What will cause engine coolant to be present in the air system tanks?

When engine coolant is found in the air system tanks, it can indicate a failure in the component responsible for maintaining the separation between the engine's coolant system and the air system. The air compressor head gasket is critical because it seals the connection between the engine's cooling system and the air compressor itself.

If this gasket is compromised due to wear, thermal stress, or improper installation, it can allow coolant to leak directly into the air system. This presence of coolant in the air system is problematic, as it can lead to various issues such as moisture contamination, which affects the performance and reliability of air-operated components in vehicles.

The other components listed may have distinct issues related to their functions, but they do not directly link the engine coolant to the air system in the same manner as a failing air compressor head gasket would. Therefore, recognizing the role of the air compressor head gasket is essential for diagnosing and addressing the problem accurately.