I have a 2002 Grand Am GT and 2 to 3 weeks ago I had a complete tune up on it. It's been running great since then until yesterday. It started having a hard time shifting gears when driving and the gas gauge ( it has a fuel tank because I had just filled it the night before) dropped from full to empty and the light came on. When I pulled over and put it in park the gas gauge when back to full, but when you shift it into reverse and drive the gas gauge will drop back down to empty and the car will shift really hard, you can feel it jerk and shift. The check engine light is not coming on to show that there is a problem but you can see and feel that there is an issue.