I timeserted by 96 STS with 110k miles. I had been using coolant for several years and last spring it got bad. I did see some buildup on the piston tops and started getting a P0300 code. I have only driven it 100 miles or so this week and it does not start smoothly. kinda rough and an occasional stumble at idle. seems to accelerate smooth though. runs ok on the highway. I was expecting it to run perfect. put a few more miles on and see what happens.