Might I ask how this is supposed to work, since the cap's lower seal should always be pressed and sealed against the bottom of the filler neck? Only if system pressure exceeds 14.7 psi should air pass through the drilled hole; of course there's already an expansion tank outlet hose for this purpose. I know from wrestling with cap issues as of late.
hi
no,no,no, sorry for the misunderstanding, the hole has to be made in the cap´s lower plate, to avoid the sealing of the corresponding seal that you mentioned
the idea is exactly the same as leting the cap open, but as you will drive the car, this way you will avoid coolant spils while you driving limiting the escape of air bubbles or eventually some liquid to a minimum
some cars that have a cap in the top of the radiator use a device with a hole in the cap in which a tube is placed that is fed from a bottle full of coolant placed as high as you can
the objective is to let the circuit move well the coolant through the intrincated and complex volumes of the M30 engine (increased if you have a CS with coolant passing through carburetors)
let me copy-paste part of an old post about this:
"""""reason for the problem: air bubbles, so incorrect bleeding, but...
how is this possible if you consider that I have been bleeding the circuit through the apropriate screw up to a dozen of times ?
the answer is: the bubbles formed inside the circuit do not move evenly towards the "exit" sign as if they were schoolboys, so you have to help them in the way out through the "other" egress mean,...eh ?...what ?
yes, say it aloud, you´ve got it ! the expansion vessel !!
but how can you do that ?, it is possible and easy, but you need a new cap, because the old one has to be tortured !
you have to drill a small hole (I did 3mm hole) in the lower part of the cap near it´s center, this hole will allow air bubbles + some coolant pass to the cilindric chamber formed by the neck of the vessel and the cap itself, in that reservoir you have the opening hole that allows excessive presure go out through a tiny tube
pepared this apparatus you start the engine and carefully pay attention to warming up of engine, tubes, radiator,...take a look on the gauge !, and so on
after a while you will see that the liquid + bubbles are starting their way out through the tiny tube, don´t worry, but look again and again to the gauge, check tubes,...
you should accelerate to force the coolant move quickly and this way take the bubbles from inside the engine to the vessel
check the coolant level, the vessel should be full anytime, you will drop away some, don´t worry, if there is enough in the vessel no harm will be produced, and you are not going to spill so much liquid in this process
after some minutes you can assume that the bubbles are away, and then, you can take off the drilled cap, and replace it by the new one, and that´s it !""""""
Is it now clear ??
regards
