W221 Heater Matrix

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Active Member
Aug 31, 2022
2006 W221 S600, 1983 VW T25/T3, 1984 Alfasud 1.5ti Green Cloverleaf, 2008 Chrysler 300C SRT Design
I have a couple of symptoms that seem to suggest a leaking heater matrix or pipework in my 2006 W221 S600. I'm looking for advice/information on how much of the dash has to be dismantled to get eyes on it and then replace it, as well as suggestions as to whether something else could account for the symptoms.

Symptoms are a very slow loss of coolant, and misting up inside the car if left parked in the sun. No coolant smell inside the car as far as I can tell. I can't find any wet or even damp carpets, but I could imagine that any moisture could still be below the carpets at the moment, if there is indeed a leak. No sign of moisture on the seats below the sunroof, or in the headlining around it. The misting up isn't consistent - the car hadn't been driven for several weeks until I took it to the NEC yesterday, about an hour's drive away, and at the start of the drive the rear screen was heavily misted up. I returned to the car late afternoon and all the windows were misted up. I ran the engine for a few minutes with the windows down to change the air. After a morning in the sun today it isn't misted at all. Could there be a coolant leak, or a rainwater drain leak into the air vents somewhere, such that yesterday's outbound journey with the heater on dried out the vents but made the interior air humid, and on the return journey everything was dry? I'm not discounting the possibility that the coolant loss is a red herring - perhaps there's a small leak elsewhere in the engine bay.

But assuming the worst, and it IS a heater matrix leak, can anyone say exactly where it is behind the dash (centre? passenger side?) and what has to be removed to access it? Google searches for 'W221 heater matrix' seem to bring up a lot of links for the heater control valve. That seems to be in the engine bay below the front screen, but could that somehow explain my symptoms?

I can add that ever since I bought the car (spring 2022) I've struggled to get the car warm on cold days - the footwell always seems cooler than the upper cabin. If I direct the air onto the screen it's plenty warm enough, but there never seems to be much warm air flowing into the footwells. Could this be due to a faulty heater control valve, or is it more likely the flaps that control the vents?

Any help to diagnose these symptoms would be welcomed.

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