98 volvo S90 fuel smell in rear of car inside? any idea please i have a check engine light on

You said you already replaced the gas cap. Did you buy a new or used one? For the cost, I would try another cap before going any further and make sure the one you get (or the one you have) is properly and completely sealing the fuel system.

I purchased a new cap with a key from AutoZone

When I installed it the cel went away for a week but came back

To really know it should be a Volvo fuel cap not a generic from Autozone or some other aftermarket supplier. Many of these aftermarket caps do not seal that well.

check lines going from gas pipe to evap canister. there is a J hose that usually breaks. jack up vehicle on drivers side and go under car and you will see evap canister. looks like a box. then also follow those lines up to the engine. they are plastic and go to another small hose, which also usually cracks. I had same issues with my 98 Volvo s90.

Check your evaporate hoses under the cover for your gas tank in the trunk. They could be cracked The hose that is connected to the canister and under the trunk could be your problem. That's what happen to me. Tom