* Cracked or damaged cylinder head gasket: This is by far the most frequent cause. The gasket seals the combustion chambers from the coolant passages. A breach in this gasket allows combustion gases (containing diesel fuel) to leak into the coolant. This is often accompanied by signs of white smoke from the exhaust, overheating, and possibly a milky substance in the coolant (from the mixing of oil and coolant).
* Cracked cylinder head: Similar to a blown head gasket, a crack in the cylinder head itself can create a pathway for diesel fuel and combustion gases to enter the coolant system. This is a more severe problem requiring a head replacement or extensive repair.
* Cracked engine block: Less common than head gasket or head cracks, a crack in the engine block can also allow fuel to enter the coolant. This is usually a catastrophic failure.
* Faulty injector(s): While less likely to directly contaminate the coolant with fuel, a severely failing injector could leak fuel into the cylinder. If there's a crack or other breach in the head gasket or head, this fuel could then find its way into the coolant.
* Oil cooler failure: Some Ford engines utilize an oil cooler that's integrated into the coolant system. A failure in this cooler (internal leaks or cracks) can allow oil and fuel (if the oil is contaminated) to contaminate the coolant.
Diagnosing the exact cause requires a thorough inspection by a qualified mechanic. They'll likely perform a pressure test on the cooling system, inspect the coolant for contaminants, and possibly perform a compression test or a leak-down test to pinpoint the source of the leak. Ignoring this problem will likely lead to severe engine damage.
