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Pelican Technical Article:

Glow Plug Replacement

Steve Vernon

Time:

3 hours3 hrs

Tab:

$60 to $150

Talent:

***

Tools:

17mm Crawfoot, 12mm, 8mm wrench, reamer

Applicable Models:

Mercedes-Benz W123 (1977-85)

Parts Required:

Glow plugs

Hot Tip:

Always ream the plug opening before installing new plugs

Performance Gain:

Better running motor

Complementary Modification:

Change fuel filters

Diesel engines do not have spark plugs but instead they work on the principle of self-ignition by spraying atomized fuel into a highly compressed, and therefore a highly heated, combustion chamber; this causes the fuel to self-ignite.

On cold engines the self-ignition temperature may not be attained by the compression and may need some help bringing up the temperature of the compressed air. This is where the glow plug comes into play. In the older engines the temperature is taken from a coolant temperature sensor and in newer engines it is monitored by an ambient air temp sensor.

The glow plug heats the air in the pre-chamber and does not enter the combustion chamber or directly trigger a spark. The glow plug heats to 1652 degrees F in twenty seconds which is why you have to wait for the glow plug light to out before attempting to start the motor. The glow plugs are only used to get a cold motor started and stops after the chamber reaches a certain ambient temperature. The glow plugs, like spark plugs, do wear out and if your diesel is having hard starting problems on cold mornings the glow plugs should be one of the first things you check.

When changing your glow plugs you should always clean or ream out the area where the plug goes as there will be a buildup of carbon in this area that if not cleaned out will eventual cause the glow plugs to be mute.

Over the years your car may have been serviced by multiple people including previous owners and this can lead to parts replaced with different size fasteners and hardware. The sizes of the nuts and bolts we give may be different from what you have so be prepared with different size sockets and wrenches. If something is different on your vehicle please let us know and share your info to help other users. If you have any questions or comments or even a different procedure you would like to share please leave it below and please leave your vehicle information.

There are five pencil type glow plugs on the 300TD and they are located on the left side of the cylinder head (red arrow).
Figure 1

There are five pencil type glow plugs on the 300TD and they are located on the left side of the cylinder head (red arrow). There is a lot of stuff in the way and changing the plugs and reaming the holes is made much simpler by moving a few things out of the way first.

You are going to be removing the fuel lines so make sure you have the proper tool which is a 17mm Crawfoot.
Figure 2

You are going to be removing the fuel lines so make sure you have the proper tool which is a 17mm Crawfoot. The fittings on the end of the fuel lines are very easy to strip or round off in the tight work space and if you round one off you are going to have to replace the whole line due to the flares at the end of the lines that preclude you from just replacing the fitting.

Make sure the engine is cold and that the pressure in the fuel system is diminished.
Figure 3

Make sure the engine is cold and that the pressure in the fuel system is diminished. There is no fuel pump relay that you can pull and run the fuel out from the lines, so be prepared for a little to spill out and make sure you catch it and dispose of it in accordance with the regulations in your area. Use a 17mm Crawfoot wrench and loosen the lower fittings on the fuel pump (red arrow).

The fittings fit snuggly over the lines, so give the lines a cleaning before loosening them; this will also stop dirt from the lines falling into the open ports on the pump.
Figure 4

The fittings fit snuggly over the lines, so give the lines a cleaning before loosening them; this will also stop dirt from the lines falling into the open ports on the pump. Here you can see the flare at the end of the fuel lines--if you round off one of the fittings you will need to replace the complete line.

Use the 17mm Crawfoot to loosen the fittings on the top off the lines to the injectors (red arrow).
Figure 5

Use the 17mm Crawfoot to loosen the fittings on the top off the lines to the injectors (red arrow). You may have to move some of the throttle linkage ball and cup joints (yellow arrow) while doing this, they should just pop off with a flathead screwdriver.

If the lines where installed correctly they will come off and go back on as a unit, but if there is any binding make sure to loosen up the line connectors when reinstalling so the lines fit both the injects and pump without any stress.
Figure 6

If the lines where installed correctly they will come off and go back on as a unit, but if there is any binding make sure to loosen up the line connectors when reinstalling so the lines fit both the injects and pump without any stress.

Remove the wiring harness (yellow arrow) to the glow plugs (red arrow) by removing the 8mm nuts on the end of the plugs.
Figure 7

Remove the wiring harness (yellow arrow) to the glow plugs (red arrow) by removing the 8mm nuts on the end of the plugs. The wiring will be brittle so brace it while turning the nut.

With everything moved out of the way you can use a 12mm wrench and remove the glow plugs from the motor (red arrow).
Figure 8

With everything moved out of the way you can use a 12mm wrench and remove the glow plugs from the motor (red arrow).

Glow plugs cannot be visually inspected for failure.
Figure 9

Glow plugs cannot be visually inspected for failure.

When changing plugs you should always clean or ream out the area the plug sits in.
Figure 10

When changing plugs you should always clean or ream out the area the plug sits in. This area will develop a lot of buildup of carbon and over time as the pre-chamber gets full of this carbon it will severely or completely block the function of the plug. The ridges on the reamer (red arrow) will safely clean out this carbon while you screw it into the head.

Use a 12mm wrench and gently screw the reamer in and out of the glow plug hole (red arrow) taking it out on each pass to clean the carbon off the reamer.
Figure 11

Use a 12mm wrench and gently screw the reamer in and out of the glow plug hole (red arrow) taking it out on each pass to clean the carbon off the reamer. Don't worry, any carbon that is dropped inside the pre-chamber will just be burnt off and sent out the exhaust. Installation is the reverse of removal.

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