What is a common symptom of valves not seating properly in an engine?

Loss of compression is a common symptom of valves not seating properly in an engine. When the valves do not seat correctly, it creates gaps that allow air and fuel mixture to escape from the combustion chamber. This loss of air-fuel mixture leads to lower cylinder pressure during the compression stroke, resulting in decreased engine performance and efficiency. In extreme cases, it can cause the engine to misfire or fail to operate altogether.

The other options do not directly correlate with the issue of valve seating. Increased fuel efficiency typically suggests that an engine is functioning optimally, which would not be the case with improperly seating valves. Engine overheating can result from various issues, including coolant loss or inadequate lubrication, but it does not specifically indicate a problem with valve seating. Excessive smoke may indicate other problems such as oil burning or fuel mixture issues but is not a direct sign of the valves not seating properly.