Heated washer reservoir always hot?
 

In my 1991 300SL, the windshield washer fluid reservoir always has engine coolant circulating, is as hot as the engine, and is causing the fluid to evaporate.

There are no leaks.

Is the flow through the coil supposed to always be flowing? Or is there some valve somewhere that controls the flow?

If there is no valve, can I simply install one? I am guessing the lines are connected to the heater core lines somewhere.

Strange. None of our three cars have ever had that problem. I thought the coolant always ran through those tubes, because in the winter it has to. Are you sure you don't have a leak? There is an o ring that usually goes bad on those reservoirs and seeps fluid.

on my '92 300SE, there's a valve on the coil that's inside the washer tank. It only opens when the washer fluid gets really cold.

I flushed mine out recently by immersing the coil in some ice water - I think the valve opened when the temp got down around 35 degrees.

The valve is purely mechanical - works just like a cooling system thermostat, I think, but in reverse.

Lots of really nasty stuff came out of the coil - maybe there's something obstructing your valve.

Look inside the tank, if you see something attached to the coil down where it would always be immersed regardless of the level of the fluid in the tank - that's the valve.

Unless you live in really cold climate where something like this is needed, you can probably safely either plug the inlet line, or remove both lines and connect them together for constant flow, but bypassing the coil.

Just be sure to use washer fluid that's rated for sub-zero temps during the winter. If your tank is in the front bumper area like mine, it gets no benefit from the underhood engine heat, so the washer fluid will freeze in at-or-near freezing temps on the road - without the special fluid.

Make sure the seal at the washer pump is good. I had to replace my pump an noted the seal where the pump plugs into the tank. If that is bad fluid will leak out around it. I did not notice mine leaking until I got in there to replace the pump and spotted the wet surface under the tank.

Last year I replaced the bottle, the pump washers, and one pump. I never even noticed the valve when I did those tasks.

There are no leaks, but the blue juice sure was steamy!
I pulled the coil and flushed it - completely clean fluid and no debris came out.

I'll be ordering a new thermovalve shortly.

Thanks everybody
Fred