A petrol engine does not cause any problems as such but the injector can be choked due to improper lubrication if the car is left unattended for a long period of time. Also, the battery could die down if the engine is left put for long as the battery is constantly charged by the alternator. To avoid this, you should turn on the car at regular intervals and rev it for around five minutes if not driving. Moreover, you should keep a check on its service intervals and provide timely service to the vehicle for its uniform performance. Other than this, there isn't much that could trouble the operation of your car in such conditions.