Usually caused by the engine having excessive blow-by which is somehow
venting to atmosphere (inside the engine-bay) then getting into the cabin
via the heater vents or unplugged holes in the firewall.
It may also be coming from your exhaust through a faulty trunk seal after
you back-off, even slightly.
Is the smell like rotten dirty socks or musty? The a/c kicks off when you
hit WOT - maybe the moisture that is coming off the a/c coils is taking odor
with it... I know when I WOT my Crown Vic, I can smell an off - musty - wet
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