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Old 02-07-2005, 07:46 PM
Posts: n/a
Gas Cap - Too much pressure in tank??

I've been hearing a humming sound from the rear of car even when everything is shut off. I found it to be pressure in my gas tank. When I removed the cap, it stopped.
Is this a problem? I have a new MB cap which I believe is not vented
(124 470 0005)
I can also hear my fuel gurgling in the tank when I start engine and gas starts to circulate. This a new sound.
Does it sound like fuel pressure regulator going bad or something?? Or normal?

It's an 87 300E



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Old 02-07-2005, 10:59 PM
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Join Date: Mar 2003
Location: Southern California
Posts: 2,233
All modern cars have fuel fillter caps with vacuum and pressure relief valves that typically open in the +/- 1-3 psi range. (In some cases the valves may not be in the cap, but on the tank itself.) They are an important part of the evaporative emission control system. Many state emission inspections include a test of cap pressure/vacuum holding capacity, and it is a common "failure" item. It's more likely to be the cap gasket (or all the adaptor seals on the pump used to test pressure), so beware. A lot of emission control test and repair facililties keep a big supply of fuel caps on hand and sell a lot of them. The valves are simple and reliable. If a cap doesn't hold pressure the first suspect is the gasket and the test setup is a close second if not number one sometimes.

I don't know if your unusual noises are related to the cap. The fuel pump output pressure is much higher than the cap vacuum and pressure release points, so I don't see a relationship. Unless there are some operating problems, I would just monitor the situation, but not spend money on "repairs" at this time.


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Old 12-23-2005, 03:10 AM
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Join Date: May 1999
Location: Sacramento, CA
Posts: 1,565
Originally Posted by MBZ OE
I've been hearing a humming sound from the rear of car even when everything is shut off. I found it to be pressure in my gas tank. When I removed the cap, it stopped.
I have this problem with my car too. Not sure what the cause is. The noise is intermittant, but it sometimes comes with acceleration, and sometimes comes after the car has been shut off for a couple of seconds. And it does go away once pressure is released from the filler cap.
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Old 12-23-2005, 02:58 PM
Join Date: Oct 2004
Location: CT
Posts: 3,495
Since I have been working on a similare issue with my SL. I was wondering what would happen if the fuel accumulator was ruptured and allowed fuel pressure to leak back to the tank. My accumulator checks out but I dont get gurgling noises just smells.

John Roncallo

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