Gas tank leak + check engine light = rumor confirmed?

Oldsmobile Cutlass 99

130K miles

Changed oil myself for the first time last month, have been checking level, seems to be fine.

Check engine light fickle. Used to go on and off every now and then, usually had to do with the gas level, but nothing definitive. If I filled up sometimes it went away, if I filled up sometimes it went on until it got to around half tank. Lately I’ve only been filling halfway. The first quarter (half of half tank) drains like urine, the last quarter lasts at least 3 times as long.

The threads on the tank the cap screws on to are definitely rusty, and the (I don’t know what to call it) gas tank shield? the part that surrounds the hole seems loose and rusty too, but is some sort of cheap plastic/thin metal.

Any of these things related? Someone told me I might have a leaking gas tank, someone else told me its the threads the cap screws on to…how much would it cost to fix these things?

I need you to clearly state what problem you want help in resolving. Why did you tell us you changed the oil? I must be working too hard as rather than say the OP cannot write I will accept that I cannot read.

I have an inexplicable talent for including as much information as seems relevant to an ignoramus as myself in no clear or concise form of language nor justification of my extemporaneous claims, thus my first post, and for that I apologize.

I’m hoping to discover the source of my apparent draining of gas in addition to the check engine light.

The check engine light started giving me issues not long after I had the tranmission pan dropped and filter replaced, yet seemed to be somehow related to when and how much I filled my gas tank, however with no particular pattern. It also has been “on” consistently not long after I changed the oil myself, something I did for the first time, which I’m hoping is not related to the check engine light, but is worthy of mention nonetheless. The car engine temp gauge also registers a few hairs above halfway, something I don’t remember, and would like to include as all relevant information.

I hope this offers some clarification.

You need to get the codes related to the check engine light. Some parts stores will read the codes for free. Get the alpha-numeric code, in the format P1234, and post it here.

If what you’re saying is that the fuel filler cap threads are rusty, that could cause the light to come on.

How do you know the fuel is draining? Do you see it pouring out? If you’re relying on the fuel gauge, you should know that they usually aren’t very accurate. It’s normal for the gauge to drop more quickly is some sectors than in others. In my car, it falls slowly through the top quarter, quickly through the middle half, and slowly again in the bottom quarter. You just get to know how it behaves. If there is a leak, that’s very dangerous, as it can cause a fire.

get the codes read. try to do this when the check engine light is on.
What you describe sounds more like a faulty fuel level sender or a faulty gauge than a leak. When you have a leak, you have a smell.

A few inches down fuller neck is a check valve and hose that goes to a charcoal canister in front of car that and gas cap control pressure and gas fumes,this is where the light is coming from.
I would put part of a clean rag down fuller tube making sure you can get it out and clean where cap screws into, get a new cap or clean old one and get a new gasket.
Do not let it get below 1 quarter and NEVER fill after pump kicks off, NEVER.
Your gas gauge is screwy, no biggie if you know it.
All other stuff has nothing to do with light.