Overheating

My 83 240 Diesel 4M/T used to run a bit cold, warming up slowly and giving out tepid heat through the vents. Thinking that the Tstat was stuck open, I replaced it with a factory thermostat from the dealership as a periodic maintenance thing.

It now runs hot! Not hot enough to boil over, but the temp gauge now reads at the mark between the 80 and 120C marks on the gauge, especially at highway speed, and it leaks coolant through the cap overflow. The engine's definitely warm! (I.E, not a faulty gauge) All this heat can't be doing the engine any favors and I'm anxious to get it straightened out.

Coincidental with some other problem? Air cavitation in the coolant? Restricted rad? Any thoughts would be greatly appreciated!

Todd S.
Oak Harbor, WA