Vannin' Community and Forums Forums Chevy Troubleshooting Heater Hiss

I wanted to see if it’s normal on a 80’s G20 for there to be a hissing sound coming from the heater controls when you turn the van off and after the heat has been running for a while. If i move the slider from “heat” to “off”, it stops right away. Otherwise, it hisses until it peters out. Had the heater core replaced early in the year, but then never really had to use the heater! Wanted to make sure it’s a normal sound. Thanks so much

Heyo .Welcome to the Site. It sounds like vacuum bleeding off. IDK if that's normal, a problem, a leak? Or not. Miy 90 G30 doesn't do it.

You can check lines and vacuum motors with those vacumm pump guns that they sell at HF for brake bleeding. If you need me to, I can crack a book if you need parts, service, or diagram info, but if a vacuum guage on the motor doesn't indicate a leak under the hood issue, And if the controls and heater all function normally?

Could mebe be a hose, or component, but if under the dash? I hope not. Gday!

I would pull the heater/A/C controls out and check the vacumn lines. One is either just loose or maybe cracked.....those lines get pretty brittle over the years.

Ugh, figured it was some kind of vacuum issue. Wanted to ask first before diving in there. Thanks for the replies

Pull the doghouse cover (leave the doghouse in place) and you may be able to tell which side the hiss is coming from, maybe tell if the hiss is from the controls side or the heater side. I like to use a piece of hose as a stethoscope to help track down leaks....

Just thinking out loud here.