Frequent Glow Plug Failure

If you are constantly having to replace glow plugs do not immediately blame the plugs! Ream the carbon!

Kent has often said that the glow plug is the achilles heal of the diesel engine. The engines are very reliable, but if the glow plugs or the pre-glow circuit fails, you will be stranded. Most quality glow plugs are very reliable and some will last for years without needing replacement. If you have recently installed new plugs and they only lasted for a few months, or you find you are having to replace your glow plugs every 6 to 12 months, look to find the real culprit BEFORE installing your next set.

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Problem & Solution

Common among these chassis:

W110 W111 W112 Sedan

Here is a list of the most common causes of pre-mature glow plug failure:

1. Excess carbon buildup in the pre chambers. This can cause shorting or overheating of the plug leading to tip burn out. Always use the correct carbon reamer when changing glow plugs. See video explaining the use of glow plug reamers in reaming out excess carbon buildup.

2. Glow cycle too long. This could be due to an incorrect relay installed, a sticking relay, or a feedback circuit not turning the relay off soon enough. In the case of a manual conversion, it could be due to consistant unnecessary long pre-glow times. See Kent's manuals on glow plug system troubleshooting for your specific model to learn more.

3. Glow relay not turning off. This is not that common but it does happen. It is usually caused by a failure inside the relay. When this happens, you may also notice your battery is not able to charge properly due to the high current drain on your electrical system. To check for this condition, start the engine and after a minute of running test the glow plug circuit with a meter or 12 volt test light. If you have voltage showing up at the wiring connnections on the glow plugs it means your relay has NOT turned off and will need to be replaced. 

4. Incorrect spray pattern of the fuel injectors. This is hard to diagnose but it can be a problem. If your fuel injectors are squirting a large stream of raw fuel this can stress the tips of the glow plugs (because of a too rapid cool down). Imagine if you heated up your glow plugs red hot and then constantly dipped them in cold water. That's right, you would burn out the element.   

Related products to help you solve the problem are listed to the right. For new glow plugs just type "glow plugs" in the search box above.