Another ST1100 Mystery From The Past

When my 92 ST1100 had about 20k miles on it, the coolant in the overflow tank began disappearing. Maybe twice a year it ran dry and I refilled it. There was no hole in the tank and the clear hose from the tank to the radiator was intact. I asked my riding buddies to look for white smoke coming out the tailpipe, but they didnt see any. I did get a wiff of coolant from time to time, but never saw any leaks on the ground or on the inside of the fairing pieces when I removed them for maintenance. Not a drop.

I just figured it was one of those things but kept an eye out for leaks, white smoke etc.

Then one winter I added a manual switch so I could turn on the cooling fan at will. Riding in 90 degree plus weather on the open road, the coolant gauge would always read about 1/3 of the way to redline. Get in stop and go traffic, and the needle would go to about 2/3 of the way. Sometimes 3/4. But it never overheated. At least according to the gauge! After adding the switch whenever I got into town on a hot day I would turn on the fan before the needle started climbing. After a little experimenting I found that running the fan for two or three minutes kept the needle in place. Then you could shut the fan off and the thermostat or whatever would keep the needle in the same place as riding the open road no matter how hot it got.

I never had to add coolant to the overflow tank again after that! Never caught a wiff of it again either!

But I never figured out why. Any ideas?

I'd guess ( emphasise guess ) the pipe from the radiator to the overflow/ expansion tank was indeed leaking either at a clip or unseen hole but by self controlling the temperature you are minimising expansion/loss at the location of the leak, wherever it may be. You don't smell it if it ain't leaking. If the coolant flow from rad to tank has minimised then so as the leak. WELL DONE.
Upt'North.
Although someone will say you should still fix the leak!

There has to be evidence of a coolant leak somewhere on that engine. Invariably, you will not see any drips of coolant with a slow leak, as it dries very quickly against any hot surface, but you would find dried residue that might appear as a white, or greenish, trail. The hoses under the carbs are mostly hidden and those hose clamps were a bit loose and notorious for causing weeping in the early years of ST production. This leak leaves a residue trail on the rear of the engine, not easily seen and a place you may have overlooked.

Why it stopped leaking may indicate something else, not good. Those hoses under the carbs live in a very hot environment. Heat from within the hose (coolant temperature) and externally (from the engine) can deteriorate the hose structure, causing it to swell over time. This may have helped to stop the slow leak, but also means that the strength of the walls of the hose has diminished. A sudden blow out (usually in the middle of nowhere) of a swelled hose is a possibility. Have you ever changed them?

[/QUOTE Never changed the hoses. Sold it in 2001.[/QUOTE]

Wish you'd said that in your first post!

Sorry! The good news is it wasn't necessary. I wonder why those hoses leak so much anyways on STs? Never had a coolant hose leak on a car in40 years.