problem in service?

Answer: Keep supplemental coolant additive (SCA) at the specified concentration

The pitting is cavitation erosion, caused by liner vibration that forms and collapses vapor bubbles in the coolant. Proper SCA levels form a protective film on the liner surface and prevent it.

Diesel exhaust fluid (DEF) used in SCR systems is made of which mixture?

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.

A diesel engine has coolant in the oil but no external leaks, and the head gasket has been ruled out. What is the best way to test the cylinder head for cracks?

Answer: Pressure test the head's coolant passages after plugging all ports, and check for leaks

Plugging the water ports and applying air or water pressure to the coolant passages shows leaks through cracks into the oil, intake, exhaust or cylinder areas.

A diesel engine has high oil pressure at cold start that drops to normal as the engine warms up. Which of the following explains this?

Answer: Oil is thicker when cold, so flow resistance and pressure are higher until it warms

Cold oil is more viscous, so it resists flow and produces higher pressure until warming thins it. The relief valve limits the peak.

On a common rail diesel, the ECM reduces power when fuel temperature returned to the tank becomes very high. Which condition is most likely to be involved in this complaint?

Answer: Excess heat from high-pressure pump and injector return flow, with an inadequate fuel cooler or low tank level

Fuel that circulates through the pump and injectors and returns to the tank picks up heat. If the cooling is poor or the tank is nearly empty, fuel temperature rises and the ECM may derate the engine.

A diesel engine has tight valve lash on the exhaust valves, below specification. What is the most likely result if it is not corrected?

Answer: The valve will not close fully and can burn from lack of heat transfer to the seat

If lash is too tight, the valve can be held slightly off its seat as parts expand with heat. This lets hot gas leak past and prevents the valve from cooling through the seat, so it burns.

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.

When checking a diesel cylinder head for warpage, which method is correct?

Answer: Lay a straightedge on the machined surface and use a feeler gauge to measure gaps lengthwise, crosswise and diagonally

Flatness is verified by placing a precision straightedge on the deck and sliding feeler gauges under it. Readings are taken along the length, across the width and diagonally.

A diesel engine overheats at highway load. With the engine hot and shut off, the technician can spin the viscous fan clutch freely by hand with almost no drag. What is the most likely problem?

Answer: The fan clutch has lost its silicone fluid and is not engaging

A healthy viscous clutch has noticeable drag when hot. Free spinning means the fluid has leaked out, so the fan will not drive airflow.

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.

Approximately at what temperature does standard DEF begin to freeze?

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.

While inspecting a pushrod, a technician rolls it on a flat surface and sees it wobble. What does this indicate?

Answer: The pushrod is bent and must be replaced

A straight pushrod rolls smoothly on a flat surface. Wobble or a visible gap means it is bent, and a bent pushrod must be replaced.

Where should a technician connect a manometer or restriction gauge to measure intake air restriction on a turbocharged diesel engine?

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.

What is the function of the bypass valve in a full-flow engine oil filter?

Answer: To let oil flow around a clogged filter element and avoid oil starvation

If the element is restricted or the oil is very thick, the bypass valve opens so oil still reaches the engine, though unfiltered.

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?

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 what point does diesel fuel begin to form wax crystals that can plug fuel filters in cold weather?

Answer: The cloud point

The cloud point is the temperature at which wax crystals first appear in diesel fuel. These crystals can clog the fuel filter.

A technician installs a new injector sleeve and measures injector tip protrusion. Why is this measurement important?

Answer: It ensures the nozzle tip is positioned correctly in the combustion chamber for spray pattern and proper cooling

Protrusion affects where fuel is sprayed in the combustion chamber and how well the tip is cooled. Incorrect protrusion can cause poor combustion, smoke or a damaged injector or piston.

A diesel engine shows normal oil pressure when cold, but at hot idle the gauge reads very low and rises sharply with rpm. Which condition is the most likely cause?

Answer: Excessive main and rod bearing clearance

Worn bearings let hot, thin oil escape too easily, so pressure drops at idle. It recovers as pump output rises with rpm.