I have a 2000 Bonneville SE and have had it since the beginning. For quite some time now the car'* performance has been going down hill. As it stands, there aren't any service engine lights or codes in the system.
Issue: When started and the engine is cold, idle speed is high at about 1700 RPM and a little rough. Once the car warms up, Idle speed can sometimes remain high or turn the complete opposite and chug and miss at 700 RPM. Not to mention the fuel economy has gone down the tubes but I don't notice any black smoke or smell of fuel. I have replaced the plugs and wires once at about 90,000 miles. The only other part that I have replaced is the idle modulator, and that had no effect. All other sensors are original and the car is approaching 150,000 miles. I know I can't rewind the clock back to 0 but I assume there must be something I can do.
So my question has a couple parts: Where do I begin? There are so many sensors on this thing and all claim to have one thing or another to do with performance and efficiency. I'm trying to avoid going into the auto parts store and spending a fortune on every sensor there is. Should I go the O2 sensor route, if so which one is more important; before catalytic convertor or after? Then there are the rest (mass air flow, intake air temp, EGR valve? fuel filter).
After all these years I still like the car and plan on driving it into the ground. Any advice you could give would be great!