DFFT Exhaust and Emissions

Question 1

A vehicle fails an emissions test for high HC (hydrocarbon) levels. What is the most likely cause?

Accepted Answer

Engine misfires causing unburned fuel to pass through the exhaust

Answer

HC emissions are unburned fuel; engine misfires cause raw fuel to exit the combustion chamber unburned and pass through the catalytic converter, dramatically increasing HC output.

Question 2

A catalytic converter is found to be glowing red hot during operation. What is the most likely cause?

Accepted Answer

Unburned fuel from engine misfires is igniting inside the converter

Answer

Large quantities of unburned HC entering the converter ignite inside it, generating extreme heat that can melt the internal substrate and destroy the converter.

Question 3

A vehicle fails an emissions test for high CO (carbon monoxide). What does elevated CO indicate?

Accepted Answer

Incomplete combustion due to a rich air-fuel mixture

Answer

CO is produced when carbon burns without sufficient oxygen; a rich mixture means there is more fuel than available oxygen, resulting in incomplete combustion and high CO output.

Question 4

A vehicle fails emissions for high NOx. What driving condition produces the most NOx?

Accepted Answer

High engine load at high temperatures with a lean mixture

Answer

NOx forms when combustion temperatures exceed approximately 2500°F (1370°C), conditions that occur under high load with lean combustion promoting very high peak temperatures.

Question 5

What is the primary function of the EGR (Exhaust Gas Recirculation) valve?

Accepted Answer

To recirculate a portion of exhaust gas into the intake to lower peak combustion temperatures and reduce NOx

Answer

Recirculated exhaust gas dilutes the intake charge with inert gas, lowering peak combustion temperature and reducing the formation of nitrogen oxides.

DFFT - Driver Fault Finding Test Practice Test

DFFT - Driver Fault Finding Test Practice Test

DFFT Exhaust and Emissions