SUPRA?!? LOL!
I have an '89 Sunbird (bayliner equivalent) 19' I/O, but it's enough that I can put down a tantrum (almost) every attempt
Like I said, let them know you're serious and drive it, or TRY to drive it, and if possible drive it some more. I would think if it's cams it will start to run worse. If it's coils it will just have a miss and feel weak.
Maybe find a local mechanic who will give it a once over. If you can find someone familiar with the v8 SHO that would be a HUGE bonus.
The dealership's mechanic I bought mine from suggested the problem (miss) I found COULD be a leaking head gasket. I thought that was a false diagnosis, but went with it anyway and got 900 knocked off their asking price. A couple $30 ign coils and the problem was solved -- of course, I had to get the cams welded.
I'm surprised nobody else is jumping in.
There are plenty of people on here who have bought and sold several of these and worked on a lot more, AND dealt with multiple cam failures. Either they're sick of these questions or I'm doing a pretty good job advising you.
