95 e300d, 205,000. #6 was an open circuit. I soaked all of them with white vinegar for three days. I plugged the block heater in and got the engine nice and warm. #6 came out easily. #3 and #4 were also pretty easy. #2 was tight. I took my time and it came out. I used my impact wrench set on low torque and just banged away. Every now and then I tried to hand wrench it out. #2 was stuck on the barrel between the threads and the element. A large surface area of carbon made moving it very tough. #1 and #5 are extremely tight and I didn't try forcing them. They moved, but any more torque and it was going to bust. So I changed 4 of 6. Add that job to changing the engine mounts earlier this week and I've had enough for a while. At least the glow system works normally now.

Regards and thanks to all who went before and posted helpful info.

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2009 E320 Bluetec 117,000
1995 E300D 306,000 Sold
1996 Ford Taurus LX 130,000 Sold
1985 300TD Still 225,000 Sold
2016 Ford Fusion 24,900