Engine Performance & Diagnostics Flashcards

Comparing the pre-cat and post-cat O2 sensor signals, which pattern points to a failed catalytic converter (P0420)?

Answer: The post-cat signal switches almost as fast as the pre-cat signal

A working catalyst stores oxygen and flattens the downstream signal, so a downstream signal that copies the upstream one means the catalyst has lost its storage capacity.

A coolant temperature sensor that always reports a cold engine would most likely cause which symptom?

Answer: A rich mixture, poor fuel economy, and possible black smoke

The ECM keeps adding cold-start enrichment, so the engine runs rich.

Early EA888 2.0T engines are known for a timing chain tensioner failure. Which DTC most often shows up when the chain has jumped?

Answer: P0016 crankshaft/camshaft position correlation

A jumped chain changes cam timing relative to the crank, which sets the cam/crank correlation fault.

On older 2.0 FSI (EA113) engines, the high-pressure fuel pump is driven by a cam lobe. Which wear item is known to cause low rail pressure and can damage the camshaft?

Answer: Cam follower (pump tappet)

The HPFP cam follower wears through, which lowers fuel pressure and can wear the cam lobe.

A failed PCV/oil separator diaphragm on a 2.0T often causes which combination of symptoms?

Answer: A whistling noise, rough idle, and lean codes such as P0171

A torn diaphragm creates a vacuum leak, which causes lean trims, a whistle, and rough idle.

When installing a knock sensor on a VW engine, why is tightening it to the specified torque critical?

Answer: The torque affects the sensor's signal, so wrong torque can cause false knock or missed knock detection

Knock sensors are piezoelectric, and how tightly they couple to the block affects their sensitivity.

Which data should be reviewed to see the engine conditions at the moment a DTC was stored?

Answer: Freeze frame (environmental) data

Freeze frame data records rpm, load, temperature, and other values at the moment the fault was stored.