Bad coolant cap (one way valve is working two ways) or you've got a slight
exhaust gas leak into the system. Probably the cap from the description.

Your otherwise good-working and well-designed cooling system is covering
for you by keeping the temps normal for now.

Jim

On Mon, Sep 9, 2013 at 6:17 AM, Richard Golen <rgolen@hotmail.com> wrote:

> Dear List Members,
>
> Here's a coolant problem for you to ponder. Recently, on my 87 Westy,
> after being driven on the highway for an hour or more, the overflow tank
> gets completely filled. Sometimes, the coolant level is so high that
> coolant sprays from the vent hole on the top of the tank.
>
> The tank slowly goes back to normal level if the van sits for a day or so,
> or if I let it idle for a while after the engine is cooled down.
>
> While driving, the temp gauge is in its normal position. I've also bled
> the system a couple of times using a "bong".
>
> Any ideas as to what could be causing this?
>
> Thanks in advance.
>
> Ric
> Sent from my iPad