View Full Version : Hissing sound when shutting car off!

01-03-2003, 05:46 PM
When I first bought my car I noticed a hissing sound when I would turn off the ignition, while this was happening the boost guage would go back to the 0 position. Is this normal or not? because my car doesn't do that anymore and I didn't touch anything!!

If its a dumb question then I guess Im dumb cause I know nothing about cars.

01-03-2003, 05:48 PM
Check to make sure the SC connections are all tight. Especially the big nut on the SC top. That noise is often a sign of a loose connection in the super charger lines. The big nut is the most likely culprit. It might have gone away due to the pipes moving around a little.

01-03-2003, 06:05 PM
On this board I learned that the super charger supposed to put out 12psi of boost. How do I check how much boost my super charger is putting out?

Is there a guage I'm supposed to be looking at? or do I use some special instrument?

Forgive me for my ignorance, I wish I learned more about cars when I was younger.

David Neibert
01-03-2003, 06:32 PM
Look at the Vacuum/Boost gauge you mentioned earlier during wide open throttle operation (with gas pedal floored).


01-03-2003, 06:46 PM
Thanks dave,

I had my super coupe for a few years now and I always wondered what that boost/vacumm guage meant. I knew it had something to do with the super charger.

So when I check to see if my super charger is putting out the proper 12psi of boost am I driving it normally when I floor it to check or what? I have a 5 speed so is there a certain gear I supposed to be in when I floor it or do I give it some gas when in neutral to see if guage goes up? Tell me where to look please. I have my car put away for a year now but planing on restoring it in the spring. Unless I go into my garage I dont remember what the whole guage looks like. I think when its idleing the guage is at the 9 oclock position, and as you give is gas the guage moves clock wise.

Someone please explain what the different parts of the guage mean?


David Neibert
01-03-2003, 08:10 PM
The "0" at the 12 oclock position means "Zero or equal to atmospheric pressure" to the left of that is inches of vacuum, to the right is pounds per square inch of boost. The gauge is showing the intake manifold pressure.

To determine how much boost your making, floor it in any gear (higher gear is easier) to put a load on the engine. It should instantly show about 8-9 pounds and climb to max boost at around 5000 rpms.

Hope this answers your questions.


01-03-2003, 10:08 PM
I just started driving my car, the vac is usually around 18-19 on idle and 20 when driving slow,etc. During hard acceleration It was on 10psi and about 5psi on normal, but that wasn't floored or at max RPM. I think my car at least can put out 13-14psi ( Cold air intake, Larger MAF, Underdrive Pulleys, Dual exhaust, Chip and a OD pulley. )

I'll find out soon enough though ;)

01-04-2003, 12:13 PM
Thanks for the reply Dave, that answers all my questions.

01-05-2003, 01:52 AM
I got the hiss as well. I have noticed that this hiss is from the vacuum controlled bypass valve that opens back up upon shutting the car off. When the car is running and being driven, say conservatively, you'll notice the vac/hg (boost) gauge stays more in the -hg area, you actually not using the supercharger as much. Sounds stupid, i know. When you get on it, and see the boost gauge climb, that is when this valve opens. Thus allowing massive flow through the blower, and making our sixers fast.

If ya don't beleive me, open your hood, start your car, and rev it a couple of times while paying attention to the inlet plenum going into the blower. You will notice a valve opening and closing. When i first got my car, this valve got stuck on the connecting hose clamp. No BOOST, freaked me out big time. It was originally intended by ford incase of blower failure. This way, you could still get air into the motor.

In other words, the hiss is the excess pressure being releived from the motor.

If i'm wrong, feel free to correct me, but this is what i've come to know.

01-05-2003, 02:35 AM
THe valve actually closes (there by routing all air from the blower into the engine and not recycling it back into the blower) when the engine is shut off or vacuum drops below about 2" Hg.