TheEVAP system isn't testingitself. It's venting the gas tank as pressure builds up.

The system vents the gas tank thru the EVAP canister. There's apressure-relief valve (opens itself from the pressure) that vents in bursts rather than just being open all the time.

 

there is no pressure release valve other than the gas cap itselt being a two way valve. the gas cap only vents should pressure be extremely high in the gas tank.

are you talking about the fuel vapor control valve?


the vent valve(in blue) only opens when the fuel cap is removed and is used during refilling

the vent float(in green) only closes when the fuel tank is completely full.

the overflow valve(in red) only closes when you keep filling up after the pump shuts off automatically.

the roll over valve(in purple under the overflow valve) only closes if the car rolls over.


the pressure release valve(in pink) only opens when both the vent float and the overflow valve are both shut. that means the client just filled up their tank with gas and either the pump didn't auto shut off soon enough or they topped off after pump auto shut off. now they take it home, both valves are sealed, fuel tank pressure builds and the pressure release valve opens slightly to vent pressure to the two-way valve.this would be a 'silent' procedure though. this provides a path to the canister which absorbs the fumes and pressure equalizes with atmospheric pressurebecause the canister vent shut valve isa normally openvalve. itis open tofresh air through a filter.

normally the overflow valve is always open venting any vapor pressure to the canister.

so in order for the pressure release valve to ever work tank has to be overfilled.

 

The EVAP systemprevents evaporated fuel from going directly out to the air. But gasoline's vapor pressure would burst the fuel tank if it can't get out somehow.

Normallythe overflow valve is open, allowing fuel vapor from the tank to go to the 2-way valve at the EVAP canister. The 2-way valve seems to open in bursts, allowing the gasoline vapor to go thru the canister, but not continuously open. That's the sound I was thinking of; Ishouldn't have said'pressure relief valve'. The action of the 2-way valve is really not very loud, so most of the time you don't notice it.

 

yeah, i've never heard two-wayvalve making noise before during normal operation.

dcsmith: any update?

 

There is not really anything to update. The car isn't here now. My daughter has it in Charlotte. This is something that the car was doing maybe 3 years ago when my other daughter originally bought it and I have noticed it many times when it is here. I never thought it was abnormal. Then I thought maybe I should find out.

 

The auto-leveling system is what I originally thought. But I doubt that is it. That's not common, is it?

 

Thanks for the update. The 2-way valve noise I was talking about is really pretty soft.Someone mentioned it on another board so I checked. On my '98 I really had tocrawl down there & listen for it. I suppose in a few cases it's a bit louder?

 

I know this is a rather old post but I am searching for an answer to the same problem (or just annoyance). I just recently picked up a 2000 Accord, F23 Vtec. The car runs fine and is in excellent shape (single older couple) but I have noticed the same "thumping" noise under the rear passenger seat (drivers side). I attributed it to the exhaust system maybe rubbing something until I investigated further.I found it has to do with the EVAP purge system which makes a constant rapid thump thump thump thump. I know its not the activation of the purge valve is self because when I unplug it the noise does not go away. I have noticed that the vent of the EVAP canister goes though a fuel filter size can that is separate from the charcoal canister. The noise seems to be coming from there.. The noise will go away if you put slight pressure on the can or remove the evap lines from it.

I would assume there is some kind of a diaphragm that is making the thumping. Has anyone else investigated this?