Here's some diagrams that might help you understand what's in your car:
Look at the diagram at the bottom center of the page and note the
unlabeled device at the top of the picture... that's a metal diverter valve
that sits in the engine compartment and has heater hoses connected to it.
The heater hoses are attached to the nipples shown at the top and right
coming out of that valve, the small hose shown as white in the diagram is a
vacuum hose that actuates the diverter valve to send hot water through
heater core under the dash when you turn on the heater, or to bypass the
heater core and send the hot water back to the engine.
note: This particular picture shows a system without a/c, but I believe
valve is present on cars that DO have a/c as well, but they just don't
on the a/c diagram (upper left on the page), they only show the airflow
I don't have a C3, so I can't verify that.
That valve can stick or become disconnected from the vacuum hose, or
be a leak in the vacuum system that prevents sufficient vacuum from
reaching the valve.
It is a common point of failure. The valve is not particularly expensive
It could be checked by applying vacuum directly to the valve to see if
then works (you should be able to feel that the water hose from the
to the heater is cool before vacuum is applied, then heats up with
You may need a vacuum pump to test with, or you can "borrow" vacuum from
another vacuum source on the engine by "Tee"ing into that good vacuum
(Simply sucking on the hose orally is not likely to produce sufficient
You could also disconnect the two water hoses from the diverter valve
them to opposite ends of one short section of copper tubing (5/8" I
believe) to bypass
the valve for the winter, then reattach them to the diverter valve in