I'm curious as to why your temperature would go up to 90 and then drop quickly when you
rev the engine. Do you have a thermostat installed ? Do you have an electric cooling fan ?
If your radiator overflow hose went all the way to the bottom of your soda can, and you fill
the radiator to the the top of the neck, the can should act pretty much like the OEM system.
You would have to partly fill the can with coolant to start with. When you first start the
engine after setting this up, the engine will blow coolant out into the can as it heats up, and
then suck coolant back into the radiator when it cools down.
If the engine overheats it will start blowing steam out the overflow and you will see bubbles
in the can and some steam being given off. If the engine has a coolant leak the level in the
can will drop slightly every the car is used until all the extra coolant gets sucked out of the
can. Shrtly after that the radiator will no longer be filled to the top of the neck and the
level inside the radiator will start to drop.
The missing coolant can only go two places (if there are no external leaks). Either into the
cylinders or into the oil. Since all engines blow a little bit of steam or condensation out the
exhaust when first started on a frosty morning, that will not make it easy to detect a
small coolant leak into a cylinder. In warm weather any steam from the exhaust just gets
absorbed into the hot exhaust and becomes invisible.
If coolant leaks into the oil, it will collect in the bottom of the pan. As the oil circulates and
the pan heats up, the water will get mixed up with the oil. As engine oil temperature is
typically higher than the coolant temperature, with extended engine operation the water
will start to boil off, turn to steam and get swept out of the engine with the blowby. This
is how it gets into your catch can.
During normal combustion there is always a small amount of water produced, and this is
what you see out the exhaust on cold mornings. A very small amount of this steam will
get past the rings with blowby and eventually find its way to the catch can. But if your can
collects 90% water that is not a good sign !!!
A second effect of water in the oil is the formation of a kind of water/oil emulsion that
will collect and condense on cooler parts of the engine. Inside the oil filler cap and inside
the cam covers for example... you will get a kind of creamy "mousse" collecting. A healthy
engine will not have any mousse inside the filler cap.
Ultimately I think you will will need to fit a new head gasket. However if the engine is
running fine it may be worth trying some kind of "stop-leak" in the radiator. Products like
this .. https://www.goldeagle.com/product/aluma ... ak-powder/
are effective on small leaks and less likely to clog heater cores etc. You don't need to add
all the product... maybe half and follow the instructions on the packaging.