I too had a bad missfire and tranny shudder under load in my 99 Escalade. I had everything except the injectors changed at least twice and finally a car dealer told me to use about a cup of laquer thinner in each tank of gas and cured the problem. I have since used Seafoam with great results. I'm now a believer in injector cleaners of any brand.

 

i have a 97 Z71 and ive used sea foam many times before and have always had opposite effects. any time ive put it into my vacuum line, gas tank, or into the oil, ive always had to do more work then needed, seems like about 50 miles after putting sea foam in, im always replacing the fuel filter and spark plugs because it gets real slugglish, i was told that it is because sea foam is designed to break free all gunk build up and it normally ends up clogging your filter and fouling my plugs, but to me it was a fair trade off having to replace a fuel filter and spark plugs knowing that there is less crap in my engine or fuel lines, recently i was board and for no aparent reason took off my throttle body, while i had it off i took a look into the intake manifolds, i could see about 1/8 of an inch of black crap built up, whether or not that has been the cause of problems ive been having lately or not i do not know, but i do know that it cant be good for the engine in any way so i planned on taking the upper part of the intake manifold off and cleaning it all out, but in order to take that part of the manifold off i would have to take the "spider injectors" off, which is a plus because that gives me an oppertunity to take them out and clean them as well, all that would have been done by now but the day before i planned on starting to take it apart my transmission decides to go out on me, my point of all that is id consider taking the manifold off and cleaning the manifold and injectors because cylinder #8's injector could just be extremly clogged and even if you used sea foam, it could be a future problem that you could have just prolonged

 

It also sounded like you could have cracked the head. FYI - i bought a real nice Chevy 2.2 King Cylinder Heads http://kingcylinderheads.com/ and I noticed they carry 350 heads as well. Good Luck!

 

I am thinking I need to replace the spider injectors also. Even though it is not clean looking down in the intake, there is gasoline setting inside it. Why the updated MFI injectors?