Heater problem with 1987 420SEL

The car was bought back in the summer and the heater blew hot air. Now that it's cold and time to use the heater it's not working.
I have checked fuses and all are fine. AC works, heater blower works,
but no hot air is coming out. If the car runs for a very long time some warm air will come out.
I have read this newsgroup a bit and heard about a monovalve. Someone also said it could be a thermostat. Could it be a problem with the heater core itself?
The monovalve repair seems to show up pretty often here. Where can I get this part besides websites?
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You can get a monovalve at any MB dealer....but you will get a much better price thru websites. Another possible casue could be a bad climate control unit on your dash, but that is a long shot. Start with the monovalve. ___________________________________________

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Check to be sure that when the car is warmed up the radiator hose is hot to the touch. If not, think thermostat.

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I would check the monovalve first but it could also be the auxiliary water pump. If the pump is seized, it will block the water flow to the heater. Does it get warmer at highway speed?

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I drove the car for about an hour yesterday in below 0 temps. Drove it on the highway and stop-n-go traffic and the thermostat maintained a constant temperature. I'll try repairing the monovalve. I've seen a monovalve repair kit through a MB internet dealer. What exactly with the monovalve that needs repairing? Could I fix without this kit?

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It is quite easy to replace the insert in the monovalve. Getting access to the valve is a little tricky. The problem is that the rubber skirt inside of the valve gets ripped, or the valve does not move up and down, open and closed. So, the kit is the insert and a ring, and it may not repair the problem. Sometimes the magnet in the valve stops working.

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