Excessive pressure typically also comes from 'blowby' gasses passing the piston rings into the crankcase - which is vented thru the head into the valvetrain/cam area. This is a 'bottom end' issue then, and would require at least a complete rebuild. To rule this out - or narrow to either this or damaged valves - a compression and leakdown test.

Steve

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'91 MB 190E 2.3
'08 RAV4 Ltd 3.5
'83 Lazy Daze m'home 5.7