Have you found any correlation to outside (ambient) temperatures and this behaviour?

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Mac
2002 e320 4matic estate│1985 300d│1980 300td
Previous: 1979 & 1982 & 1983 300sd │ 1982 240d

“Let's take a drive into the middle of nowhere with a packet of Marlboro lights and talk about our lives.” ― Joseph Heller, Catch-22

No, it seems to be completely random.

Although, as I am thinking back on the saga of this issue and gathering information from the driver of this car (wife) I'm beginning to develop a hypothesis.

Since I have owned this car (4 years) there has always been a small leak in the heater core that would require me to occasionally top off the coolant reservoir over the course of the Fall/Winter. Once warm temperatures return the level will remain constant all summer until the heat is required again. This has never been a problem since the level drops fairly slowly and I've been pretty diligent about keeping it topped off. That is until this Winter when my wife has been driving the car almost exclusively.

It seems that she ignored the low coolant light for a while and sometime in February made a stop at Autozone and had them top of the coolant with some 50/50. Interestingly, when I took a look at the coolant jug there was less than 1/2 gallon remaining so the level had obviously dropped rather low.
I'm now wondering if it dropped low enough to introduce some air pockets in the coolant system and just maybe that would explain the intermittent problem I'm having.

This is admittedly a bit of a swag but I've run across some threads where air in the system has been proposed as a cause of similar issues, so maybe???

__________________
Joe

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1980 300SD - 495k miles - 'The Ambassador'
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Former Family Members
95 C280
73 280SEL
90 300D
87 300SDL (X2)
86 560SEL
84 300D
80 300SD

Ether - I'm having the EXACT same problem with my 87 300sdl. Exact. I replaced the mono valve also (the old one did have a tiny tear in the the diaphragm). And the same thing happened.

I pulled the plug and drove it, expecting full heat all the time, but it went right back cold (at about 2200 rpm for me). I was thinking the CCU because when it is extremely cold outside - below zero- and I start the car, the CCU clacks and clicks and the Recurc button lights up and off and the heater fan starts and stops till the car warms up.

I'm willing to buy a new one, but I would really like to have a good idea that that is the problem. My antifreeze appears to be full - I've even put the car on an incline to see if I could get things to move, but still nothing.

Hopefully someone here can help us both. I will be watching this thread! Good Luck

There's a lot of this going around. Myself and another forum member in the Memphis area are both working on what sounds to be the same issue.

Vstech --- I sure could use a w126 monovalve assembly from your parts car.

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daw_two
Germantown, TN

Links:
Sold last car --- 05/2012 1984 300D Light Ivory, Red interior
No longer selling Cluster Needles Paint
No longer selling New Old Stock (NOS) parts

Past:
3/2008 1986 300SDL "Coda"
04/2010 1965 190D(c) "Ben"
& many more