I’ve been reading a bunch about what appears to be a recurring question regarding the expansion tank/coolant neck caps and their proper placement. I know some people are still running the pressurized cap on the filler neck and I was trying to discern what the fluid dynamics look like under both scenarios (caps in stock placement or reversed). After closely examining my car and how the flow of fluid seems to be intended here is my theory.

The pressure relief cap (pink arrow) is designed with the same 2 valves as most any modern coolant cap. The first valve is controlled by the spring and smaller rubber gasket. when the pressure under the cap gets to 13psi (10psi for UK caps) the pressure is released. The 2nd valve is controlled by the brass colored metal valve (looks like a tiny tea saucer). This valve allows fluid or air to flow in the opposite direction of the 1st (when the system pressure is reduced (cooled) and the system vacuum pulls air/fluid back into the system.

If the caps are positioned in a stock manner with the pressure relief cap on the expansion tank:

– Coolant is allowed to overflow in the expansion tank freely through the drain hose in the filler neck (red arrow) without any specific delta in pressure required. It is also allowed to flow freely back into the system from the expansion tank as there is no valve for it to pass through on its path from the expansion tank through the fitting identified with the yellow arrow and back up the drain hose. As the pressure in the expansion tank reaches the required 13/10psi the valve lifts and allows air/fluid to escape depending on how full the expansion tank is. Unless the expansion tank is completely full, or an extreme over-pressure/boiling situation develops, fluid should never rise to the level where it would vent to atmosphere (through the green arrow vent hole in the filler neck of the expansion tank). When the system cools air will be pulled from the atmosphere through the green arrow fitting and back into the system to equalize the pressure through the 2nd valve mentioned above.

If the caps are positioned opposite of stock:

– Coolant must reach 13/10psi before it is vented into the expansion tank. The expansion tank always remains un-pressurized because the other cap does not seal the system from the atmosphere and air is free to escape through the port identified by the green arrow. In a over-pressure/boiling situation the pressure relief cap allows coolant to escape through the drain hose (red arrow) and down into the expansion tank which fills up displacing the air and venting it to the atmosphere. In an extreme over-pressure scenario when the coolant fills the expansion tank to the filler neck, the coolant will be vented overboard. When the system cools again, the fluid is drawn up the drain hose through the fitting identified by the yellow arrow and then into the filler neck by way of the brass secondary valve.

In both scenarios:

  • Pressure is mainntained in the system until the prescribed threshold.
  • Fluid is drained first into the expansion tank and then out to the atmosphere in an over-pressure situation.
  • Fluid can be drawn back up into the system by vaccuum created as the system cools thus keeping the system full at all times.

Results: I don’t think it matters what way around the caps are placed for proper engine cooling to occur because both the filler neck and the expansion tank neck have a lip on the inside (blue arrows) to accept the gasket of valve 1. However, the system was designed to have the pressure relief cap on the expansion tank. We can tell this because the expansion tank is made to withstand the 13/10psi of the pressurized system. It is metal and there is no other reason for it to be metal other than to withstand the pressure of the system. Also, the expansion tank neck was made with the proper lip to accept the dual gaskets of the pressure relief cap. It appears the the design intends for the expansion tank to act as a part of the entire system instead of merely an overflow tank.