Engine a little hard to start and runs rough at idle

I just bought this car there's no check engine light on but I check the engine with a tester anyways and no codes. But car is little bit hard to start every time I go to drive it and it vibrate's at idle when in gear. But overall drives good. The person I bought it from said she just had a new fuel rail put on and spark plugs were changed a few months ago. What could be causing the vibration and the hard starting? She told me that when it's hot outside that its hard to start But It is hard to start every time. Thank you