I have a 1997 Cobalt 190 with a merc 5.7l efi. After running about an hour or so I have errataic tach movement, hear a mettalic pop or backfire, and loose power over 2500 rpm's. The temp gage maintains 170 to 175 degrees. No alarm goes off. Boat has less than 300 hrs. Was told tach may be...
