I have a 85 300D. Seems as though when the heater is on, the slower I drive the hotter the air gets. As I reach highway speeds, the air cools dramtically.

What is the engine temperature doing?

I think the problem is in your monovalve. The monovalve sits towards the back of the engine on the passanger side and it has a black plastic top which is an electrical connector. Heater hoses run through this thing also, so it is easy to find. I think if you pull that top off, clean up the electrical connector it might well solve your problem. I had to do that about once a year on my 85 300D.

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Junqueyardjim
Christianity, if false, is of no importance, and if true, of infinite importance. The only thing it cannot be is moderately important. C.S. Lewis



1983 Mercedes W123 240D 4 Speed 285,000 on the road with a 617 turbo, beautiful butter yellow, license plate # 83 240D INDIANA

2003 Jaguar Type X, AWD. beautiful, good mileage,
Mom's car, but I won't let her drive it!

Nothing...............

You may also find it is damaged/leaking internally. There is a rebuild kit available.

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David Bruckmann, Oakland, CA

Current:
66 Citroën DS21
72 MB 280SEL 4.5
73 MB 450SLC
76 Citroën 2CV6
76 MB 300D
79 MB 300SD
Past:
70 Citroën DS21 (170kkm)/71 Citroën DS21 (137kkm)/72 Citroën DS21 (502kkm)/78 MB 300D (1,200,000 km-ish)/79 MB 300D (425kkm)/79 MB 300TD (260kkm)/81 Citroën 2CV6 (120kkm)/88 Merkur XR4Ti (209kkm)/81 Peugeot 505 GRD (350kkm)/84 MB 300TD (385kkm)/85 Toyota Camry (330kkm)/86 Renault 9 1.7L (155kkm)/02 Golf TDI (74kkm)

If the temperature is normal, I agree that you should look at the monovalve.

x3 on the monovalve. it's very easy to install the repair kit and you have the classic symptom of decent heat at low speed and no or low heat once the road speed gets higher.

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punkinfair

thats interesting...usually the monovalve breaks open. You know how when you stop holding something it tends to fall. The monovalve holds back the heat so when the diaphram rips it lets heat in. I suppose its possible to break closed but that would be a bit on the rarer side.



Pull that puppy and check the rubber diaphram. Then give it 12 volts and see if it moves.

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77' 300D, "Cartman" SOLD @ 150K (didn't know what I had)
83' 300SD, "The Superdon" 325k+ @ 28mpg
95 E320 wagon, "Millennium Falcon" 231k+ @ 24 Mpg
95 E300D, "Sherley" 308k @ 33.69 Mpg, currently anticipating a head
99 Suzuki Intruder "Trudy" @ 45 mpg

For whatever reason, they do have a failure mode where they work at low speed and not at high speed. Ask Brian, he's been through this.

Once a monovalve is replaced, how long do they last usually?
Anyone have a newer one break yet?

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jeepers, if mine was that easy to access i would pull it out and put in a brass ball valve
summer off and winter on! win win

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[B]AlanT
BELEN NM


93 dodge cummins truck~old reliable
01 Volvo AWD XC70 Turbo Wagon
74 450 SE on the way out!
Recent: 1972 Toronado 56k on the clock!

IF it plugs in, lights up, makes noise,
I'M ON IT!

I replaced mine once, years ago; they should last for a while.

monovalves can fail open, closed, stuck in the middle... lots of failure modes.
another possible issue is the aux pump could be seized. if it's seized, the heat would work low, but not high speeds. if it's the pump, it can burn out your push button unit, so find out for sure.

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

That entire analysis is quite flawed. Hands down, the most common affect of a torn diaphragm is normal heat at low rpm and poor heat at high rpm. The diaphragm doesn't block the normal flow of coolant; it seals the solenoid side of the valve. A torn diaphragm allows pressure to build on what should be the dry side of the diaphragm. The pressure overcomes the force of the solenoid and causes the valve to close, resulting in poor heat when coolant flow is high, as in during high engine rpm conditions.

Actually, it's the other way around.

I think I'm on my 6th monovalve so that's an average of about 4-5 years each. Some lasted longer than others.

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Charles
1983 300D, bought new, 215k+ miles, donated to Purple Hearts veterans charity but I have parts for sale: http://www.peachparts.com/shopforum/mercedes-benz-cars-sale/296386-fs-1-owner-83-mb-300d-turbo-rebuild-parts.html