Carbon can be the root cause of a problem, acting as a heat soak, causing hard starting and premature heating coil failure. A proprietary glow plug reamer set should be used with silicone free grease to remove the carbon (Fig 3). A melted or broken heater tube tip is a sign of over voltage (fault with the control system), or abnormal combustion (this can be the result of incorrect injection timing, flow rate or excessive lubricating oil entering the combustion chamber). A swollen heater tube tip can also be a sign of over voltage. Using the same tool you used to remove the old glow plugs, install the new glow plugs being careful not to over-tighten them (this will make them harder to remove next time). Closely examine the glow plug and heater tube surface. A slightly ‘shiny’ appearance indicates the heater tube has been caked by carbon and ‘scuff/swirl’ marks are where the plug has been unscrewed through the carbon. If you have to use a bicep-building level of force to remove a glow plug they might either be damaged or have been in the car for too long.