Help us help you. By posting the year, make, model and engine near the beginning of your help request, followed by the symptoms (no start, high idle, misfire etc.) Along with any prevalent Diagnostic Trouble Codes, aka DTCs, other forum members will be able to help you get to a solution more quickly and easily!
I'm guessing that your gas tank EVAP vent solenoid is stuck closed.
Find it and with the car off or unplugged, you should be able to blow through it. Try to gently access it since sometimes accessing something like this changes its state, meaning you could knock it enough to open it. If when you perform this test you can't blow through it, it is stuck closed and should be replaced.
Completely agree with the above post.
I would also add that it's possible for the charcoal canister to be blocked. Usually this happens if you tend to over fill the tank after the pump has stopped, the gas can make it's way into the canister and turn the fine charcoal granules​ into a brick.
After you test the components mentioned by members above and if all checks out ok, you may also want to test the fuel tank float and vapor control valve.
The float is integrated into the tank on the first gen MDX but the vapor control valve comes separately. I have had to replace 3 tanks due to a stuck closed float not releasing vacuum.
PM me your email and I can send you the troubleshooting steps for this issue.
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