W123 bypass hose
 

I have been dealing with hose leaking. I have replaced the short bypass hose with new replacement hose. It was a very tight fit. I didn't shellac it or anything. I just put 2 new ss clamps on it one on top and one on bottom. The issue is if you run the heater, and warm it up it will leak out the top of that hose. I did a running test last Saturday like 100 miles round trip, and it didn't leak at all without the heater on. If I run the heater, it will force coolant out the top of the hose. The hose that was on there, that I replaced was just mushroomed out at the top. It's trying to do the new one the same way. I don't understand why it would have more pressure etc. with the heater in the string? I don't understand why it would make any difference. Any help would be appreciated!

Was there a load of powdery corrosion around the housing?

If so you need to clean it off.

Hylomar might help with the sealing if you have some / can get some

For sure, the difference is not pressure related. It might be flow related; since the heater core acts like a radiator, running the heat might flow more coolant through the bypass instead of to the radiator. Either way, neither flow or pressure is the root problem; the poor hose connection is the problem.

Re: Thread Title---"W123 bypass hose"

W123 indicates a chassis type.
Is an engine involved?
Is a specific year model involved?
Is the bypass hose a part of the chassis, or part of the engine?
Are guesses and assumptions substitutes for facts?

Both of the bypass hose nipples on mine were not only white with corrosion but deeply pitted.
I wire brushed and pick out all of the white corrosion and filled in the pits with JB Weld (the slow curing one that can take the heat).
That was back in 2008 and have had no issues in that area.

Wet it with WD40 and install. It will melt the rubber a little and weld it onto the pipe.

Thanks for the help on this. I felt that the issue was still with the sealing on the hose to the pipe connection. I will have to re-do it like you suggested. I appreciate the feed-back!

Water Jacket Hose Leaks
 

This is a really common problem, often your best move is to remove all those alloy hose nipple bits and wire brush them until clean and shiny .

Then you may or may not have to either build up the deeper pits with Epoxy (JB Weld etc.) then sand them smooth using 1,000 or finer grit abrasives or, by the parts new and remember to change the dang coolant every three years or 30,000 miles like you're supposed to .

I had to suffer this on several of my earlier W123's both my NA 617 and my '82 616 engine (still have that one) needed replacing of almost all the detachable hose nipples .

I used silicone hose for that, from ~3 ft I bought on ebay. I recall 42 mm, but don't quote me. My nipples weren't very pitted, but silicone might seal better since more flexible. I got it because it should last much longer. I doubt there is more pressure with the heater on. In a 300D, (only W123 diesel I know of), there is just the mono-valve in the heater loop. But, even closed, the line pressure would be about the same at the bypass location. There is also the Aux Water Pump (unless you removed it like me), but doubt that builds much pressure.

It's hard to believe, but there is a four-cylinder diesel version of the W123. It's known as the 240D.

...and 220D...
...and 200D...

(But I'm sure it wasn't meant like that!)

Cooling System Pressure
 

Bill ;

No worries, everyone only knows one when first they begin .

One of the first things taught to Mechanics (and often the first thing they forget) is : in any hydraulic system, absent valves to create differences, the pressure remains equal throughout the system .

So, all the coolant hoses have the same pressure .

I'm surprised the silicone hose didn't kink when you forced it into the 'S' bends necessary .