My grandpas corvette is having issues with overheating (antifreeze boiling) and an idle issue where it wants to jump and fall quickly. The temperature also climbs rather quickly, trying to exceed 230. We took it out for a drive and got it warm, the temps started climbing a little higher than what they are supposed to and the idle when we put it in park was bouncing probably between 1-3k rpms. We drained the anti freeze and put some more in and when we cranked it back up it started boiling and the temp gauge hadn't even exceeded 215 degrees. We turned it off to let it cool and then cranked it back up to move into the garage, it started backfiring which its never done. Needless to say I'm clueless on what the issue could be. We are pretty sure all the vacuum lines are sealed.

 

Not sure if you’ve found the issue yet but if your leaking any coolant or oil then sounds like you’ve got a bad head gasket. If not then you may have multiple issues, one being an air leak of some sort possibly and the other being blocked coolant passages or bad fans maybe, C4s are known to run very hot and especially during the summer when it gets warm out but if your seeing anything over 230-235 then it’s an issue.

 

Replace the T-stat, pressure check the radiator & cap.

If all looks good and still an issue, pull spark plugs & run compression check on each cylinder.

 

Welcome to the forum. Did you ever successfully solve this problem?