About 1 minute after turning car off there is a thud sound that I traced to the gas tank. You can actually see the metal of the tank at the bottom flex abit when this happens. Either gas or air is surging back into the tank. 10 months ago, I had a new gas tank and sending unit installed. I just put on a new gas cap to see if that helps, no change.

Can you guys please help me solve this issue. Is it the fuel pressure regulator going bad? I have a check endine with the po440 code- Evap system. I just replaced all the bad vaccum lines. Could there be a defective purge/vent solenoid causing the problem.

Any help much appreciated as a gas issue is kinda 'unsafe'

On further inspection, it appears the gas tank is actually flexing inwards when the car is running. You can see the gas tank has a concave shape at the bottom. Then when you shut the car off, the metal tank rebounds back to its normal shape. I guess the mechanic gave me an elcheapo gas thank. Either that, or the fuel pump/system on these bonnies have a very high pressure spec and pull alot af vaccum pressure.

Or could my fuel pressure regulator be going bad and deliverying too high amount of pressure.?????

I think what is happening is vacuum pressure is being built up in the gas tank when the system pressurizes and then released when engine is shut off.


Guys please, I need help.

Is the charcoal canaster clogged or the solenoid gone bad.


No one else has had a vacuum build up on a Bonnie????

I was hoping someone more familar with the EVAP sysem would reply.
Did you check for kinks in the hose from the fuel tank to evap canister?

The answer lies in this info:



(If the purge solenoid is open and the vent solenoid is plugged, vacuum builds up in the canister and fuel tank)

So I have an open purge solenoid and a plugged vent solenoid pulling vac on the tank. Now to find out if 1 or more solenoids are bad , a short circuit somewhere or the PCM has a glitch. Or the fuel tank pressure sensor is bad.



Gas Cap Codes:
Loose gas cap can activate warning light
by Jim Kerr

Two solenoids are used to control the flow of fuel fumes from the carbon canister into the engine. A purge solenoid opens the canister to engine line, and a vent solenoid opens the canister to atmosphere so clean air can be drawn into the canister to aid purging the fumes. The computer tests each solenoid electrically, and then tests it functionally. If the purge solenoid is open and the vent solenoid is plugged, vacuum builds up in the canister and fuel tank. A pressure/vacuum sensor inside the fuel tank signals the computer of a vacuum build up and sets a code. The MIL engine light comes on if the test fails twice.

The so-called "gas cap code" occurs because there is a small vacuum leak in the system. To test to see if the system is intact, the computer is programmed to place a vacuum inside the fuel tank by opening the purge solenoid and closing the vent solenoid. It then closes both solenoids and measures how long a vacuum is held in the tank. If the vacuum does not hold for several seconds, then a code is set. Any leak that flows more air than a .040 inch hole would, will set a code. Loose hose connections, leaking solenoids, or a loose gas cap are common causes of this code.

Simply tightening the connections or gas cap may be all that is needed, but it is not that simple. Once the MIL light has come on, the system must pass its tests three consecutive times before the light will be turned off. It can be also cleared with a scan tool at the dealership. Conditions to run the test are very exact, so it can take a lot of driving to turn the light off again. For example, on a specific vehicle to run the test once, the engine must be started cold, the fuel tank level must be between 24% and 75% full, the outside air temperature must be above 7 degrees, C, and the engine must be operated for several minutes. If all the conditions are met, the test runs. Pass the test three times and the light goes out.

Many drivers do not understand why sometimes the light comes on with a loose gas cap, and the next time it will not. It all has to do with the conditions programmed into the vehicle'* computer to run the diagnostics.

It may be a nuisance to have that MIL come on brightly on the dash, but it helps us protect our environment. Keeping that gas cap tight will help keep that light off too

I still need help.

Canister Purge and Vent solenoid check out ok.

Charcoal canister not plugged up.

Vaccum lines all good.

Gas tank vent hose good.


I'm left with a few possibilites : defective vacuum check valve at gas tank.
defective (FTP) Fuel tank pressure sensor

What appears to be happening is the PCM is stuck in diagnostic mode, sampling the system for leaks by pulling vacuum on the system.


I gotta tell you, this really sucks...............

I don't have access to the fsm'* till I get home, I can look this up then

Thanks>

You should pull the vent solenoid out and clean it out. Sometimes they get stuck and set a code although they check out electrically OK.
Sounds like yours may be stuck closed.