Sounds like you have something burning inside. Have you opened the system yet? I would start there looking for plastic parts that appear melted. Also look at the components not just on the visible side you’ll really need to take the logic board out. You might as well clean the old thermal paste off and apply a fresh coat.
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Follow this guide [https://www.ifixit.com/Guide/MacBook+Pro+15-Inch+Unibody+Mid+2012+Logic+Board+Replacement/10766]
Sounds like you have something burning inside. Have you opened the system yet? I would start there looking for plastic parts that appear melted. Also look at the components not just on the visible side you’ll really need to take the logic board out. You might as well clean the old thermal paste off and apply a fresh coat.