Hey Mare,

I would say perhaps contaminated gasoline, but I do not think that would cause the RPM needle to go nuts when the key was in.

$300. sounds very steep just for someone to look at it. You must live in a very large metro area, or west coast. Are you getting these quotes from the Mercedes dealer?

You may want to post what region you are in if you are comfortable with that. Someone here may can recommend a good repair shop to you. There is also a thread on here somewhere that you can actually look at yourself to see if you see a local shop there.

Good luck.
Carry on.