I have a 99' Oldsmobile with the 3.1. I am getting a CEL with code P0440 .

I started getting a thunk from under the car shortly after start up and shortly after turning the car off. It seemed that the thunk was coming from the gas tank, this started out being periodically then got to be every time I drove the car.

Also around the same time I started getting a CEL light that would come on periodically. Pulled the code and got a P0440.

In trying to diag the problem I kept the gas cap off, this eliminated the thunk problem. Which I believe to be over pressurization of the gas tank. But I would now always have the CEL of course for the code p0440, and every once in a while P0452. Also I noticed that if I tried to fill the vehicle while it was running gas sprays back out of the filler.

In the past 2 years I have replaced the fuel sending unit and intake manifold gasket for a coolant leak and replaced gas cap with auto zone unit.

I appreciate any help you can offer! Thanks.

The banging you hear is the gas tank "oil canning" due to pressure. You probably need to replace the purge solenoid.

That is what I was thinking. Which solenoid? There is on on the back side of the motor and there is one on the canister.

Thanks!

Not sure. I would start by removing the hose that goes to the solenoid on the engine, and see if the clunking goes away ... similar to what you did in removing the gas cap. If removing the hose clears up the clunking the solenoid was the restriction. If it doesn't clear up the clunking, repeat the process for the other solenoid.

Thoroughly inspect the EVAP system for any obvious signs of wear or damage.

the vent valve by or on the canister is what does the venting is likely stuck shut.

Could also be a fuel tank vent valve, and possibly even your gas cap. How long ago did you switch to an aftermarket cap, and is that when the problem started?

Read this thread:

https://www.gmforum.com/1992-1999-91/fuel-tank-vent-valve-fuel-pressure-sensor-locations-276365/

Cap is sealed to vacuum if it wasent it would leak when it does the test

The gas cap is a one way check valve. It is sealed to vacuum, but not pressure. In other words it should hold vacuum but release pressure. Aftermarket caps quite often fit okay, but do not have the design features the OEM caps have and are not really an acceptable substitute.

So last night I pulled the purge valve and tested it. It was a little sticky so I replaced it. I also put the OEM gas cap back on based on you'alls recommendation that an aftermarket unit is no good.

I had the CEL cleared and took a little drive. The CEL has not returned nor is there any pending codes. Also I have not heard the gas tank "oil canning". IF the CEL returns I will post back up.

Thanks for the help!