WTF? VW stall warning

MAKG1

MAKG1

Touchdown! Greaser!
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As a driver of old vehicles, occasional odd sounds, vibrations, and smells come with the territory. But this one is new.

Driving down to Palmdale for yet another handholding session, I hit 72 MPH on I-5 near Firebaugh. It's hot out, the air conditioner is blasting full, and the radio is on. The, without warning, the stall warning goes off. Or at least it sounds EXACTLY like the electric warning on the Warrior I tried to fly this morning (no drop on the right mag, bug that's another story). As hard as I pushed the steering wheel, I couldn't get the nose down. :)

My first thought was alternator whine, but turning off the radio had no effect. The sound only exists at that one speed; any change and it goes away. It's a B-flat, around 930 Hz. The engine note was also B-flat, four octaves lower (just shy of 4000 RPM), so I'm ready to believe it's a resonance. The sound is coming from the center of the dashboard. All that's back there is the climate control box (evap and heater core), the radio, and an odd vacuum powered set of ducts and doors. I wonder if it's leaking vacuum.

It's a 1990 VW Jetta with an 8 valve 1.8L gasoline engine, West-German assembled, and with enough miles to drive to the moon.
 
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MAKG1 said:
As a driver of old vehicles, occasional odd sounds, vibrations, and smells come with the territory. But this one is new.

Driving down to Palmdale for yet another handholding session, I hit 72 MPH on I-5 near Firebaugh. It's hot out, the air conditioner is blasting full, and the radio is on. The, without warning, the stall warning goes off. Or at least it sounds EXACTLY like the electric warning on the Warrior I tried to fly this morning (no drop on the right mag, bug that's another story). As hard as I pushed the steering wheel, I couldn't get the nose down. :)

My first thought was alternator whine, but turning off the radio had no effect. The sound only exists at that one speed; any change and it goes away. It's a B-flat, around 930 Hz. The engine note was also B-flat, four octaves lower (just shy of 4000 RPM), so I'm ready to believe it's a resonance. The sound is coming from the center of the dashboard. All that's back there is the climate control box (evap and heater core), the radio, and an odd vacuum powered set of ducts and doors. I wonder if it's leaking vacuum.

It's a 1990 VW Jetta with an 8 valve 1.8L gasoline engine, West-German assembled, and with enough miles to drive to the moon.
Click to expand...

It sensed a vacuum on the return trip from the moon. Try to fly without stalling in a vacuum. D'uuh. Sheesh. I should teach this **** for a living.
 
Sac Arrow said:
It sensed a vacuum on the return trip from the moon. Try to fly without stalling in a vacuum. D'uuh. Sheesh. I should teach this **** for a living.
Click to expand...

What's the angle of attack in a vacuum? I'm having trouble figgering the relative wind. ;)
 
MAKG1 said:
The sound only exists at that one speed; any change and it goes away.
Click to expand...

Easy fix.
Just do not drive that speed :D
I will send you my bill.
 
ron22 said:
Easy fix.
Just do not drive that speed :D
I will send you my bill.
Click to expand...

I'd hate to see what this little bugger does at 88 MPH (assuming the hamster doesn't spontaneously combust before then).

If it's a vacuum leak, I'll go searching for it, as those eventually cause "interesting" driveability problems on mass-air fuel injection systems.
 
Its one of two things:

1. Your windshield has a small leak that the air is screaming through.
2. Your radiator cap has failed, and the air pressure at that speed hits just right to cause the air escaping to vibrate the rubber on the seal just right.

I've had both happen, and they both sound identical. Also - you can't trust where the sound comes from - its probably reverberating through the body of the car.
 
SkyHog said:
Its one of two things:

1. Your windshield has a small leak that the air is screaming through.
2. Your radiator cap has failed, and the air pressure at that speed hits just right to cause the air escaping to vibrate the rubber on the seal just right.

I've had both happen, and they both sound identical. Also - you can't trust where the sound comes from - its probably reverberating through the body of the car.
Click to expand...

My guess is an air leak too,

Some experimenting with duck tape could prove useful in finding such a beast
 
The windshield is definitely not original. I have seen any water leaks (even in a car-wash), but they can be hard to spot on the bottom.

The rad cap is holding pressure nicely, and nothing has exploded.

Interesting ideas. I hadn't thought of either of those.
 
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