What symptom may occur in an engine if the exhaust valve leaks during operation?

When an exhaust valve leaks during engine operation, one of the primary symptoms that can manifest is a backfire. This backfire is often a result of unburned fuel entering the exhaust system. The leaking exhaust valve allows some of the air-fuel mixture to escape from the combustion chamber into the exhaust system before it is completely burned. This unburned mixture can ignite when it comes into contact with the hot exhaust gases, resulting in a backfire.

Additionally, the presence of a leak can lead to abnormal airflow and pressure changes within the system, which can further contribute to this issue. Other symptoms like overheating or excessive smoke may also be associated with different problems within the engine, but backfiring is a direct consequence of a leaking exhaust valve due to the nature of how combustion is affected.