I replaced my heater core on my 2000 GMC Sierra last fall and disconnected the AC line while I did it. I was careful not to purge the AC system during this time. I reconnected it and in the spring recharged the AC system. I have very cold air but know I am getting an intermittent hiss (like a pressure release valve is opening up) from under the hood on the passenger side. Only happens when the AC is on and I still have lots of cold air. Any thoughts?

 

Is it a loud, violent noise or just a barely audible hiss?

Allan

 

Sometimes it is very loud like a truckers air breaks.

 

Okay,

One of three things is going on and I'll list by order of probability:

You're overcharged with freon.
The filter (or some other component) is plugged.
The relief valve is actually bad (doubtful).

I'd guess that you have too darned much freon in 'er.

Allan