I had some heat trouble with my 300SDL recently, but all has been fixed. After searching tirelessly for some answers, I may have some for you if you ever suffer this same situation:

I was having the trouble of "no heat at high rpms". Basically I had heat at idle / below 45mph / under 2000 rpm. I replaced the monovalve with an MTC unit and that did not help.

Well, turns out that I had a very loose (but still connected) hose coming from the tee above the BFS that goes to the vac reservoir. Check that line! It cured my problem right away. It is also worth mentioning that my CCU is no longer clicking on and off relentlessly, although I don't know why vacuum would affect that, I just thought it was bad solder joints. I'm guessing that there was not enough vacuum available at higher rpms to keep the mixing flap open for heat.

Secondly, while trying to solve this issue, I found that many have had issues with the MTC valves being too weak or fitting too tightly. Did you all know you can disassemble the monovale quite easily? I ended up just taking the diaphragm from the MTC unit and putting it on my original. You just need to remove the small c clip and everything will come off. With those only being $15 or so, it's still worth it IMO for the new diaphragm.

Anyway, hope this helps someone. Winter is coming!

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Dan
2005 E320 CDI - 246k
1987 300SDL TD05-16g, Herlevi pump, Elbe manifold, 2.47 LSD - 213k
Past: 1987 300D - 264k