I have a 2004 Cobalt with the 496 Mercruiserbig block engine in it. Where is the OBD computer hook up port? I have been having some random power loss, and shuttering issues. The symptoms will disappear as soon as they appear. The cam sensor and crank sensor have been replaced. Almost seems like a "bad gas" symptom. Suggestions?? I wanted to see if any codes have been thrown, without being raped by a dealer or marina.