Does it also slip in reverse?
From the description, it sounds like the trans is a goner. If the fluid is nasty, and smells burnt, it's done for. IF the fluid is still red colored, and has no burnt smell, it's possible the pressure regulator valve in the pump is stuck, but there's no way to correct that without removing the trans, and opening it up. Either way...the trans is goin to need to come out and be repaired. No magical elixirs or snake oil will help.....
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Joel Adams
C3VR Lifetime Member #56
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"Money can't buy happiness -- but somehow it's more comforting to cry in a CORVETTE than in a Kia"