2 issues: Gas tank pressure?? & slight high pitch hum behind seats

mmolto · #1 (**permalink**) 04-20-2008, 08:22 AM

Hello,

Any feedback on any of this is appreciated.

[Issue 1]
Just curious, when I go to fill up gas in my S4 I get a loud swooshing (suction / pressure release type sound) when I open the Gas cap. Is this supposed to happen? I'm trying to determine if i have a potential fuel venting issue.

[Issue 2]
Possibly related... but not certain yet. I have started to notice a slight high pitch humming sound coming from what sounds like the rear shelf behind the seats. It seems to last for a few seconds then it goes away (but could just be hidden by the engine noise) I hear it clearly when hit the gas when I'm about to proceed from a complete stop:

I only notice it after the car has warmed up. Does not do it when it is cold.
I do not have to be in motion to hear it. I only have to press on the gas. Car can be in neutral.
It was louder yesterday when my fuel level was low (less than 1/4 tank), but after I filled up, the noise was still there... but much quieter.
The car runs fine and I do not experience any sluggishness. It still drives great with lots of punch.

I suspect I'm starting to have fuel pump issues (either in-tank or the main one) but I'm no mechanic.

Any thoughts out there? Any feedback is appreciated.

Thx

Michael

**Tifosi** · #2 (**permalink**) 04-20-2008, 08:41 AM

Quote:

Originally Posted by mmolto

[Issue 1]
Just curious, when I go to fill up gas in my S4 I get a loud swooshing (suction / pressure release type sound) when I open the Gas cap. Is this supposed to happen?

mmolto · #3 (**permalink**) 04-20-2008, 09:32 AM

Quote:

Originally Posted by Tifosi

It is a vacuum, or is it pressure when you remove the cap? (it may have bearing on issue #2)

*
Do you get a hestiation, surge or power drop when the sound occurs?

*
For giggles you can take it out with the gas cap off to see if the noise occurs.

If it doesn't, then it's definitely the vapor system.

**Gubi** · #4 (**permalink**) 04-20-2008, 10:53 AM

A *little* bit of hissing is normal. The valves of the vent system take a fraction of a PSI to open, but a fraction of a PSI over a large volume of tank space can result in a lot of air flow.

The vapor system is in the trunk area behind the flap where the jack is. There's a big overflow tank and two valves. One valve is open to atmosphere and lets air into the tank, the other controls flow forwards to the charcoal canister. If you're sure the problem is vacuum, then it's likely the inlet valve.

Pull off the inlet valve and make sure air flows through easily into the tank, if not clean it out with some carb cleaner. Then disassemble the other valve by gently popping it apart (note how all the parts go together and what direction it goes into the car) and clean it out as well. See shamelessly stolen pic below.

Your other noise sounds like what happened to me when the in-tank pump died. If you do a search you'll find some threads on this.

**ghnl** · #5 (**permalink**) 04-20-2008, 05:46 PM

Another possibility for the source of the hissing sound behind the passenger (right side) seat could the MAP (Manifold Absolute Pressure) sensor. There is a vacuum line that runs from the intake plenum along the inner sill to the MAP sensor which is located behind the vertical panal behind the right side door. The MAP sensor is known to whistle or hum if it develpos a leak.

**Gubi** · #6 (**permalink**) 04-20-2008, 06:30 PM

'91 Spider. There is no MAP sensor.

**ghnl** · #7 (**permalink**) 04-20-2008, 06:37 PM

umm, then never mind.

(sorry, I didn't make note of that info...)

mmolto · #8 (**permalink**) 04-20-2008, 07:53 PM

Quote:

Originally Posted by Gubi

Your other noise sounds like what happened to me when the in-tank pump died. If you do a search you'll find some threads on this.

**Gubi** · #9 (**permalink**) 04-20-2008, 08:04 PM

Suddenly happened. I was sitting in the car idling when I suddenly got a hum like you're describing that did not go away. It was the sound of the main fuel pump straining because the in-tank pump had died.

It's easy to test. If you pull the trunk carpet and look at the top of the tank, you'll see a bunch of wires for the in-tank pump and the fuel gauge sender. The pink-white wire is 12V to the pump.

With the car OFF, disconnect the pink-white wire. Get yourself a remote starter switch at the auto parts store and connect it from positive on the battery to the now-vacant connector on the fuel tank where the wire was. Make sure you're ONLY clamped onto that connector and not touching any of the other connectors or the metal of the body. If you trigger the switch, you should hear a loud hum from the in-tank pump running. No hum and its toast.

**Tifosi** · #10 (**permalink**) 04-21-2008, 02:56 PM

Query for potential future use @ Gubi:

When your in-tank pump went, did the gas cap still produce an vacuum whoosh when removed, or does it actually require that 'extra' pull to accomplish the feat?

I understand if you don't recall because that was on page 14 of the priority list at the time, but it could have quite a bit of bearing on how such diagnosis are conducted.

DavidH · #11 (**permalink**) 04-21-2008, 03:28 PM

Quote:

Originally Posted by Gubi

note how all the parts go together and what direction it goes into the car) and clean it out as well.

**Gubi** · #12 (**permalink**) 04-21-2008, 04:09 PM

That's correct: white side to vapor tank.

DavidH · #13 (**permalink**) 04-21-2008, 04:42 PM

Lucked out again. Thanks, Tom.

**Gubi** · #14 (**permalink**) 04-21-2008, 05:34 PM

Quote:

Originally Posted by Tifosi

When your in-tank pump went, did the gas cap still produce an vacuum whoosh when removed, or does it actually require that 'extra' pull to accomplish the feat?