i got a 190E 2.0 1988. changed engine oil y'day. i used 15W-40 as recommended. today when i started the vehicle i found that it was idling too high. how can i reduce it. i tried lifting up that air cleaner casing but i couldn't fine any screws. please help me. in a bit of a mess. it idles around 1500rpm. can it be cause of the new oil?