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. Make sure the engine oil level is at the max line. If it is an automatic, make sure tranny fluid is correct and not overfilled. Air filter housing might not be put back correctly. Many mechanics have a hard time putting our air filter housing back correctly. And if you suspect bad gas, put in a bottle of gas stablizer. Is the Check Engine light on?
Edit. Why did you think you needed to do something to the timing (belt) chain? How many miles on the car?
