I have an OM662 Ssangyong engine. When i first start the car and have all the auxiliaries turned off, the car engine begins idling rough at around 550 rpm. After a few seconds, it goes up to around 1000 rpm and the car works normally. When i turn on the ac, the car rpms go down to 550 again and it takes around 10 minutes for the car to reach around 850 rpm and it feels sluggish. it never reaches 1000rpm again Not sure whats going on? Maybe a vacuum problem? The car should be idling at around 750.