Tune up?
 

Can you tell me when to consider if I should need a tune up or not?

Tune ups consist of things like:
- New plugs
- New plug wires
- Replacing the fuel filter
- Replacing the air filter
- Flush the coolant (which should be done fairly often before a "tune up" interval)

If you are at or near 100,000 miles on a modern Monte Carlo, yes you should consider these things.

GM normally gives a suggested maintenance schedule with these cars. If you still have it, that guide usually won't steer you wrong.