On 6/24/05, Ed Anderson <eanderson@carolina.rr.com> wrote:
Al, all I can tell you is that the pressure would go immediately (within 2 seconds) to 21-24 psi. Then as the engine warmed up the pressure would drop to a nominal 8 psi or so. It seem likely that the pressure sensor was responding to small variations in the coolant volume. But, certainly open to another explanation. It was not coolant/combustion chamber leaks of that much I am certain. Tracy found the same thing, have not heard any other similar reports until John Slade mention his coolant pressure build up.
Ed
It makes sense to me Ed. Heat the coolant just a little, and it needs to expand just a little. In order for any of the coolant to flow out past the cap (even just a little coolant), it must reach the pressure rating of the cap.
Once the engine is warm, there is no longer any need for the coolant to leave so it does not reach the full rated pressure of the cap.