Go to Career and choose the car you want. Then go to tune car and choose test drive.

Choose any track with a pitstop (I always use the Nissan Speedway track). Then go in, pause the game and choose tune car in the pause menu.

Then when you are in the tune options menu, just press A without changing anything and then just click resume game at the pause menu.

You can then drive freely in the pitstop. Go out the way you came in and keep going on with your lap.

The game will say "pitting", but you actually won't. If you do this then your engine won't take any damage.