Uh...not so fast, guys.  Even if the thermostat made a tight seal, which it
may since it has a rubber gasket, there's a honking great pipe coming
*back* from the radiator -- or is this special VW directional pressure?
<g>  In fact, looking at the diagrams and drawings on 19.10 and nearby, it
appears that the thermostat main shutoff -- the one with the gasket --
operates to prevent water from flowing back from the radiator to the pump,
not the other way around.  I believe the other smaller-diameter end
obstructs the flow from engine to radiator, but it's hardly a tight
seal.  This would certainly reduce transient response when cold, but
shouldn't give a problem in measuring static system pressure.  Response
time would depend on how well the thermostat seals and how much volume (a
pint, maybe?  I dunno) would have to pass through to equalize the
pressure.  Might be different with plastic vs metal lines.

There must be some pressure drop across the system when the pump is
running, though, so you'd get an RPM-related pressure change of some sort
that would increase as you put your measuring point closer (in circuit
terms) to the pump outlet.  If that's an issue it looks to me as though the
best place to tap on an '86 would be right off the rear bleeder valve, i.e.
tee into the small outgoing line from the valve, leave the original valve
open all the time, and add a valve inline for the bleeder function.  That
would give you a nice shutoff in case your pressure line leaked.  Or tap
into the thermostat cover itself.  But otherwise I think either the
radiator or heater lines would be ok to tap.

For quick-and-dirty testing (not an installation) just wedge open the
little round return valve inside the expansion tank cap, and run the
overflow line to the gauge instead.  Just remember this loses your
overpressure relief.  If you have a gauge like mine with a valve-controlled
outlet you can deal even with this.  Manually, of course.

david

At 11:11 10/23/99 , Alistair Bell wrote:
>Doh! of course Per!
>
>Ah well , it seemed like a good idea.
>
>Alistair
>
>
> >The heater core is in line with the engine, not the radiator, so it will
> show
> >the same pressure that's currently in the engine. The radiator doesn't
> >have any
> >pressure until the thermostat opens.

David Beierl - Providence, RI
  http://pws.prserv.net/synergy/Vanagon.htm
'84 Westy "Dutiful Passage"
'85 GL "Poor Relation"