The head gasket sits between the engine block and the cylinder heads, sealing the combustion chambers from the coolant passages. If this gasket fails, combustion pressure can force diesel fuel (or coolant) into the other system. Since diesel fuel is less dense than coolant, it's more likely to end up in the coolant system, rather than vice versa (though both can occur).
Other less common, but possible causes include:
* Cracked cylinder head or block: A crack in the cylinder head or engine block could also allow diesel fuel to leak into the coolant system. This is usually a more catastrophic failure than a head gasket leak.
* Oil cooler failure: The oil cooler is part of the cooling system and can fail, allowing oil or diesel fuel contamination. However, this usually manifests as oil in the coolant, rather than just diesel.
* Incorrect repair: A previous repair might have introduced diesel fuel into the system, but this is less likely.
In short, the most probable reason for diesel fuel in your 7.3L's coolant is a blown head gasket, and this is a serious issue requiring professional attention. Driving the truck with a compromised head gasket will cause further and potentially irreversible damage.
