Testing and Replacing Glow Plugs

The glow plug is one of the most important features of diesel engines. Almost like a first defense against the cold, glow plugs help your diesel engine start in the cold and prevent damage from freezing temperatures. Testing and replacing these little gadgets should be on every diesel owner’s winter prep checklist. 

In a nutshell, slow starting, and complete failure to start. Once the vehicle does start, missing, rough idle, hesitation, and lots of white smoke until the engine warms. If you have a four-cylinder, one glow plug bad will cause a slight hard starting and a miss. Two will cause a very hard start and very rough operation. Three bad plugs make the vehicle almost impossible to start and keep running. The same applies to larger engines. If your vehicle starts OK if you plug in the block heater overnight, but will not start if you do not, then you probably have a glowplug issue unless your engine has very low compression.

Step by step test:

1. Grab a multimeter.
2. Set your multimeter to ohms.
3. Find resistance value of your multimeter
4. Test the voltage of your battery.
5. Locate the glow plugs.
6. Remove the caps or connectors from the glow plugs.
7. Clip the multimeter’s negative lead to a ground point in the engine. 
8. Connect the positive lead to the top of the glow plug. 
9. Evaluate the multimeter’s reading. (Tips for step at bottom⬇️)
10. Replace the glow plugs (If one or more of the glow plugs is faulty, replace all of them. Never replace a single plug. If the area around the glow plugs has dirt or buildup, clean it first).

Step 9 walkthrough:

• Subtract the resistance value of your multimeter from the resistance value of your glow plug. For example, if your plug's resistance measured .9 ohms and your meter's resistance measured .2 ohms, the real reading for the plug is .7 ohms.
• All the glow plugs in your engine should have similar readings. One glow plug with a higher resistance can affect your engine's performance--even if the plug is still good.