Whence the wolf-ghost howl?

A caller on today’s show reported a sound like a wolf ghost when a) the tank is partly empty and b) she puts on the brakes. Click and Clack some kind of sympathetic vibration in the gas tank amplifying some vibration in the brake system. Here’s another thought

When you put on the brakes, gas is pushed toward the front of the tank. To make room for the gas, vapor is pushed toward the back. If the gas tank were just an open tank, this push would cause the gas to slosh around. To prevent this, gas tanks have baffles in them to retard this motion. Thus when you are decelerating, gas and air are moving through these baffles. Now suppose there is a loose place in one of the baffles so that the air rushing through it sets up a vibration. It would make a sound, like some kind of windy harmonica, or maybe a wolf-ghost howl.

How to test it? Any acceleration that pushes gas around in the tank should have a similar effect. One alternative is to go around a curve fast enough that you feel pushed towards the outside of the curve. If you hear the howl in such a case, it supports the theory.

How to fix it? Don’t fix it! Sell tickets to hear the sound! Make a CD! Auction the car for big bucks: the car that stumpped both Click and Clack.

When the fuel pump is fully submerged in the gas the pump vibration is absorbed in the gas. when half full and braking the gas moves to the front of the tank,only partialy submerging the pump hense the harmonic vibration=(Noise). Now the alusive wolf comes to life. The pump will get louder as it gets older. Which my turn the wolf whistle into a growling bear. this can be fixed by removeing the pump and striking it through the heart with a wooden stake. It will have to be a hard wood such as oak is the metal will resist a willow branch. You can try having the pump exorsized, we have tried this in the past but have had a zero sucess rate. Mabey we should try a different priest. After buring the corpse install a new pump that has been blessed with holy water. ( avoid the full moon as not to atract spirits that are looking for a place to inhabit ). Good luck. Bill from Baton Rouge

That sounds plausible. I didn’t know the fuel pump was inside the gas tank!

If it is the fuel pump, it will be heard on a steep downhill incline even when not decelerating. It will be audible when the gas tank is almost empty. It will increase when you step on the gas and the pump works harder.

If it is vapor flowing through the baffles, it will be transient. If you are decelerating over a long time from a high speed, the sound will die out. It will not be heard on a steep incline after the first few moments. By then the pressure will have equalized in the different parts of the tank.

I had the same wolf howl in my 1980 Citation, it was actually gas vapor escaping under pressure from the gas cap and would get more persistent the closer I was to empty (and since I was a poor college student I was close to empty a lot!).

"…when you step on the gas and the pump works harder."
This is true on some cars that are returnless fuel systems. However, most cars are a return system. On these, the fuel pump always pumps at the same rate. Whatever fuel that is not used by the engine, is returned to the tank via the fuel pressure regulator.