When I bought my car it has a collapsed gas tank. It is about 9 gallons now.
I am wondering if I need to do something with it?
Tnx.

Hi

Yes, you will need to replace the vent valve which is located underneath in the area of the fuel tank inlets/outlets. The 86-88 (124's primarily) would develop an issue with the vent valve from time to time. Without proper ventilation the tank collapses due to the suction power of the fuel pump!

Do they check the vent valve on the emission test? The test was Ok.
I read somewhere about some kind of a regeneration module? Could it be a problem?

The fuel tank vent valve is not a device that is normally tested during a state emissions test. The check (or vent) valve has two functions, 1) allow the fuel tank to draw (breathe) outside air into the tank as the fuel is depleted 2) closes to the outside air when the tank is under pressure (at rest) in order to direct the fuel vapors to the charcoal canister.

The regeneration valve is working correctly, that is why the fuel tank is collapsed. The regeneration (Purge) valve is connected to the intake manifold on one side and to the charcoal canister on the other. When certain conditions are met the valve is operated electrically which in turn opens the valve to a vacuum source (Intake Manifold) in order to evacuate the fuel vapors form the charcoal canister. Since a vacuum is present a ventilation device needs to be incorporated in order to allow outside air to enter into the system, otherwise the result is a collapsed fuel tank.

So, The Fuel Tank Vent Valve is connected to the intake manifold on one side and to the Charcoal Canister on the other:

Air Intake Manifold<--Fuel Tank Vent Valve<--Charcoal Canister

And, if Charcoal Canister is plagged, the fuel tank collapses because of vacuum force from Air Intake Manifold.

What is the part number for Charcoal Canister? How to teach it to replace?

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The cannister being plugged would not cause the tank to collapse. It would prevent the suction from getting to the tank. The Fuel Tank Vent Valve is (or was) probably the culprit.

Back to your original question. If you can live with 9 gallon capacity, you can leave it as is. I wouldn't myself because I do a lot of distance driving and need larger capacity. I would remove the tank, purge it and see if I can pressurize it and pop it back into it's original shape. Be sure to leak check it afterwards to make sure it didn't develop a crack somewhere. Don't do any of this until you get all the gas fumes out.

Wes