Dad jsut got home from a long trip in his 81 300CD and had plenty of heat in town but nothing on the road.. any ideas?

functioning tstat.

tom w

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

What was the engine temperature reading when the heat was cold?

The temp is normal about 85 degress C and the t-stat is new.. is it possible to put it in wrong in a 617?

It is possible to install the stat backwards on the 617, but I doubt that's the problem if the engine temp is normal. I don't know what would cause these symptoms. I guess I would start with the monovalve. I would try unplugging the monovalve, you should get maximum heat with it unplugged (fully open).

I have the same problem with my 300D. I have tried a new thermostat, monovalve and by-passed the auxilliary water pump. The thermostat did the best job so far with getting the temperature up but it still goes cold if I go over 3750 RPM's. It will not get hot again until I let it idle for a couple seconds. If I am going down the road, I can shift into neutral for 3 seconds and then back into drive and its good again. I think I may have a bunch if chunks of stuff or old stop leak clogging the heater core and it will like fall off until the flow comes up then it will clog the tubes again. Either that or there is a flap of hose hanging in the flow path somwehere that pops out if the flow comes up. Sorry for the long reply, I have the same issue as you and these are the things I suspect on mine. Eric.

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1984 300D, 246k, veggie, sold
1982 300SD 301k

The fastest and easiest check to make is for a torn monovalve diaphragm. Don't bother with any thermostats, heater cores, etc. until you perform this 10 minute check.