I used to have three Supras back in the day, 87, 90, and 90 turbo. Sorry I can't help with the problem your having, but a great site to visit for Supra information is
http://www.supras.com/sogi/sogi.php3
Just to let you know the 87 and 90 Supras treated me great and I had very few problems with them. My 90 turbo was another story. I blew the head gasket on it and then blew the engine and I had tons of other problems with it. It seemed like I was always having the thing towed to the shop. I also have had tons of problems with the local Toyota dealership, so have my parents with their Toyotas. It might just be the dealership where I am at, but I thought I would let you know. I have also found that Supras are much more expensive to fix than most cars. A brand new turbo engine from Toyota for thyour year is over $7,000.
Inferno is dead on about the head gasket problem. The supras.com sight even has a section devoted to the 87-92 Supra head gasket problem and a place where you mail your blown head gasket stories to Toyota.
Hope this helps