So, do you mean that under all conditions, there is coolant flowing through the heater core on its way to the radiator?

The coolant won't be flowing if the monovalve is closed, but the heater core will be subject to the pressure present in the cooling system.

Unless one's sense of smell is significantly compromised, detecting a heater core leak while driving won't be difficult.

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Ah, okay.
And is this right?: The monovalve is closed by default (no power is applied to it) but opens and allows coolant to flow to the heater core when power is applied (pushbutton unit is activated into Heat mode)?

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My sense of smell is working just fine.
Which then tells me: If I am not smelling coolant in the cabin, then there still must be a leak of coolant somewhere in the engine compartment.
Sheesh...what a puzzle and nuisance!

I still have the cooling system pressure testing tool so I may take another stab at the problem by re-pressurizing the system and looking for leaks again...

The monovalve is closed electrically and is spring-loaded to the open position when not powered.

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When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.

Ah hah!
My understanding was only completely opposite of the facts. Facts duly noted. Now eating humble pie.

Thanks for steering me right!
:-)

and if measuring with a meter, the monovalve will have +12v all the time the key is on, but the -12 is modulated by the climate control to energize the valve closed...

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
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My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

If it really bothers you, switch to NPG and run the system at zero pressure. It would be a lot easier (and cheaper) than changing a heater core that is probably not leaking. If it was leaking enough to drip from the evaporator drain, you would definitely smell it and your windows would be fogged with the residue.

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'83 300D, 126K miles.

vstech:
Thank you for the info. about the electrical. Got it.

rscurtis:
I'll plead ignorance: What would you mean by "NPG"?
Also, yes, I agree, if there was a heather core leak, I would likely be smelling the coolant and there is no such smell in the cabin.

Update:
I was conversing with a long-time M-B tech. in my town today and this is what he told me:
1. The expansion tank cap may not be holding pressure and could be causing a little leak. Consider replacing it. It was last replaced 9 years and 44,000 miles ago.
2. Check the tightness of the clamps for the hose coming off of the expansion tank.

So, I have a little more detective work to do.

Thanks-

Just for kicks, stick the probe of a volt meter into the coolant and ground the other probe. Any voltage approaching .3V indicates electrolysis is going on and the coolant needs to be changed. So says the destructions in a heater core that I had. The core manufacturer was obviously trying to prevent premature warrantied failure.

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85SD 240K & stopped counting painted, putting bac together. 84SD 180,000. sold to a neighbor and member here but I forget his handle. The 84 is much improved from when I had it. 85TD beginning to repair to DD status. Lots of stuff to do.

NPG- non-aqueous propylene glycol, or waterless coolant. Home » Evans Cooling. Watch the video with Jay Leno.

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'83 300D, 126K miles.

Where is the evaporator drain on a w123? ThNk you.

The drain is inside the car, just above the transmission tunnel, behind the dash. There is typically a short foam piece that carries the draining condensate out of the car through a hole in the floor. It drips on top of the transmission.

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1979 300D 040 Black on Black - 1985 300D Maaco job (sadly sprayed over 199 Black Pearl Metallic) on Palamino

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Sometimes the pressure tester itself leaks. Test again and consider it fixed unless you find wet, lose fluid or smell coolant.

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85SD 240K & stopped counting painted, putting bac together. 84SD 180,000. sold to a neighbor and member here but I forget his handle. The 84 is much improved from when I had it. 85TD beginning to repair to DD status. Lots of stuff to do.

So, I have pressure tested the system three times:
The first time, I found several hose clamps (as I mentioned above) that were leaking due to not being tight enough. I tightened the clamps and eliminated the leaks.
The second time, I pressure tested the system about 14 hours after driving the vehicle. I kept the pressure on (none added) for about 30 minutes. No leaks found.
The third time, I pressure tested the system about 18 hours after driving the vehicle. I kept the pressure on (none added) for about 30 minutes. No leaks found.

As I also mentioned above, I plan to replace the expansion cap and check the tightness of the hose connected to the expansion tank.

At the present time, no signs of coolant found anywhere, no smells of coolant.

Good stuff from the sound of it.

__________________
1979 300D 040 Black on Black - 1985 300D Maaco job (sadly sprayed over 199 Black Pearl Metallic) on Palamino

http://i.imgur.com/LslW733.jpg

The Baja Arizona Oil Burners Send a message if you'd like to join the fun
Left to Right - UberWasser, Iridium, Stuttgart-->Seattle,, mannys9130

Visit the W123 page on iFixit for over 70 helpful DIY guides!