OK so here is my issue I have heat when the car warms up, it stays hot untill you start driving then somewhere around 40mph it begins to blow cold(ambient outside air temp) sometimes heat will come back if I return to idle or low speed. I tested the sampler motor (can hear it running with ign. in on position) & it will hold a small piece of TP to the intake grate. Lots of ppl pointed fingers at the mono valve so I removed the connector leaving it in what I believe to be the "open" position. Same symptoms. Thinking I may have a wacky vacuum issue I tried running the heat in park and held the rpm's between 2500-3000 heat stayed working just fine..... I have noticed that the blower motor sometimes ramps up and down when on the high setting is this normal? or perhaps a sign the CCU is dying.....PLEASE HELP!

What is the engine temperature gauge showing while all this is going on?

A bad thermostat which is sticking open might cause similar behavior.

Monovalve has always been a mystery to me because I haven't had to fix one yet. Removing the power should have resulted in full heat, but if the monovalve insert is torn, could that cause your symptom? I don't know...

__________________
Respectfully,
/s/
M. Dillon
'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
'95 124.131 (E300) "Sapphire", 380k miles
'73 Balboa 20 "Sanctification"
Charleston SC

temp gauge shows right in the middle as it should. Vehicle tested at full operating temp.

I thought I responded by I don't see my response. The monovalve can be wide open but with a torn diaphragm you're not going to get sufficient coolant flow through the heater core.

Sixto
MB-less

Had same problem. It was the mono valve.

Well I replaced the mono valve and problem solved! This site had been a huge help in getting my new MD diesel sorted! Thanks sixto i know you are well respected around these parts!