1992 Jetta motor problem.

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I bought a 1992 gas Volkswagen Jetta 4cylinder,I had some work done to it and it was running fine until I started to drive it and the motor shut off and the battery light came on,so I got it stopped and the engine wouldn't turn over after that.A few days later it started but when the engine is running is sounds like a diesel engine,my buddy is a mechanic but hasn't really worked on Jetta engines before and he is puzzeled.We are not sure what the problem is.If anyone knows about Volkswagen Jetta engines could you respond to this.Thanks