Toyo radiator ( model # 60300 AF ) 's cap puts too little pressure on coolant

If you have bubbles in the coolant, then you have a vastly more serious problem than the radiator cap. Possibly a blown head or manifold gasket that is permitting exhaust gas to leak into the cooling system. It is absolutely NOT normal to have any bubbles in the coolant...given no internal leaks, dissolved gasses are driven off in the first couple of minutes of normal operating temperature after a fill.

| Possibly a blown head or manifold gasket Other symptoms will appear, if either gasket cracked.

| dissolved gasses are driven off To where ? They cannot escape when radiator cap is closed, & will re

-dissolve into coolant ( when cool ), esp CO² ( quite soluble in cool water ).

| normal operating temperature Pls define. My brother's Mercedes 280 has a maximum water temperature of 79° C, but his C200 - 90°C. Japanese cars' tmprtre gauges lack calibration , I can only guess what their water's max tmprtres are.

Under normal conditions the coolant cycles to/from the reservoir when the engine is run. Whatever bubbles collect in the radiator should get cycled out to the reservoir each time the engine warms up. I am more than a little concerned if bubbles appear on a continuing basis more than a few days after the system is filled. OTOH, if it is the head gasket dying the symptoms will soon come on strong, as you say. Within a month or so you should know one way or the other.

A common shadetree check for head gasket failure into the coolant passages is to remove the radiator cap (with the engine cold), start the engine, pinch off the hose to the reservoir and place the palm of your hand over the radiator opening. If combustion gases are leaking into the coolant you will feel the steady rise of pressure against your palm within a couple seconds and there will be no mistaking the pressure within less than 10 seconds. By the time you can feel fluctuating pressure the gasket is very far gone - I don't recall feeling that on a car that still ran.

Mike

| Whatever bubbles collect in the radiator should get cycled | out to the reservoir each time the engine warms up.

Only if cap ( like my Toyo cap ) puts too little pressure on coolant , & coolant gets hot enough for bubbles' pressure to exceed cap's. Even then, not all bubbles will escape ; when bubbles escape, remaining bubbles' pressure will drop til < cap's, then no more bubble will escape.

The amount of pressure your cap provides is directly correlated with he temperature at which it is intended to open. The is technology from, oh I'm guessing, the 1920s. All of the dissolved gas is quickly vented form the system and, if any remains, it isn't causing a problem.

I cringe to ask what you think the problem is here.