Engine Mechanical problem
2003 Subaru Outback 4 cyl All Wheel Drive Automatic 75,000 miles
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I just got this car from a deceased relative. It had been about 10,000 miles since the last oil change so the first thing I did was change the oil. I added a detergent when I changed the oil because it had been so long. It was running fine before hand but as soon as i changed the oil the engine started to slow down just like i was running out of gas. I would press the peddle down harder but the engine wouldn't respond accordingly. Then lots of white smoke pouring out the bottom of the car. It stalled once but i was able to start it and get it home.

People have suggested that its just all the dirt getting cleaned out of the car because it was so long overdue for an oil change. either way, what should i do?