Here check this out Troubleshooting GPR & GP's
or this 7.3 glow plug system diagnosis
Glow plug removal:
This is a straight-forward service. After removing the valve covers and unplugging the glow plug(s), loosen the glow plug a couple of turns (10mm deep socket, 1/4" drive). Then push a 4-6" piece of vacuum line over the end of he glow plug and use the hose to unscrew and remove it. Use the hose to install and screw in the glow plug, then tighten with the socket. The socket will contact the rocker arms if used to remove the glow plug. If the glow plug is difficult to turn you may be able to loosen it by working it back and forth--turn counter clockwise one turn then clockwise half a turn--until it turns freely enough that it can be loosened with the hose. If the glow plug probe is severely carboned up or swollen, the barrel may screw out of the head leaving the probe stuck in the hole. In this instance you may be able to remove the probe by removing the adjancent rocker arm and push rod to gain enough room to get ahold of the probe with a pair of needle-nose pliers. Have someone hold the pliers to prevent the probe from falling into the cylinder and use a long punch to loosen the probe by tapping it down. Once the probe is broken loose, work it up and down in the hole until it can be pulled out--some WD 40 or other solvent may help to loosen any carbon on the probe once it can moved, just remember not to use too much and to remove any residual by cranking over the engine with the glow plug out. You may be able to dislodge the seize probe by cranking over the engine, just remember to reinstall the rocker and pushrod if removed, and place a blanket or fender cover over the glow plug to keep it from shooting out. If the probe falls into the cylinder or cannot be loosened, the head will have to be removed to extract the probe. To prevent damage, remove all the glow plugs on the head to be removed and install them after the head has been reinstalled.