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I vacuumed the inside for weight reduction.

I have a Type-R one in mine. Revs really high compared to a Vti-R one. I'll be sitting on close to 3,500 RPM on 100kms.
Good thing I have another car for freeway driving.

It's most likely the O2 sensor.
Try taking it out and using some carby cleaner on it.
Otherwise they cost around $80 brand new.

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nice! welcome!

Seam sealer is good for fililing gaps, it's flexible, paintable and easy to use.

Like Dougie_504 recommended. Best to sand the old colour to a haze but use 240 grit dry paper (so the primer can bind).
Hi Fill it using Primer with at least 3 coats, sand it using 240 grit and use...

that's nuts!