Here's why:
* Head Gasket Failure: The head gasket seals the combustion chamber from the cooling system. A blown head gasket allows coolant to leak into the cylinders, especially under pressure (when the engine is running). This is exacerbated if the failure is between a cylinder and a coolant passage. Because it's in all eight cylinders, it suggests a widespread failure of the gasket, rather than just a single localized leak.
* Cracked Cylinder Head: A crack in the cylinder head can allow coolant to leak into the combustion chamber. This is less likely to affect *all* cylinders simultaneously unless the crack is extremely extensive.
* Cracked Engine Block: Similar to a cracked head, a crack in the engine block can cause coolant to enter the cylinders. This is even less common than a cracked head, and a crack large enough to affect all cylinders would be a major problem.
How to distinguish between these:
A pressure test of the cooling system and a compression test of the cylinders are needed to diagnose the problem accurately. A visual inspection might show signs of coolant in the cylinders or external leaks, but a pressure test will pinpoint the leak source more definitively. A leak-down test can help determine whether the problem lies in the gasket or cylinder itself.
In short, while other less likely scenarios exist, coolant in *all* eight cylinders strongly indicates a significant problem with the head gasket, possibly coupled with a head or block crack. Immediate action is needed to prevent further damage.
