84 300SD blowing air, but no heat

Hi,
I was wondering whether anyone encountered the following problem: when the heater is turned on, only the cold air is coming out of the vents. I noticed sometimes when the car idles, the air gets hot only by the
windshield vent. Also, the air always gets cold once the car starts moving and never gets hot again.
Thermostat? Heater core? Where? How? How much?!
:)
Thanks!
Mia
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Is the engine's operating temperature 80 degrees C.?
Are both heater hoses hot?
Your car's heat control valve has a screen. It could be that the valve or its screen needs a look.
The valve CAN be rebuilt with a kit if that's necessary.
There's an electric auxiliary water pump that runs only between certain temperatures; it's NOT the problem here.
Alternatively, the climate controller may not be giving the heat valve the ON signal. Check the fuses before tearing into anything.
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Heat control valve is called "the monovalve" .
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check the monovalve insert and solenoid. Hank

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I had same problem. Check the monovalve first, which is easy to remove. Mine had a hole in the rubber gasket. They are not expensive at parts.mbz.org.
Jim
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I've the same problem in my 85 but it's intermittent. I've no idea where a monovalve may be though :)
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Yeah, where is this monovalve in my car?
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I looked around inside the engine compartment and read my manual ... Nothing. I called the mechanic ... He said that because the air is blowing, and the temp gauge is fine, it can't be anything serious. Time to get some heat!
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The monovalve is under the hood a liittle off center on the passenger side. 4 screw hold the cap that has the rubber insert. Hank

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Well, here's a lesson to all of us newbies:
My heater would blow cold air only, guys in this group suggested replacing the monovalve, I didn't listen, I paid the mechanic $170, the monovalve really WAS the problem. I got heat, I am happy, but I wish I got out of work an hour earlier and figured it all out myself instead of spending the cash. Although, I haven't seen my mechanic in 9 months, so I guess it was time ... :)
MJ
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