Why are you changing the glow plugs? Unless you have a failure there is really no reason to change them. If one fails the engine will run just fine. The glow plugs are just used when starting. One cylinder with the bad glow plug will fire OK after the rest of the engine starts up and run. I had one glow plug fail because its power supply had too high of voltage that burned it out. The only indication was a check engine light. The engine ran well with the glow plug burned out.

Chuck