im thinking it might be the tune...im probably wrong though....lol...thanks yall

 

The tune is in the computer system, not in the engine. If you are using the HHR's original ECM, PCM etc., then it will run off of that tune.
My guess is you've got a bad sensor that you swapped over between engines or a plug in not making a good connection. MAF or MAP sensor would be my 1st thought.
Try pulling the codes and see if it has triggered a stored code. You may find one that will point you to the problem.