Check Engine 5-8 light came on.

Well the first thing your gonna have to do is pull the codes out of the Motronic 2.7. I don't know for sure if it is same process as pulling them from the 348 Motronic 2.7. But you are still gonna want to pull the codes before you do anything else.

As for the high idle, have you disconneted the battery lately?

 

went to WWOC and they simply repalced the sensor (something I could have don myself cheaply had I known) and never happened again...but it could be many reasons, some costly so get the code and then the cure.

 

I had this happen on my '95 F355 B at least twice. In my case it was the slow down light 5-8. Both times it was the connectors that interface with the ECU that monitors the catalytic converter temperature.

Your problem could be similar: connector corrosion; but getting the codes is the best approach if you can do it. Given that your trouble includes high idle, it may not be as simple as mine.

Have you done an archives search on this forum? I'd be surprised if this hasn't happened to others.

Good luck