Two replies suggested the cap could go bad, but my first thought was that the
thermostat doesn't open enough or at all. With only the engine's volume, the
hot, expanding water has to go somewhere, possibly the expansion tank. A stuck
thermostat answers the overheating, too.

PerL
87 Syncro 112i

Eric Small wrote:

> Ya'll,   three weeks ago I performed my annual coolant maintenance on my '86
> GL (180K miles).  I emptied the usual 7+ of the total 18+ quarts and
> refilled with Autobahn coolant/water.  I have done this many times but this
> time a problem.   Everything seemed fine for several days of city driving
> but when I got out on the freeway ...at speed...  coolant began to get
> pushed into the expansion bottle.  Once in the expansion bottle it does not
> get pulled back in as would be normally expected with the expansion cycle.
> If I go too far the expansion tank will overflow.  I have bled the system
> twice and still the same problem.   Also of note, the van is running hotter
> than usual.  I know... all indications are that there is still air present,
> but I have never had this problem in the past.  Also,  I don't believe there
> is any leakage into or pressure from the cylinders, etc. as all other
> operations, fluids, and emissions are normal.
>
> Does this make sense?  Any thoughts?
>
> Eric Small
> '86GL    180Kmiles