I have a 1979 W123 250 with no air con. When I'm driving along, even if I have the demister on full, the driver's half of my windscreen fogs up. Also, once when I went through a car wash the whole inside of the car fogged up extremely quickly once I turned the fan on. I suspect water might be getting into the heater/demister system. Any ideas of where to look. Any advice/help much appreciated by a novice merc owner.

Thanks,
Joseph.

Not so familiar with the non-AC versions, as all my W123s have AC, but generally if the fogging is accompanied by a sweetish, antifreeze odor (assuming you are using antifreeze) it's probably coolant seepage from one of the heater cores. Your clue about fogging up the driver's side would tend to confirm this as these cars have left and right heater cores.
Otherwise, water leaks into the interior can build up in/under the carpets & floormats causing serious fogging until the interior dries out.

Happy Motoring, Mark

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DrDKW

Pinhole leak in a heater core. I have had this problem before and the symptoms sound like an exact match.

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Whoever said there's nothing more expensive than a cheap Mercedes never had a cheap Jaguar.

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Does indeed sound likely to be a leak in the heater system on the diver side of the climate control. Does this happen even with the fan turned off? Also might it be possible that the diver side heater control does not completely close the heater flap? The cable might be sticking somewhat.

If you are in a humid environment (not sure about NZ in that respect) you are likely to have the interior fog up irrespective of wether or not you have the demister on or off. In my opinion "demister" is a meaningless term sans air conditioning. I've never been in a car without A/C where the inside did not fog up to some extent when humid. That is until I wound up in Phoenix AZ which is usually bone dry anyway.

- Peter.

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2021 Chevrolet Spark
Formerly...
2000 GMC Sonoma
1981 240D 4spd stick. 347000 miles. Deceased Feb 14 2021
2002 Kia Rio. Worst crap on four wheels
1981 240D 4spd stick. 389000 miles.
1984 123 200
1979 116 280S
1972 Cadillac Sedan DeVille
1971 108 280S