Hi, I'm new to doing this forum thing but I'm not sure where else to turn. I bought an 02 3.8l Camaro in June. And for about 2 months now I have lost 150-200 miles in my fuel range and about 3weeks ago it had an issue to where I was accelerating and it thudded twice and then was fine, then did it 4-5 more times. I've replaced everything fuel wise and ignition wise, all o2 sensors....I just had it at the shop and they tested the cats and o2 sensors and everything was fine. They checked the transmission and everything was fine.....I don't really know where to go from here to get my fuel mileage back

 

check brakes, check oil in rear diff. where is thud coming from, front, rear, right, left?

 

I assume there are no codes or pending codes? "thudded" is not a very good description. Did it feel like a miss, you can feel the car sort of miss or did it feel like you hit something and the wheel jerked.

The computer system will see misses and not set a check engine light. It need to see a certain amount in a limited time frame. A scan tool will track how many misses you have per cylinder. It will also show you plus width to the fuel injectors and output for the O2 sensors.

I am all about replacing all maintenance items before you start really diagnosing and issue but this is sounding like drop a parts bomb on your car. One think mechanics hate is working on a car that just had a bunk of parts replaced. As a general rule, one thing fails at a time. Most diagnostic trees assume one failure. If you really want help you should list what parts you did replace. You did not replace everything in the fuel and ignition system. That would be the entire computer system and if you bought all that new you would have spent more then the car is worth.

 

Change PCV valve?