This makes much more sense to me than low coolant.

__________________
2009 ML320 Bluetec
1985 300CD
1981 300TD

Past Mercedes
1979 300TD
1982 300TD
2000 E320 4Matic Wagon
1998 E430
1984 300SD
1980 300SD

Did you get it fixed?

__________________
It is a truism that almost any sect, cult, or religion will legislate its creed into law if it acquires the political power to do so. Robert A. Heinlein


09 Jetta TDI
1985 300D

I've been experiencing the identical problem in my 85 wagon. Does anybody have the MB part number for the monovalve?

__________________
-1985 300TD - 324k mi on chassis. MB "Tauschaggregat" 617 motor + 4-speed conversion
Alaska Roadtrip 2009
-2002 E320 Wagon - 197k mi
-1998 E320 Wagon - 310k mi - retired

1 147 213 007

I just recently replaced mine and it worked great for 3 weeks and then I started having the same symptoms as the OP.

Here's the thread I created on the topic with a variety of replies:
Strange Heater Behavior??


****Maybe the following info can help asprince too:


My symptoms:

1. driving along and heat is great. Come to a red light and stop car, heat is great.

2. Light turns green and I pull away. When RPMs go above 2000-2500 rmps or I reach about 60 mph, the heat begins to fade and goes completely cold.

3. I turn off the PBU and wait about 10 seconds. I turn the heat back on and it fades back to hot (as long as I keep the RPMs lower). (return to #1)



I was told to pull the glove box and watch the fresh air flap.



I have the glove box out and have tried watching. What exactly should I be looking for?

This pic shows the heater in the off position:


When I turn it on, the rod moves in the direction of the arrow until the black lever piece is in a vertical position. It doesn't seem to change at all even when the heat begins to fade out.

This pic shows the other pod that's visible. Is this the one I should be focusing on?


I can't tell what's happening with it at all. If I know what to look for, I can have my wife drive the car while I look at the potential problem spot. It's too hard to lean over and see what's happening while I'm driving. The car needs to be running over 2000 RPMs for the heat to fade out.

__________________
Chad
2006 Nissan Pathfinder LE
1998 Acura 3.0 CL
OBK#44
"Pleasure in the job puts perfection in the work." - Aristotle (384-322 B.C.)

SOLD
1985 300TD - Red Dragon
1986 300SDL - Coda
1991 - 300TE
1995 - E320
1985 300CD - Gladys