the M104 is known to have a head gasket problem. Even the M103 has a problem, but less so than in an M104.

Notice something about these 2 gas engines and the diesel engines mentioned earlier? They are inline-sixes.

An inline-6 has a very long block, and when the head and block are of different material, the expansion differences between the two are more emphasized, than in one with a shorter block (e.g., V-6 or V-8). And the more the relative movement between head and block, there is a higher probability of gasket failure.