Since I only had 4 working glow plugs going into winter in Michigan I started the job last Friday to replace them. 7 came out great but 1 turned hard all the way out until the outer sleeve broke off. I sprayed seafoam deep creep in the hole over night but it didn't even seep in. I tried pulling it with vise grips and pounding it down and all that did was break the electrode off. After soaking it some more I was able to tap what was left of the tip in about 1/16" and the seafoam made its way down around the glow plug piece so I put a board over the hole and fired it up. Within 5 seconds it blow out and I was back in business without pulling the head.