Air Induction and Exhaust Systems Flashcards

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?

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.

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.

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.

A turbocharger with variable geometry vanes has vanes stuck in the fully open position. Which complaint is most likely?

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 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?

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.

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.

Which tool is commonly used to verify that DEF has the correct urea concentration?

Answer: A refractometer

A refractometer measures how much light bends in the fluid, which shows urea concentration. Proper DEF reads about 32.5%.

During an active DPF regeneration, what is the main purpose of the diesel oxidation catalyst (DOC) located upstream of the filter?

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.

Which finding most directly points to a failed compressor-side oil seal in a turbocharger?

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.

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.