I have heard that a crack in an exhaust manifold can cause harm to exhaust valves. The reason told to me was that cool air can be sucked in through the crack which can cause shock cooling damage to the valves.
I may have also read that leaks between the head and the manifold are more harmful in this way to the valves than are cracks in the body of the manifold. I assume this would be because cracks in the manifold are further from the valves than a leak between head and manifold???
Can anyone shed any light on this? Thanks.
Tom