Just got a 2005 Duramax with a seized injector to fire up after three days of fighting it.
I was about to pull the head when I finally got the stuck #7 injector to pop free by soaking it in a mix of ATF and acetone for 48 hours and using a slide hammer I borrowed from the shop down the street.
My uncle swears by that mix for every stuck bolt in his garage. It's saved him from buying three new tools this year alone. Sometimes the cheap fix is just the right amount of wrong to work.
Remember my buddy's old farm truck? He did the same thing, soaked a stuck caliper bolt for a week with that mix. It finally broke loose right as he was loading it up to take to a shop.