Humming noise from gas tank

[CindyLynn]

Hi I have a 1994 Chevrolet Camaro V6 3.4 and it has started to make a soft 'humming' noise from the gas tank after I turn the car off. Some times it does it, but most of the time it don't. And it eventually stops after a few minutes or so.

Any thoughts on what this may be? I appreciate your feedback!!!

Thanks!!!

[bellaCuba]

Are you sure it's coming from the gas tank? Maybe it's the antenna for your radio? I think your model they come electric & they go up (if radio is on, key on) & down (when key off). Maybe the antenna motor is going bad? I can't think of anything else back there that would still have power once key is off, besides the antenna for a few minutes. Fuel pump in tank is instant shutoff.
Let us know what you find.... very curious!

[Blue Bowtie]

Originally Posted by bellaCuba

Fuel pump in tank is instant shutoff.
Let us know what you find.... very curious!

Actually, the fuel pump is not an instant shut-off. The auxiliary oil pressure switch supplies power to the fuel pump regardless of the PCM, fuel pump relay, or ignition switch, and continues to do so until pressure at the switch drops below 4 PSIG.

This symptom is not really that uncommon. It becomes a real problem if the oil pressure switch has sludge in its ports and takes minutes to turn off instead of the usual 2-5 seconds. The diagnosis can either be performed with a fuel pressure gauge connected to the fuel rail or by disconnecting the electrical connector at the oil pressure switch. As mentioned before, removing the fuel pump relay is useless since the oil pressure switch bypasses that as well.

If you discover the pressure switch is sticking, remove and clean it or replace it, then stop using oils with excessive polymers such as conventional Quaker State / Pennnzoil.

[CindyLynn]

Originally Posted by bellaCuba

Are you sure it's coming from the gas tank? Maybe it's the antenna for your radio? I think your model they come electric & they go up (if radio is on, key on) & down (when key off). Maybe the antenna motor is going bad? I can't think of anything else back there that would still have power once key is off, besides the antenna for a few minutes. Fuel pump in tank is instant shutoff.
Let us know what you find.... very curious!

No, it's coming from the gas tank area. And I don't have an antenna that goes up up/down on/off when I turn the key on. Just a regular old antenna.

It hasn't done it in a while, but I'll keep you posted if it starts to do it again.

[bellaCuba]

That oil sending unit is also activated by the key.... Key OFF = no power. No such thing as risidual power on these. Now... let's say your fuel relay goes bad, the oil sending unit will use it's leg to power the fuel pump, as long as there is sufficient oil pressure. Key On ONLY. I've had so many Chevys... all old with the "oil sending unit/ fuel pump power" circuit.

On the schematics, it also proves... Key On ---> to PCM fuse ---> PCM ---> fuel pump prime,
OR
Key On/Start ---> PCM fuse ----> oil sending unit---> PCM ---> fuel pump relay ---> Fuel Pump.

https://www.dropbox.com/s/xgebc8yf5n...34.44.png?dl=0

What's causing the noise? Maybe tank is relieving excess pressure...if the cap is starting to go bad, that is one possibility. When you turn off the vehicle & it's still humming, try removing the gas cap. If the noise goes away instantly, you know the culprit is with the excessive pressure. There's an EVAP system connected to your gas tank in order to keep a certain amount of pressure & relieve the rest within itself to keep from going into the atmosphere. Just a brief explanation, but any 1 of those parts could be failing...which is a more likely scenario IMO.

[CindyLynn]

Originally Posted by bellaCuba

That oil sending unit is also activated by the key.... Key OFF = no power. No such thing as risidual power on these. Now... let's say your fuel relay goes bad, the oil sending unit will use it's leg to power the fuel pump, as long as there is sufficient oil pressure. Key On ONLY. I've had so many Chevys... all old with the "oil sending unit/ fuel pump power" circuit.

On the schematics, it also proves... Key On ---> to PCM fuse ---> PCM ---> fuel pump prime,
OR
Key Start ---> PCM fuse ----> oil sending unit---> PCM ---> fuel pump relay ---> Fuel Pump.

https://www.dropbox.com/s/xgebc8yf5n...34.44.png?dl=0

What's causing the noise? Maybe tank is relieving excess pressure...if the cap is starting to go bad, that is one possibility. When you turn off the vehicle & it's still humming, try removing the gas cap. If the noise goes away instantly, you know the culprit is with the excessive pressure. There's an EVAP system connected to your gas tank in order to keep a certain amount of pressure & relieve the rest within itself to keep from going into the atmosphere. Just a brief explanation, but any 1 of those parts could be failing...which is a more likely scenario IMO.

Thanks for the info, BellaCuba!!! I will definitely try removing the gas cap the next time this humming noise starts. Because as I think I have stated before, the noise occurs when the Key is OFF, and removed complete from the ignition.

I will keep you posted!!!!

[bellaCuba]

Originally Posted by CindyLynn

Because as I think I have stated before, the noise occurs when the Key is OFF, and removed complete from the ignition.

You most definitely stated that. My big explanation, in the beginning, was more geared towards Bowtie and the latter more towards you. I should've directed my paragraghs better, but at least you also see the reason behind my statements/logic too.

[CindyLynn]

Thanks again for your input!!! I'll let you know how it goes!!! (Maybe just maybe it won't happen again lol)

[brcidd]

My money is on the residual oil pressure keeping the fuel pump active-- seen it on my S-15 truck-- fuel pump would run 5-10 seconds after pulling key out of ignition- again due to oil pressure bleed off- takes awhile to get below that oil pressure switch thresh hold of about 4 psi.

Don't believe it? put a test light on fuel pump primer lead-- then you can watch what happens on shutdown...

[bellaCuba]

@brcidd, that's certainly a great idea. Thanks for putting that out there. I appreciate the input!