Would you spend $500 on a 10 y.o. car to diagnose problem that may reveal a quick, cheap fix or a massive (expensive) problem that involves a cracked engine block?
Its gonna cost $500 just to find out what is wrong?? If so,you might want to find a new mechanic. Mine charges $40 to run a computer check that will narrow down the problem...I don't think I've ever paid over $200 to find out what was wrong with a car. And thats when his hourly pay went into effect and he had to pull a bunch of stuff out to get to something else.
I don't know if you have them out there.. but AutoZone does free diagnostic checks. They can tell you what's wrong with the car or what the codes the computer is throwing... not like they can fix it for you.. they are just a parts and accessories type of store.
I think AAMCO does it too. Definitely look around and kick your mechanic to the curb. Evil money grubbing punks!
I have a rear seal on my engine that is bad and they diagnosed it for nothing at my inspection.
I already knew it was bad because I had taken it to another mechanic and for under $50 he had told us the same thing I just had not decided at the time if I wanted to spend the money to repair it or not. Just recently I decided I had to and I needed it inspected so they noticed the leak and checked what the problem was for nothing so now I have a second diagnoses stating the same thing.
I really think you can find someone who will diagnose your problem for a lot less.