A fuel tank that is overfilled typically causes which issue in an engine?

An overfilled fuel tank can lead to a rich running condition in an engine. When the fuel tank is overfilled, fuel can expand and escape through the vent system or get into other parts of the fuel system, potentially overwhelming the fuel delivery system.

In a rich running condition, the air-fuel mixture in the combustion chamber contains more fuel than the optimal ratio for complete combustion, which is typically around 14.7 parts air to 1 part fuel for gasoline. The increase in fuel can result in incomplete combustion, causing the engine to run less efficiently. This condition can lead to issues like increased emissions, poor fuel economy, and potential fouling of spark plugs.

Moreover, a rich mix can exacerbate problems related to engine performance, such as reduced horsepower and increased carbon buildup on engine components. Though fuel leakage is a concern with overfilling, the primary issue regarding engine performance relates directly to the rich mixture effect on combustion.