Hey, I just recently aquired a 1999 seville sts. I even just drove it back about 2000mi from AZ to MI with no problems at all. However this will come into play latter, the guy who was helping me drive decided to put E-85 into it for two fillups when passing through iowa, and I think once in nebraska. Anyways after a few weeks at home with it, I was driving home up a hill (been running premium now), when I got a loud ping with the service engine light on, after I stoped the car began sputering and died. Well I can get it to start sort of, with the throttle open aways and this is how I did some testing to find the problem. I get little to no vacuum, cylinder six shows zero compression, low compression on other cylinders, (I havn't tested the other bank yet for obvious reasons. I also tried pumping air into cylinder six while in the compression stage, and It will not hold any pressure it is leaking through the valves. Because of the low to zero vacuum the engine has been running very rich. So far, I think the E-85 burning hotter could have started destroying the valves. The car did have many rough idles after the E-85, but once the engine warmed up it went away. So far the mechaninc said, due to the hastle of rebuilding this engine, it might be easier just to get a new one. So far the cheapest I have found is $4,300 on the internet. I have access to many tools including a 5-axis CNC mill if I need to totally upgrade the block or even make new pistions on that thing. Anyways, has anybody ever had this same problem or can think of a know issue that would cause it. Oh, one more thing, I looked at a graph for the burn information on the spark plugs or whatever it was called. Thats how I knew to check cyl 6. It showed a very low peak with no duration to the spike. The cylinders next to it which had decent compression didn't have this, and the other cyl from the other bank were also very low. Again, thanks for any help I can get.