85 300D Overheats, but shows no signs of steam in the exhaust or water in the oil. Starts & runs well and has good oil pressure. No sign of exhaust gas bubbles in coolant resevoir. Former owner said he drove it about 100 miles with a broken generator pulley, he replaced the pulley, but now it overheats, has no electricity, but runs fine. The generator and water pump seem to be turning OK. Water boils out of resevoir after about 20 min of running.
Will appreciate any advice as to how i can tell if the head gasket is blown or some other cause, and if there is relatively minor damage to the gasket, could an additive like "Mendtite" be a temporary/semi-permanent fix?

Forgive me if this has been covered before, i did search for about 1/2 hr.

sir edmund the green