drift240_2009

Ok i have a 50/50 exhaust. Theres some aftermarket some stock and i have sealed the welds that were bad and now the car seems to be smoking some funky color. Heres what i see: I get sometimes the burning oil color, then sometimes a mix of oil and coolant. This happens under either light or hard acceleration. Now I have had a shop check for any warpage,leaks,cracks etc and nothing seems to be leaking. They even did the UV dye to see if theres leaks anywhere but couldnt find any. Can someone help me solve this? Plus the car backfires hella lot when im sitting at a certain speed and sometimes i can do it on command.