Coolant Leak

[Harry]

I'm at a loss and hoping some unfortunate soul has shared this experience.  My 230SL has a coolant leak that I cannot find.  It leaves a small puddle on the floor just behind the drivers side wheel.  It is trickling down the firewall behind the tire (in front of the driver).  Under the hood, the small shelf under the brake booster is wet.  I believe that it must be associated with the heater because I seem to notice it when I have had the heat on.  And sometimes, it is dry.

I recently had the heater core redone and pressure tested before putting the car on the road and before any leak.  When I look through the access hole on the upper firewall (under the removable rubber plug above the stem on the core), I can find no evidence of wetness.  In particular, if the core were leaking, I don't see how it could not be into the car since the core is directly above the heater box.

But - I also cannot find anything along the firewall or in this general area that carries coolant.  Very frustrating.  I'm at a loss.  What are your thoughts/experience?

Harry

[Rolf-Dieter ✝︎]

Hello Harry,

I don't know the configuration of the engine Bay Area of your car since I drive a  280sl, however your description of the leak made me think.

What I share here with you is only a hunch on my part, still I like to share it with you. You mentioned that the leak happens when the heater was on. Naturally hot water would enter the heater hoses and heater. So if this would be my car and issue then I would do the following:

A) I would drive the car without the heater on then park it and see if a leak develops.

B) If a leak developers I would let the engine cool down to ambient temperature then clean up the leak and place a clan paper or cardboard in the same area and see if the leak continues.

C) If you see no leak at A or B then I would continue with D below. Should you have a leak at A or B then I would open the hood and see if I could trace the leak to the origin (perhaps a loose hose clamp or a faulty hose).

Keep in mind that rubber hoses deteriorate and the heat will expand the hoses again and again as you well know.

D) I would then run the car again with the heater at full blast and repeat basic let steps A and B again to confim that the leak only happens when the heater is on.

Keep also in mind that when you drive the car water/coolest leaks my be moved to a different area in your engine bay (fan is operating, draft from the road etc. this easily makes other areas wet and may mislead you as to were the source of the leak is), thus my "B" recommendation.

As I said, it's only a hunch of mine I think you are dealing with a bad rubber hose or a loose hoes clamp that lets some of the coolest out while it expanded then cooles down.

Hope this helps somewhat, good luck in finding the source. Do let us know when and if you find it.

[Harry]

Rolf - thanks so much for the pragmatic recommendation.  I had a very similar approach laid out but I began doing something that you probably assumed I had already done.  I started the car and closely observed for leaks.  My plan was to cycle the heater controls and continue to watch for the errant leak to see if I might encourage it while I was watching.  I did so and I'm thrilled to report that I did identify the leak! It is actually a trickle on the bottom of the line that has the valve in-line.  The leak is on the bottom of that line, where the hose connects.  It is very close to the firewall and the leak is very well obscured from view unless you are really hunting for it.

Such a relief, and should be so easy to repair.  Never-the-less, thanks again for your time and expertise.  Normally, coolant leaks are pretty easy to find so this one was particularly frustrating.  Happy ending though!

Thanks again,
Harry