Why did you remove the T-stat?
Put it back in. The engine is designed to make its best power and fuel mileage, once it has gotten to the proper running temp. Your car now is not getting into open loop and stuck in closed loop. I always get them confused, open and closed, but either way your car is not getting to the one and stuck in the other. Computer controlled cars need to get to temp, not like cars years ago that were not computer controlled.
A couple other things are, change your oil and check your air filter.
