I have an 84 300d. The heat works great if I keep my speed under 45 mph. Above that, the fan still blows air but not hot. When I slow down, the air instantly gets hot. I have searched some threads but have not read anything relating to this problem. If I read correctly, the water valve is vacuum operated but the default is open. If I have a vacuum leak would not the valve default to open?

Thanks in advance for your input.

Steve

You must have been reading a selection from the fiction section!!! The heater valve is electrically operated.

Replace your monovalve diaphragm. Chances are about 100:1 that your heat will work fine after that.

I second that, replace Monovalve guts.

__________________
I'm not a doctor, but I'll have a look.

'85 300SD 245k
'87 300SDL 251k
'90 300SEL 326k

Six others from BMW, GM, and Ford.

Liberty will not descend to a people; a people must raise themselves to liberty.[/IMG]

X3

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

I say clean the screen on the monovalve first (but buy a new insert anyway).
And, watch how it comes apart, and goes back together.

__________________
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

Even if it defaults to open, with a bad diaphragm you won't get good heat at high engine speeds.

Sixto
87 300D

That is why I was thinking dirty screen. The insert is just to have on hand for when the diaphragm breaks (which it will eventually do).

__________________
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

And if the mono valve doesnt fix it, you might be just too low on coolant. lack of heat at higher speeds is classic low coolant level.

__________________
81 300CD (sold) 1972 280 SEL 4.5 (sold) 1966 250 S 4 spd (sold) 1974 450 SL (sold) 86 BMW 325ES (sold), 1973 280C (sold) 1988 300 SE.

Really? In my head it doesn't seem like that should be the case. If you were low on coolant, wouldn't the flow be less at slow speeds causing even less heat below 45mph?

__________________
2009 ML320 Bluetec
1985 300CD
1981 300TD

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

Higher speeds robs the heater core of coolant, thus less heat.

__________________
81 300CD (sold) 1972 280 SEL 4.5 (sold) 1966 250 S 4 spd (sold) 1974 450 SL (sold) 86 BMW 325ES (sold), 1973 280C (sold) 1988 300 SE.

Normally that is the case, but these cars need an Auxilliary water pump to maintaijn a consistent temp of coolant going to the heater core. At higher speeds in my 1983, the coolant seems to bypass the heater core on the freeway without the Aux pump kicking in. Actually, this might be your problem too, an Auxilliary water pump failure.

You have a leaky diaphragm in your monovalve. I think the higher coolant pressure at higher revs pushes coolant through the tear and starts to close the valve.

__________________
Michael LaFleur

'05 E320 CDI - 86,000 miles
'86 300SDL - 360,000 miles
'85 300SD - 150,000 miles (sold)
'89 190D - 120,000 miles (sold)
'85 300SD - 317,000 miles (sold)
'98 ML320 - 270,000 miles (sold)
'75 300D - 170,000 miles (sold)
'83 Harley Davidson FLTC (Broken again) :-(
'61 Plymouth Valiant - 60k mikes
2004 Papillon (Oliver)
2005 Tzitzu (Griffon)
2009 Welsh Corgi (Buba)

That's an interesting theory!!!

How, exactly, does high speed "rob" coolant?

In theory, an aux pump failure would cause exactly the opposite symptoms. Its purpose is to improve coolant flow at low rpm. The water pump needs little help at higher rpm.

That is why I am having problems understanding why low coolant would cause LESS heat at high rpms. I ran around all of last winter with NO aux water pump and the first part of this winter. I also wired the aux water pump to a switch on the dash.

When I have no aux pump, it takes quite a bit longer to warm up in the car (I have a thermometer in the vent). It also only puts decent heat out at high rpms.

So in my experience it has been the opposite. High rpms gives me heat the best. So low coolant should not cause the OPs problem.

High RPMS= high coolant flow = best heat

The problem must lie in the monovalve or electrical system somewhere.

__________________
2009 ML320 Bluetec
1985 300CD
1981 300TD

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