96 Grand Mar. Trans Over filled

I found the 96 GM I just bought has an over filled trans. Would this explain why it is shifting in and out of 3rd and 4th ? The level , when hot is over the high fill marks by about a half an inch. What should I do? Drain it all out and change the filter too ? How many quarts of Mercon does it take ? I don't have an owners manual. Thanks DM fr. NJ.

Changing just the fluid and filter will take just a little over 4 quarts of fluid. Unless you have a way to siphon it ourt of the fill tube, or remove the drive shaft and jack up front of vehicle, there is no way to just drain a little fluid. I would opt for the fluid and filter change.

I did syphin off about 16oz (1pint) using a small clear diameter tube in the dip stick hole. When I was done the trans shifted a lot better than it had been. I looked at the fluide I sucked out and it is old, but not burned. It still shifts a little funny around 30mph. I have a owners manual now and think I will drain the trans and change the filter too. Its due for the change according to the manual. Also at highway speeds I notice a engine speed / performance slight differance and I think it is the o2 sensors doing there job . The engine runs strong and there are no misfires at all. I am surprized just how much faster this Grand Marc. is over my older ones I use to have (1977 & 1978 with the old 351 Widsor engines in them). Thanks for the help;Dan in sj.