I just bought a 2005 (or 2006?) Audi A4 1.8L that has a very small coolant leak coming from the valve at the top of the coolant pipe that runs over the top of the intake manifold. I see pictures of engines that appear to have a cap over this valve; mine does not have a cap. The guy I bought it from said he tried to stop the leak with some glue, but that failed. He said I could probably just cap it off with some JB Weld, but I am skeptical...

My question really is just: What is this valve for? I suspect that because it is at the high point of the coolant system, it might be to bleed off air bubbles that get trapped in the system. It might also be to easily connect a pressure gauge gauge to it when diagnosing problems. I suppose it could also be an over pressure release valve, but I doubt that... If anyone can tell me what that little valve is for, that would be greatly appreciated. I am trying to determine the importance of replacing the entire coolant pipe at this time...

Thanks, Damian