Hi. I 've got a 1991 Chevy Camaro Z28 that I have owned for 10 years now. I have always had this mystery hum come from it. Whenever I shut the car off and let it sit for at least 4 hours or more a loud intermittent hum comes from the gas tank area. The sound itself sounds like an aftermarket fuel pump. Has anybody had this problem before? I would really appreciate any your responses. Thanks.

 

remove the gas cap and see if it still happens. It could be pressure building up and releasing. Are you sure it's not the fuel pump running? It may be the oil pressure sender or something malfunctioning and causing the pump to run. Never heard of something like this after it's sat for so long. If the pump is the culprit, try disconnecting the oil pressure sender down by the filter after you shut the car off and see if it still happens then. If that fixes it, replace the sender. If that's not it, I don't know where to turn. Kyle

 

I seem to recall hearing of it happening. Never seen it myself, though.

The oil pressure switch receives power from the ignition. It's not likely to be the culprit.

The relay, though, is energized by the ignition and is supposed to "time out". I'm not completely sure, but I believe that power is not switched. If it is switched, then...

But, it does sound like the pump is continuing to run.

 

It's the vent valve.... a little plastic thing in front of and just inward from the left tire on the end of a tube, about 1½" diameter x 2" tall. Needs a new one.

Nothing wrong with the pump, gas tank cap, or any of that.

 

I had a similar problem. It was the vent like RB said.

 

I have the same noise right now! I'll try that and see if it fixes it. When mine does it, it is VERY loud but only for a second or two...
I looked all over for anything but would have never known to look for that.
Thanks!

 

It's not an oil pressure sender, it's an oil pressure switch! it tells the comp. that the engine is cranking in case the fuel pump relay takes a dive! That way you won't get stranded out on the road.
When the fuel pump relay quits, the switch will turn on the pump but crank time will be longer before it starts.

 

I went to autozone today and they had a hard time coming up with the part, what is the correct part name for this? Vent valve? They couldn't find anything under that, not that they could find their own **** either but... let me know.
Thanks

 

Go to a GM dealer.

 

Not to start any battle, but I belive the auxilliary oil pressure switch is supplied with power directly from the battery, so an intermittent switch can cause intermittent pump opreation: