I had a similar problem with my '88 GL, and I think that Per Lindgren hit the nail
on the head as to cause.

I tried a new pressure cap first. That didn't do it, so replaced the thermostat,
problem solved.

Over the years a problem that has appeared fairly consistently for me is a failed
or failing thermostat. They can introduce some strange symptoms. IMHO thermostats
should be replaced routinely as part of a preventive maintenance plan.

John Rodgers
"88 GL Driver no longer in New Mexico

Per Lindgren wrote:

> 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