I tried this and got about 1/2 inch more in the bottle, but eventually the little tube from under the radiator cap blew off. I guess our seal hole drilled in the cap was too tight. The cap for the coolant reservoir in my car is not that tight so air can enter and leave it as the engine heats and cools. So I tried leaving the cap on the reservoir bottle loose so it wouldn't build up pressure. I got about an inch of additional coolant enter the bottle from the radiator. Not having a thermostat installed kept the temp from going above about 1/4 high whereas with the thermostat, it usually runs about 1/2 way.
I should re-install a thermostat to get a better idea of performance.
Agreed , can put the t-stat at boilng water to see if it opens fully, conversely can put t-stat in ice water to see if it closes. This does not check for calibration but only if it functions.
I know that coolant is supposed to return to the radiator when the engine is off and cooling down. I don't understand how it does that since having a vacuum in the radiator would tent to make the cap have a better seal.
At boiling temp the pressure is increased above atmospheric pressure of 14.7 PSI so when it cools off it goes in vacuum assuming the radiator seal is not leaking. Water changes a bit of volume that adds to pressure, although you will always have a bit of air and rubber hoses to expand and contracts so the radiator would not bust.
I noticed that there were bubbles of air entering the bottle from the tube.
That is air entrained in the water also some air bubble when hot water flashes to steam at atmospheric pressure
Having the back-pressure on the cap overflow caused very little coolant to escape the radiator, but I'm not sure if that is good for the cooling system to go above the 13 psi rating on the cap.
It is okay for diesel engine as it is designed and more efficient running a bit higher than gas engine.
In the past it would lose enough coolant to make the gauge go all the way to hot when running a brush hog on a hot day.
just watch your gauges, make sure seals are okay in your water pump and don't allow the temp needle to go over 1/4 passed the middle and read temp using an infrared temp gauge.
using a jar of an old Peanut butter. Make sure to consume PB first. Drill the cap just slightly smaller than OD of the rubber overflow hose to have a tight fitting. Put enough of antifreeze in the jar , about 1/2 inch and do make sure the end of the tube is in the liquid. Over flow when engine is hot is normal. it does not appear you have busted head gasket.This marvel of engineering will collect the overflow when engine is hot and gets sucked back in when the engine slowly cools off. I have had that for more than 10 years. You do need some high quality bailing wire to mount the jar in place securely.
