Have a 2009 hyundai santa fe with 2.7 engine. At about 6k miles it started to have a dead zone in the acceleration ( I was thinking fouled fuel injector) After a dealer visit they said they could find nothign no codes returned. Now at 17k I went back as it got worse. This time they said they did see evidence of a problem and we did a fuel system service, injectors etc. While it improved drivability the car now has cod starting problem, it really coughs and chugs for about 30 seconds. Otherwise runs ok. Does it need another service or is a sensor like EGR or a fuel filter problem the issue?