Copied from the sticky in the General Nitro Forum:
What you need to know about glow plugs:
Glow plugs are very important to the performance of your engine. You can also read your glow plugs to determine if you are running too rich or too lean.
A brand new glow plug has a wire or filament that is shiny. If after running the wire is still shiny and the bottom of the plug is wet, then you are running too rich. You are probably getting only 85% of the maximum engine power.
If the wire turns starts to turn gray, and the bottom of the filament is slightly wet, then you are almost near the peak engine power.
Once you have a gray wire, and the bottom of the plug is dry, you are at 100% maximum power.
Once the glow plug wire starts to distort, you have exceeded the maximum power and are running too lean. Adjust to a slightly rich setting.
Hot plugs are used for hot summer days, while cold plugs are designed for cold "winter" days.
LINK FOR GLOW PLUG TROUBLESHOOTING:
http://www.rc-truckncar-tuning.com/Glow-plug-reading.html
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