Here is the problem Audi 100 S 1994 No heat while driving Temp gauge stays on cold, but if you let it sit and idle for say 20 to 30 min. temp gauge goes up a little and it blows heat but then drive down the road very little heat and temp gauge goes back down to cold.

 

Odds are that your thermostat is bad.

 

What do you mean by " stays on cold " when driving? do you mean its in the normal medium or its all the way freezing? also do you hear any weird noise from the engine bay when idleing?

 

its your thermostat. Its stuck wide open so when you let it sit for a the time you stated there is not as much air flowing through it so the coolant warms up. You'll notice too that your gas mileage will be down. Get that replaced.

 

Odds are you need to replace the timing belt also because the thermostat should be done while your in there.

Get a TB kit and do it all.

 

Like others have said, thermostat man, check it out. Unfortunately, the car has to go into service position to get to it. You might as well do the water pump, thermostat and timing belt while you have the front end ripped apart.

It's important not to let this go, the CTS will continually tell the ECU that the engine is cold, causing you to run rich all the time. You run rich long enough and you'll start to causing a lot of build-up on the oxygen and EGT sensors. Mark my words...they aren't cheap.