Low compression can be a sign of which engine issue?

Low compression in an engine typically indicates that there is a serious issue affecting the ability of the cylinders to create the necessary pressure for efficient combustion. One common cause of low compression is a blown head gasket. The head gasket is responsible for sealing the combustion chamber and maintaining the integrity of the engine's internal pressure. When it fails, it can allow air or combustion gases to escape from the cylinders, which leads to a decrease in compression.

A blown head gasket can also result in other problems, such as coolant leaking into the combustion chamber or oil mixing with coolant. This not only impacts the engine's operation but can also lead to more severe engine damage if not addressed promptly.

In contrast, while a clogged air filter, a full fuel tank, or a dirty spark plug can cause issues in the engine's performance, they do not directly lead to low compression in the same way that a blown head gasket does. A clogged air filter may restrict airflow, but it wouldn’t affect the cylinder pressure; similarly, a full fuel tank is unrelated to compression issues, and dirty spark plugs primarily affect ignition and combustion efficiency rather than the pressure dynamics within the cylinders.