| Machinehead
I need your help in explaining what do you mean by saying:
1. I'm still getting air in the expansion tank. Its like when the car is turned off and cools the water level doesn’t balance out and find its natural, even, level as the air in the tank is not escaping, caps open when hot not when cold!
2. I can only think that the aftermarket radiator doesn’t have the tube in the right place and is obscured by the cap. the cap does seal the hole ; again when hot ( under pressure it opens up)..see below.
My question is why is it important to balance and find the natural water level in your expansion tank? Is the car overheating? Is it loosing coolant? What is the exact problem the car has other than this balancing of fluids between the cap and the expansion tank? To sum: If the car is not overheating I won't worry about it.
Just in case I have misunderstood your post please read the following:
A. Please note that the expansion tank cap is like a spring-loaded valve "REACTING" to system pressure. As such it does not have to follow the laws of gravity in balancing the water level as implied above. Thus Its main function (other than the obvious) is to keep the RIGHT system coolant level at CERTAIN PRESSURE.
B. After market caps? Since PRESSURE is the main factor here I would make sure to replace it with an exact cap with the same pressure rating/setting.
C. Due to the relative cheap pricing on caps I refuse the aftermarket even if it was as good as the dealer's.
D. BMWs, at least the 5 series, don't run cooler than other cars in fact they do run on the hot side.
In short: When your cap is sealing coolant and keeping it under preset pressure; it is doing its job. Laws of Gravity do NOT apply here.
Regards
billb | 
