What is the effect of a stuck closed EGR valve on engine performance?

A stuck closed EGR (Exhaust Gas Recirculation) valve can lead to several performance issues in an engine, primarily due to the role that EGR plays in controlling combustion temperatures and emissions. When the EGR valve is functioning correctly, it recirculates a portion of the engine's exhaust back into the intake manifold, which helps lower combustion temperatures and reduce nitrogen oxide (NOx) emissions during the combustion process.

When the EGR valve is stuck closed, the recirculation of exhaust gases is inhibited, leading to a higher concentration of oxygen in the combustion chamber. This can result in excessively high combustion temperatures, which can contribute to an increased tendency for engine knock or detonation. Engine knock occurs when the air-fuel mixture ignites prematurely in the cylinder, which can lead to performance issues and potentially engine damage if not addressed.

Thus, the effect of a stuck closed EGR valve is an increase in engine knock due to the higher combustion temperatures resulting from the lack of exhaust gas recirculation. This is the primary reason why the choice indicating increased engine knock is the correct answer.