Hi everyone,
I'm new to this site because I hope to start fixing my own car and I have a problem I don't know what to do with. I have an 89 240 GL with 200k miles on it. It was running beautifully and then last week the engine started shaking one time when the car was cold. I let it warm up, and the problems lessened. Then 2 days later, the check engine light went on and the car started idiling really badly. The next day, the car wouldnt even start. I figured this was a problem way beyond a beginning fixerupper like me and I took it to a mechanic who replaced the spark plugs and got it started. They also tried replacing the air mass meter which didn't fix anything. The car runs now but still has a poor idle. 2 tanks of gas before the problem started, I put a Techron engine cleaner in the fuel tank. Would that have done anything? Does anyone have an idea what to look for? What would give it the poor idle? Is there a way to tell if timing belt is going bad or do they just break? Is there a way I could check why the Check Engine light went on myself?

Any help would be GREATLY appreciated