Hissing under the dash
Thread Starter
Burning Brakes
Joined: Apr 2009
Posts: 807
Likes: 2
From: VIRGINIA BEACH VIRGINIA
Hissing under the dash
Well I finally got my car running and noticed a hiss under my dash. It is coming from the area around my light switch and bypass valve. The lights do not come up. The vacuum and lights worked fine before I was working on my car and I didn't do anything in the interior. What is the best way to determine if the switch or bypass valve is bad. I checked all the hoses and the are connected.
Dan
Dan
Burning Brakes
Joined: Sep 2009
Posts: 1,124
Likes: 140
From: Western New York
Sorry, I couldn't resist. Just take a length of hose and put it to your ear. Start the car and move the other end of the hose around under the dash until you pinpoint where the hissing is coming from.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
Vacuum hose or fitting problem on the bypass valves, most likely. Remove the lower steering wheel cover, then remove two sheet metal screws which mount the bypass valve plate to the underside of the dash. You should be able to see what the problem is at that time (if it is on the bypass plate). Other possibilities are: the wiper solenoid/switch mounted to the backside of the tachometer gauge and the headlight switch. [...hope it isn't the latter item...]
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
Top 10 Most Expensive Corvettes Ever Sold on Bring A Trailer
Brett Foote
10 Things Every Corvette Owner Needs (2026 Edition)
Michael S. Palmer
8 Most "Only Corvette Owners Understand" Quirks and Problems
Pouria Savadkouei
10 Reasons the C6 Z06 is Still A Performance Benchmark After 20 Years
Joe Kucinski
How Much Horsepower Every Corvette Engine "LOST" in 1972
Joe Kucinski
Top 10 DOs and DON'Ts for Protecting Your Convertible Top!
Michael S. Palmer
Top 10 Most Explosive Corvettes Ever Made: Power-to-Weight Ratio Ranked!
Joe Kucinski
150 hp to 1,250 hp: Every Corvette Generation Compared by the Specs That Matter
Joe Kucinski
8 Coolest Corvette Pace Cars (and Replicas) of All Time
Verdad Gallardo
Le Mans Master
Joined: Sep 2004
Posts: 5,116
Likes: 5
From: Livonia MI
Probably a good idea to get a Mity-Vac test kit for all the under dash vacuum operated stuff in these car's. Better still, a motorized vacuum pump from an HVAC service guy. A lot better than having to run the engine.
Thread Starter
Burning Brakes
Joined: Apr 2009
Posts: 807
Likes: 2
From: VIRGINIA BEACH VIRGINIA
I will pull the bypass valve today. C3 Dave, I don't think the hoses got switched. I haven't drive the car in a couple of months and everything worked fine. I am wondering if I maybe swapped some hoses down by the headlights when I had the lights and radiator support out. Damn.
Thread Starter
Burning Brakes
Joined: Apr 2009
Posts: 807
Likes: 2
From: VIRGINIA BEACH VIRGINIA
7T1vette, I think you are right. Got underthere today and the headlight switch and bypass valve are not leaking/hissing. Something else is. I guess I will pull part of the dash and see what it is. It is not the vents and I disconnected the brake booster and that was not causing it. I am maybe thinking something with the heater/ac controls. More later.
Race Director
Joined: May 2006
Posts: 16,528
Likes: 53
From: Dayton, Ohio
7T1vette, I think you are right. Got underthere today and the headlight switch and bypass valve are not leaking/hissing. Something else is. I guess I will pull part of the dash and see what it is. It is not the vents and I disconnected the brake booster and that was not causing it. I am maybe thinking something with the heater/ac controls. More later.
Burning Brakes
Joined: Sep 2009
Posts: 1,124
Likes: 140
From: Western New York
Like I said at the beginning of your post, just take a 4' length of small diameter rubber hose, stick one end in your "ear" (that's with one r not two) and then move the hose around under the dash, and under the hood, as Roger suggested! Don't start taking things apart and making extra work for yourself(and the chance to screw something else up) until you've pinpointed where the hiss is coming from. If you'd have done this first, you'd have it fixed by now. Good luck! And, we did get dumped on today. I snowblowed the driveway twice today and it's still coming down.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
Vacuum does operate the heater/a/c servos/doors in the interior. There is a vacuum distribution manifold attached to the left side of the heater control [in the lower console]. With the engine running, move the left thumbwheel through all the possible settings and listen for the 'hissing' to change in sound. If it does, that indicates that you have a vacuum leak problem in that system and not under the driver's side dash area. Noting the thumbwheel setting(s) where the vacuum leak disappears will provide clues as to where to look in that system for the leak.
Melting Slicks
Joined: May 2009
Posts: 3,079
Likes: 315
2024 C3 of the Year Finalist - Unmodified
I had a hissing under my dash, so I hooked up a long hose to a secondary vacuum source (my wife's car) and zeroed in on it. Turns out it is the vacuum actuator on the driver's end of the heater box. When I reach up and press on it, the hissing stops. Evidently those actuators (there are 2 - one also on top of the box) have a shaft seal that hardens over time. Next time I rip the dash apart I plan on replacing it. This actuator controls the defroster flap...not necessary in Miami. Meanwhile I just disconnected the vacuum line for the interior heater controls where it tees inside the engine compartment and plugged off the source.
Thread Starter
Burning Brakes
Joined: Apr 2009
Posts: 807
Likes: 2
From: VIRGINIA BEACH VIRGINIA
found the hose. It was inside the engine compartment. It was one of the hoses that comes from the cruise control. blocked it off for now until i find out where it goes. Forgot that I had replaced the cruise control last month and must have left a hose off. Thanks all. Glad I did not have to start pulling dash parts
