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From: pwv@fluke.UUCP (Pat Vilbrandt)
Newsgroups: net.auto
Subject: Re: hissing sound under dash, driver's side:  ideas?
Message-ID: <765@vax1.fluke.UUCP>
Date: Sun, 14-Oct-84 17:30:47 EDT
Article-I.D.: vax1.765
Posted: Sun Oct 14 17:30:47 1984
Date-Received: Wed, 17-Oct-84 06:40:21 EDT
References: <1522@wateng.UUCP>
Organization: John Fluke Mfg. Co., Everett, WA
Lines: 38

> I've got a 1979 Chrysler Newport (we're talking Detroit coffin here...) in
> beautiful condition...I just took ownership, and the fellow at the garage was
> quite impressed with the shape it is in.  A minor point, though, has been
> bothering me...there's a very faint hissing sound coming from somewhere in
> the dash, on the driver's side.  ...

Big pig iron sleds of the vintage you describe have a "finger tip touch"
climate control system that uses engine vacuum to operate valves and open /
close dampers for the defroster, heater and air conditioning.  (Some GM iron
had this feature back in the 60's as well.)

Your problem sounds very suspiciously like a vacuum servo / valve / line /
other leaking under the dash.  Since engine vacuum drops under acceleration
and is almost non-existent with an open throttle, it is definitely vacuum
related.  I don't think it has anything to do with the brakes (Careful Here!
I'm just guessing.)  The amount of vacuum allowed to be "drawn off" the engine
manifold for accessories must be limited for performance reasons, so the
hissing could stop just because all vacuum is going to recharge the power
brake booster (tho I doubt it, now that I've thought about it more).

If it is brake related, it probably is the power brake vacuum booster leaking
(this is the big, round canister mounted on the firewall under the hood right
in front of the driver).  The brakes will work just fine with the booster
leaking, it would just require more brake pedal pressure to stop the car,
depending on the volume of the leak.  This would be the easiest to check for,
just remove the vacuum line from the booster, PLUG IT, and start the engine.
If the hissing is gone, you've found your problem.  I wouldn't recommend
driving the car like this, for it will take *a lot* more brake pedal pressure
to stop the car.

Good Luck!

-- 

   Pat Vilbrandt
   John Fluke Mfg. Co., Inc.
   Everett, Washington USA
   { uw-beaver, decvax!microsoft, ucbvax!lbl-csam, allegra, ssc-vax }!fluke!pwv