A known issue with the pre-'97 6.5L TD's was burned-through head gaskets in
the rear-most cylinders. This was caused by inadequate coolant flow and was
fixed in 1998 with a higher flow water pump with twin thermostats and a
crossover. I don't know just how much water you are losing over time, but
you should have your cooling system pressure tested and consider this a
highly potential cause.
The post-'98 water pump set-ups can be retrofitted onto earlier year motors
so if you are going to have the motor repaired I highly recommend this
modification as well.
Good luck - Jonathan
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