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Replacing Glow Plugs

Pictures and Writing
by Sean C. Roland

Symptoms of Bad Glow Plugs

  • Misfiring when starting and during warmup
  • Hard starting
  • Glow Plug light on dash blinking after startup, or not lighting at all
Ordering Plugs
  • Order all the glow plugs needed.  FastLane has them in stock, and the page can be found at http://www.mercedesshop.com/catalog/index.php3.  Order either Bosch or Beru plugs, both are Original Equipment Brands (OE).
  • If your car is post-1980, you should have the pin type plugs. 1980 was the transition year for the plugs, so check to see which ones you have. The older loop plugs are larger and are connected by a loopy look wire, while the pin type is connected by separate wires. The car in the illustration has the newer, pin type plug. 
  • While you are buying plugs, consider buying a new hand pump if you have not replaced yours already. This will come in handy when replacing the fuel lines. 
  • Locate the glow plugs (See Illustration #1). They are on the drivers side of the engine, below the fuel injectors and metal fuel lines.  I recommend you remove the metal fuel lines while replacing the plugs. Space is very tight and it is easy to drop tools without the lines removed. Some people are able to replace the plugs without removing the lines, which will save you from using the hand pump later on.  

Illustration 1:

 

Fuel Line Removal  (skip if you are not removing the fuel lines)

  • Loosen the bolts on the fuel lines at the injectors and the Injection Pump.  I found using the proper size wrench was the best tool.

  • Unscrew each of the bolts until you can slide them up fuel lines. 

  • Carefully lift the fuel lines and remove them from the engine compartment.  DO NOT BEND THE FUEL LINES, IT WILL RUIN THEM! (See Illustration #2)  

Illustration 2:

 

 

Glow Plug Removal 

  • Remove the small nut (size 8) on the glow plugs that hold the electric line on the plugs.  Save the small nuts, as new ones do not come with the plugs.  Push the wires aside so as not to obstruct your view (See Illustration #3). 

Illustration 3:

 

  • Loosen and remove each plug using ratchet set (size 12). The ratchet can be used on plugs 1-4 but a wrench is needed on plug 5 due to space constraints (See Illustration #4). 

Illustration 4:

 

  • Clean the electric contacts and the small retaining nuts before reassembling.

Glow Plug Replacement 

  • Use a reamer to clean each glow plug hole before replacement. 

  • Replace each plug and tighten until fair resistance is felt. 

  • Put the wires back on the glow plugs, and replace and tighten the retaining nut. 

Fuel Line Replacement 

  • Carefully place the fuel lines back in place, and line them up.

  • Tighten each nut on the Injection Pump and Injectors slightly to ensure proper alignment, then tighten all the nuts fully in both locations. 

  • Locate the hand primer pump (See Illustration #5) 

Illustration 5:

 

  • Unscrew the pump and start pumping!  It will usually take about 200 pumps to rid the system from air.  You will know you are done when the clear fuel return line starts to fill, and a bubbling sound can be heard. 

  • To gauge your progress, you may want to crack one of the injector fuel lines nuts and watch for diesel.

  • This process can also be accomplished by cranking the engine for a while with the fuel lines open, but this may wear down your battery.  

The Last Word 

Hopefully after replacing the Glow Plugs, the light on the dash will quit blinking, and your starting will be much easier. 

If not, you may also want to check the Glow Plug fuse. It is located in the relay box on the drivers side of the engine compartment, bolted to the side of the compartment. The fuse is a small, exposed strip. 

 

Good Luck!