The two were having sex with no clothes on inside the car in Aaonla locality of Bareilly, police said.
The girl is a BA third year student in a college here. She has been admitted to a private hospital, while Pushpendra's body has been sent for post mortem, IG Bareilly zone Devendra Singh Chauhan said.
Police said, the preliminary inquiry showed Pushpendra died after inhaling carbon monoxide generated inside the car. The girl was gasping for breath, when Pushpendra's uncle opened the door of the garage on hearing the engine running.
Police is awaiting the girl's recovery in order to record her statement.