Ugh, that's the classic Craigslist gamble. It feels like every "great deal" online hides a massive, expensive problem the seller didn't mention. I've seen it with used furniture full of bugs, not just cars. The cheap price is always a trap for some huge headache you'll have to fix.
I had the same problem and finally just bought a new one. The time and money I wasted trying to fix a junk one wasn't worth it. A clean start saved me more hassle.