In my limited experience with an American car, this is caused by low
coolant level.With the engine at idle, the level is too low to reach the
heater core. As the RPMs come up, the water pump flogs the coolant more
strongly, and some of it reaches the heater core inlet.

Possibly also the water pump itself - or the belt driving it.

Bill

The cabin heater matrix has a thermostatic valve in. When the
temperature comes back down (while driving) it is allowed back into
the cabin.
Does the temperature rise up to an indicated red on the gauge? Is the
radiator fan working? Check this by baring back the terminals on the
radiator thermostatic switch and bridging the terminals with something
metal (insulating yourself from the voltage, of course). Does it come
on at about 2/3 - 3/4 up the gauge? Check the fuse.
One final things - there is a small hose that runs from the top of the
radiator to the coolant expansion tank - feel it. Is it cold? If so,
it's blocked ... most likely at the expansion tank end spigot. That
may cause an airlock where the radiator thermostatic switch is, so the
radiator will not work ... overheating.

You can change the thermostat - go for an 89 degree one. Might as well
anyway for what they cost - check out the things above and report
back. We should be able to get this one sorted between us.

"John" <john@myhmg.org> Jan 26, 2008 at 09:13 AM

a

I second the previous comments about coolant levels, temp senders, fans
and the like. It may also be worth checking for a collapsed hose -
especially the very long one. Cheers


--
Message posted using http://www.talkaboutautos.com/group/alt.autos.saab/
More information at http://www.talkaboutautos.com/faq.html

John ("John" <john@myhmg.org>) gurgled happily, sounding much like they
were saying:

No, you've got the RIGHT 'stat installed.

The genuine Saab 'stat has three stages, not the usual two.

1. Block only, warm-up.
2. Full circuit, normal.
3. Rad & block only, over-temp. Heater is cut out of the loop.

First call would be to change the coolant and flush it all through -
there's a block drain under the exhaust manifold (probably seized) and
the rad drain under the RH (US passenger side) of the rad.

When you've got nice shiny new coolant in, turn the ignition on (don't
start the car), unplug the two plugs from the rad temp switch (top LH of
rad) and short between 'em. The fans should come on. If not, that's your
problem. If they do, then start the car, and run it up to temp. If the
fans don't come on as it gets to half-scale on the temp gauge, then the
switch is at fault.