Recently I had a problem with heater deciding that while it was somewhere in the teens outside, it would not put out any warm air. After some checking I discovered that the thermostat was bad. I replaced the thermostat (and just added more coolant mixture versus flushing with the outside temp.) and once again had heat. That lasted about two weeks. I am not getting any heat under any conditions – defrost, EC, etc. although it does blow out plenty of cold air…
I have done some searching and found some leads – the mono valve and auxiliary water pump (the main water pump was replaced about 20k ago). My questions are:

1.Is there an easy way to check these to determine if they are the culprit?
2.What other possible causes are there?
3.I have always had problems getting heat from the floor vents. Doing some research it seems that could be attributed to a vacuum leak. I checked online for a mity vac and was overwhelmed by the different models available. Any suggestions on which would be preferable and where to get it? I would like one that is versatile (and reasonable) so that I can use it in a variety of situations in the future.

Thanks for any suggestions.

I have a 300SD and have the same problem, I have learned from another forum that there is a pump or valve in the heater line near the front right shock that may be the culprit in my case, but it is too cold and snowy to get under there to check. Think I will get an early oil change as an excuse to get her up on a hoist.

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Helpful if you list your model and year, however this does sound like classic monovalve issue. The failure is either all heat or no heat. I assume that your engine is at proper operating temp, your thermostat is opening and your radiator hoses feel hot past the thermostat. This is a 10 minute fix on most models
The auxillary water pump only serves to allow coolant to flow while at idle, and not a factor in the "no heat issue"
If your floor vents are not opening you will not get footwell heat. I can give you some advice but will need a bit more data.

Sorry - '89 420 SEL

The temperature is where it should be and the hoses are warm past the thermostat. Should I replace the mono valve or purchase the repair kit - how do you tell which is the right choice?

Any information regarding the lack of heat in the floor vents would be appreciated.

Thanks for the reply.

Ahh .... 89 420 SEL, the car of my dreams
Before you do this repair, just make sure your in cabin air sensor is working. Best done with car running ACC unit on place small piece of tissue paper of very small piece of paper over the screen. It should stay in place due to draw of air.

Unless you see leaking around the monovalve enclosure , the insert is all you need, a 10 minute fix. Make sure the car is cold, open radiator cap, remove electrical connection remove 4 small screws ,pull out and replace.

If you remove the undersides of the footwell area you will be able to peer in with a flashlight and determine if the footwell flaps are opening or stuck closed.
If stuck closed could be the vacuum actuator or broken lever arm. A little bit of a PIA to fix but can be done . On my 84 I have this issue and I get around it by manually opening them in the winter for footwell heat and close them in the summer so my feet don't freeze from the AC