Feel your heater hoses as the car warms up and after it warms up. You'll get a good idea what's happening. The hose coming out of the head by the filter housing will be the first to get warm, from there it goes into the firewall to the heater core, then out to the monovalve, down to the aux pump, onto the t-stat housing.

In my 83 when I ran WVO, for heat, I Tee-ed into the hose coming out of the head with 3/4" brass Pex Tee, and I used 5/8 heater hose for the system and used the same 3/4 Pex ball valve.

Thanks for the info, Jarod! If you look carefully at my 2nd pic under my "Epic Fail" post above, you can see that I already have a 3/4" valve similar to the one you suggest. The other parts you recommend I'm sure I can find locally at any hardware store. IF my mechanic does nothing, I likely will do this myself, tackling it this time from the monovalve side. That said, my mechanic, upon seeing the W123 diesel's hose layout believes it would be more correct to put the control where I was attempting to, thereby blocking it at its source if you will. I still may attempt this if he does not; luckily my town has two hose companies here that appear to have bendable heater hose. Always a plus!

Funola, your thoughts are exactly what I would expect of how the heat flows in the system---thanks for confirming my thoughts. Say, in my diagram with this post, do I have what you did drawn correctly? Eliminates having to snake any new hose around the oil filter; great idea!

My mechanic also believes that my "double Bowden cable" or Pex valve idea (as I diagrammed in the second part of my childlike drawings below) is a valid one. This would completely eliminate any possibility of water flow into the core, I believe. 97 320's thoughts were that it could create issues with too much pressure in the system otherwise---my mechanic doesn't believe so, as he mentioned to me, in any standard manual setup, if you turn the temp control all the way down, that shuts ALL heat off to the heater core, but doesn't really affect the operation of the cooling system.

One other note; apparently 5/8" hose will work---from what everyone else has described above. Guess I just need to "finagle" it a little more to do so.

You need only one valve, either location will accomplish the same thing, i.e. stop flow through through heater core. No flow no heat. I would not install 2 valves for same reasons 97s320 mentioned.

Your second diagram is not what I did. I Tee-ed into the hose there to get heat to my wvo system.

I had said that putting a valve at the inlet and outlet of the of the heater core might cause an over pressure / vacuum condition in the heater core as temps rise and fall.

If the core is full of hot water and both valves are closed, as the water cools to room temp it contracts. This will lead to a vacuum that could collapse brass end tanks on the heater core. ( If you have thin brass end tanks. ) I don't know for sure if a collapse will really occur so I recommended one valve. ( If you keep the mono valve in circuit, when the engine is shut off the valve opens allowing pressures to equalize. )

I use a 3/8" pipe brass valve and 6" long piece of 3/8" pipe cut in half. The OD of 3/8" pipe is slightly over 5/8 and slightly under 3/4 making it work on both sizes.

When you are making things fit, don't be afraid of making a 300* loop of hose rather than a 60* bend that might kink.

Thanks for the input, 97 SL320! What you say makes sense; will be interesting to see how my mechanic tackles this---it's at the shop at the moment. I should know more by late this afternoon.

Isn't 3/8" pipe too small to fit the coolant hose?

Pipe is sized to a nominal trade ID not OD. ( just like wood 2 x 4 doesn't measure 2" x 4". )

A 3/8" pipe measures about 0.675 ( this OD isn't tightly controlled like with tubing so it may vary. )

Actual 5/8" 0.625"

Pipe 3/8" about 0.675" OD

Actual 3/4" 0.750"

Chart

Steel Pipe Dimensions - ANSI Schedule 40

This needs to go in the wagon hall of fame next to Pwogaman's $450 wagon save.

What a testament to the durability of the original MB carpet and MBTex.

Nice job, can you work some magic on my 87?

Well...new problem arose today. The existing A/C manifold hose had a cut in it, so...A/C repair is dead until I get one. Believe I have local options to have a new hose made; had difficulty online finding the correct A/C manifold hose for this car. At the moment, monovalve fix is on hold; but...there WILL be an update on this...hang in there! :)

UPDATE: Use clamps instead?
 

I said I'd get back to this post; after experimenting for awhile, I believe my "solution" to this issue works, along with my post regarding Climate Control settings on the ACCIII to keep warm air out of the cabin's ductwork when using windows in warm/hot weather.

My solution? Instead of installing a Pex valve, which involves the chopping of hoses which may or may not be difficult to find replacements if necessary, I went the clamp route (see pic below). First, I unplugged the aux water heater, then put the clamp up on the hose near (but before) the monovalve. No heat was felt out of the ductwork doing this. Later, I placed the clamp on the hose near the aux water pump, and had the same result.

Is it possible that simply unplugging the aux water pump was the "fix"? Sure. Is it also possible I might permanently damage the hose by leaving the clamp? Sure. But to both questions, I think the clamp is a reasonable, non-alteration solution to excess heat in the cabin in warm weather. YMMV.

UPDATE: Clamp a success
 

Just letting folks know that I used that clamp successfully all summer (since August anyway), but the other night when the temp was dipping down in the low 40s, I decided to remove the clamp and test my heat. Back to normal operation, with no markings or damage caused to the hose itself!