I have a bizzare phenomenon happening in the cab heating system. (85-300TD W123)
I have the radiator covered for the winter to keep things warmer in our winter months (especially with a veg-oil system). When I am driving on a highway and get to just over 100km/hr (60mph) or so the heat stops coming out of the vents and the engine temp rises fairly quickly. The fan continues to blow however.

I have to decelerate to less than 80KM/hr (50mph) in order for it to switch back to blowing hot air and decrease the engine temp. Would there be a particular flap in question that can be oiled or something?

I could also use more heat period. I cant seem to gte a nice heat coming out and when the car is idling its almost cold. I need to bring up th rpms to get heat pumping through better. Any ideas?
Thanks

Losing heat on the freeway is typically associated with a monovalve failure, I would use that as a starting point as it's a fairly painless and inexpensive job.

Thats perhaps good news.

Where would I find the monovalve. Is it something serviceable or normally needing replacement if its failing?

Fastlane has a repair kit. It's actually a pretty common failure on these cars.

http://catalog.peachparts.com/ShopByVehicle.epc?q=1985-Mercedes--Benz-300d-Climate--Control&yearid=1985%40%401985&makeid=63%40%40MERCEDES+BENZ&modelid=6193%3AED%7C10000135%3AMBC%7C1504%40%40300D&catid=242213%40%40Climate+Control&subcatid=242328@@Mono+Valve+Repair+Kit&mode=PD

Thats better than $100. but Im not sure its worth it yet for me to change it. Maybe next fall. wethers too miserable still to be messing around in there without a garage.

Would this monovalve kit possibly improve the overall heat as well? As I was saying, when Im at an idle the heat decreases quite a bit and will only pump out heat again when the rpms go up. Or would this be more of a coolant flow issue? (perhaps bunged up heater core)

It's possible it would help, but I imagine it's pretty cold where you live and idling a diesel won't generate as much heat as running it, at least in my experience. I've always noticed this on my diesel Mercedes as well, I think the heater draws so much heat from the motor idling will decrease the heater's performance.

Try covering the thermostat housing with some cardboard. If you truely do have to slow down to get the engine temp back down, just waiting a few minutes isn't enough, then there's more going on than just the monovalve.

I'm thinking maybe the thermostat won't open because it's getting a blast of frigid air at 100 km/hr that it doesn't get at 80 km/hr. Covering the housing with cardboard to deflect any such blast should change the behavior enough to tell if that's part of the story.

Personally, I'd be more concerned with the engine temp than the cabin heat, but then I don't live in Winnipeg

__________________
'83 300DTurbo http://badges.fuelly.com/images/smallsig-us/318559.png

Broadband: more lies faster.

So the engine's working hard, can't get airflow due to the cardboard and the temp increases? Sounds normal to me.

If the underhood "outside" temp sensor also gets hot due to the lack of air under the hood it may shut off the monovalve to prevent the coolant from overheating the "hot" outside air coming into the air intake.

Is your Aux Pump connected? It's supposed to pump coolant through system at idle. I'd recommend the 1 A fuse mod first, if it's not currently plugged in.

However, the same phenomenom may be occurring at idle: the underhood temps go up due to the lack of airflow, the CCU closes the monovalve since the outside air is already "hot".

I recommend you simply unplug the electrical connector on the monovalve and see what happens. If it doesn't get electricity it stays open in the "full coolant through the heater core" mode. This assumes it was/is intact, not gunked into position by crud, not torn, etc.