bad idea, need to fix the issue, or good idea if and only if you are going to eventually fix the issue.
any water inside is not good, and holes into the cabin are bad for many reasons, CO2, noise, you will not be able to use any type of covering over footwell

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Ron
2015 Porsche Cayman - Elizabeth
2011 Porsche Cayman - Bond,James Bond
Sadly MERCEDESLESS - ALways LOOKING !
99 E320 THE Queen Mary - SOLD
62 220b - Dolly - Finally my Finny! Sadly SOLD
72 450SL, Pearl-SOLD
16 F350 6.7 Diesel -THOR
19 BMW X5 - Heaven on Wheels
14 38HP John Deere 3038E Tractor -Mean Green
84 300SD, Benjamin -SOLD
71 220 - W115-Libby ( my first love) -SOLD
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81 380 SL - Rest in Peace

That's CO (Carbon Monoxide) that you're worried about. We'd all be dead if CO2 was poisonous. And diesels produce far less CO that gassers, so it's not even much of a problem. I also don't think these holes would be large enough to make a difference in any of those factors, especially with a carpet over it.

Does it leak when it rains, even if you're not driving? I was trying to find a leak, did the hose test, nothing. Took it for a drive with sister in the back seat and no carpets. Once i drove through a puddle, water gushed in

If it were me, I'd track down the problem or live with it for the time being. Drilling holes might not even solve the problem, because your carpets would probably soak up any water that gets there.

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1983 240D 3.0T 4-speed manual, now sold

1989 Subaru GL Wagon 5-speed Touring Edition