I have 96 gmc 1500 vortec v8 that misfires on cylinder #6 and it idel's high all the time and the vacuum system is off

I had a cold start up miss on my 97 Vortec and had suspected a head gasket or cracked head because it would clear up after it warmed up . I had noticed that the plug on #6 had white deposits on it leading me to suspect that it was coolant .I found another person had posted the same symptoms as those I had experienced and found the problem to be a vacuum leak on the intake gasket on Cyl 6 . Bingo I blocked off the PCV valve and blocked the tube off of the inlet to the Right side valve cover . I started the truck and put my thumb over the hole in the valve cover and that confirmed that there was a vacuum in the crank case . The vacuum leak caused cyl #6 which is the reason for the white on the plug . It disappeared when the eng warmed up and the gasket expanded enough to stop the leak . I replaced the intake gaskets with Felpro Ultra Dry and that has completely solved the problem . It's been a year now and wish I could have had the correct diagnosis right away . This may not be your problem , but it is another thing you can look into and maybe at least rule it out . Good Luck I had a cold start up miss on my 97 Vortec and had suspected a head gasket or cracked head because it would clear up after it warmed up . I had noticed that the plug on #6 had white deposits on it leading me to suspect that it was coolant .I found another person had posted the same symptoms as those I had experienced and found the problem to be a vacuum leak on the intake gasket on Cyl 6 . Bingo I blocked off the PCV valve and blocked the tube off of the inlet to the Right side valve cover . I started the truck and put my thumb over the hole in the valve cover and that confirmed that there was a vacuum in the crank case . The vacuum leak caused cyl #6 which is the reason for the white on the plug . It disappeared when the eng warmed up and the gasket expanded enough to stop the leak . I replaced the intake gaskets with Felpro Ultra Dry and that has completely solved the problem . It's been a year now and wish I could have had the correct diagnosis right away . This may not be your problem , but it is another thing you can look into and maybe at least rule it out . Good Luck