Nothing persuades a spark plug to come out like a 3/8" impact wrench. I use a lower powered model that delivers lots of blows but very little force so the plug doesnt break. It usually only takes 4-5 seconds before it comes right out. I always run a chaser down the threads to remove any rust residue and clean up everything before replacing the plugs.

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Matt
2003 Jetta Wagon TDI 145,000MI
1999.5 Jetta TDI 287,000MI
2001 Suzuki SV650S 26,000MI
2008 Yamaha Vino 125 11,000MI
http://www.crazymatt.org