Glow Plugs

Have you seen the curly yellow Glow Plug light on your dashboard?

It should show before startup, but never after the engine is running

What is a Glow Plug?

A Glow Plug is a heating element that heats the fuel and air within a diesel engine, ensuring quick starting in cold conditions. They are pencil shaped and are installed into the cylinder head of the engine. There is one for each cylinder. There is a control unit which controls when and for how long the glow plugs are heated by electrical current.

In a Mercedes Sprinter or Vito, the Glow Plug light will illuminate on the dashboard before starting, indicating the pre-heating phase. If this Glow light continues to illuminate after starting or whilst driving, it is indicative of a fault with the Glow Plugs.

How does a Glow plug Work?

Glow plugs look like long metal pencils and they are screwed into the engine cylinder head. They have a positive wire connected to the external end, which will cause the tip of the glow plug to glow red hot when powered! Similar to a toaster heating element. 

This red hot tip allows the diesel fuel to combust when the engine is cold, improving starting performance and emissions.

heated glow plug

The Glow Plug light stays on after startup

If your Glow light is illuminating after the engine starts, your engine ECU has detected a fault with the glow plugs or control unit. At this stage we recommend that you contact us promptly. We have a variety of diagnostic techniques to locate any faulty glow plugs specifically (for example cylinder No.1 + No.3) or control unit. 

The sooner that you get your faulty glow plugs replaced, the likelihood of your glow plugs becoming seized or stuck in the engine will be reduced. 

Symptoms of faulty Glow Plugs

If your Glow Plugs are faulty, you may notice the following:

  • Difficult Starting: This is the most common glow plug related symptom and will be most apparent on cold mornings
  • Engine Misfiring and Smoke: After startup, you may notice a rough running engine or smokey exhaust, which will indicate poor combustion.
  • Engine Light: Not all models will show an engine light if there is a glow plug fault, However it is common for the V6 models.
  • Glow Plug Light ON: This is the most common indication of a fault. Remember that the glow plug light should not show after the engine has started.
mercedes glow plug warning light and control unit relays and electrical multimeter on workbench

Repair Methods for Glow Plugs

We recommend the removal of faulty glow plugs as soon as your Vito or Sprinter shows the Glow Plug dashboard light after starting. First, we will diagnose which glow plugs need replacing and whether the glow plug control unit relay will need replacement.

Note that it is often not necessary to replace all 4, 5 or 6 glow plugs at once. Eurostar Diesels will remove the faulty glow plugs and reinstall new glow plugs with anti-seize lubricant. The repair can be completed in less than a few hours and help your engine run optimally.

In Summary - Glow Plug Light

If your glow plug warning light shows AFTER startup, then you have a fault and you should have the problem diagnosed and repaired. The good news is that you don’t necessarily have to replace all 4, 5 or 6 glow plugs. We’re always happy to chat so give us a call or send us a message!

glow plug dashboard light

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