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i bought a really nice 1 owner 1990 300 se it has 140 k on it and everything works great except the heater, it gets warm at idle and then turns cold when you start driving, i have replaced the thermostat and no help, any ideas would be greatly appreciated, thanks.
 

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this web site is awesome and so are the members, thank you so much for the information on the monovalve, replaced it today and warm as toast, i really appreciate you taking the time to help, i had no idea what a monovalve was, thanks again.
 

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I have a similar issue with a 1990 300se. Was not getting any heat or it would come on once in awhile. Replaced the monovalve and was getting hot air in the front vents and defrost but cold air from the rear vent. After rechecking a few days later will get hot air intermittingly. Have been having problems with the heater for a few years. Anyone have any ideas on what could be causing the issue?
 

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That's what I was thinking too but was gonna wait til someone else said it :grin I usually leave mine shut as nobody rides back there anyway
 

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Hi,
Does anybody know the flow direction in the three openings to the mono valve, one at the bottom with a small branch tube and a V shape arrangement at the junction where the plunger supposedly operated. I am assuming the lower tube is for the coolant from the engine/water pump and the two above, one to the heater core and the other from the aux pump. could someone please confirm. I have my MV unplugged but still get heat mostly at low speeds, but have had head at high speeds some time, weird.
Ken
 
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