REF: 1997 A4 2.8Q

I know I have a slight leak somewhere in my system, but by no means has it ever been an issue...My coolant level is perfectly fine right now, but the temp is becoming an issue - or so it says at least.

Over about a 4 mile stretch of city driving (30-45mph), a few stop lights, my temp sensor gradually increases up to about 11/12 of the entire gauge and the light comes on. However, after I drive about a half mile, it will drop down to about 8/12 or 9/12 and then go back up...repeat this process for whatever driving I am doing. When I get back, I'll check the tank and it will be fine with fluid...There is a slight hissing noise that is coming from behind the intake manifold in the back of the engine...

This has been happening for about a week now. Thoughts? Engine Coolant Temperature Sensor (ECT Sensor)? Thermostat? Pressure issues?

Side-Note: It is about 40 degrees outside (avg for the last week) and when I turn the heater up to anywhere above 70-ish (no specifics) and put it on auto, the A/C 'snowflake' signal comes on so I have to switch it to manual. And now I no longer have any heat coming through the system at all...other thoughts? Thermostat? Heater core?

Here are all of my symptoms:
- Coolant Temp sensor is slightly eratic
- Minor coolant leak
- No heat
- Hissing noise in back of engine
- A/C turns on when heat should be on
........

Any thoughts ASAP please! Thanks!

 

So I wrote the beginning post right after I came inside. About an hour later now I went out and checked, sure enough, there is a coolant puddle under my car. I popped the hood and hear the "hiss" still. I removed the coolant cap and found that the hissing stops once all of the pressure is released from the system...

I think that the leak is pressure related now, so I'm thinking to temporarily fix it, I will just need to release the pressure after every time I drive..

More thoughts on what the leak might be now though? I need to be able to fix it myself because I can't afford a mech visit...

(I still need the help with the temp issue too though...)