Hello,
With your statement of it becoming worse the hotter the engine becomes I would be led to believe that it's in the valvetrain. Due to the miles of your vehicle, it would not be unheard of. In terms of valves, they typically get tighter as the engine wears. The valves utilize a hydraulic lash adjusters meaning they are non-serviceable they are a replacement. Since the lifters use oil pressure to maintain proper adjustment you could have a low oil pressure issue. A symptom of valves becoming "loose" is a tapping or clicking and a rough idle.
I have included a breakdown of the top end of your engine (11) is your valve lifter
Please let us know what you find,
Dustin
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Thursday, September 7th, 2023 AT 5:46 PM