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FAQ

 FAQ #36
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What is a Glow Plug and what can go wrong?
A glow plug is simply a starting aid on a diesel engine.

Once the engine has started it relies on air,fuel,and compression. There are a few types of glow plugs in use in vehicles. The older "loop" style were large and had a simple loop of a heavy special wire protruding from the end that got red hot and heated nearby air up in either a pre-combustion chamber or on some diesels like older Perkins in the intake manifold.

Perkins actually had a fuel line with a valve connected to the intake style that when the element got hot enough it would open the valve and let a drop or two of fuel hit the hot element to vaporize it and aid starting. Both of these types had very long pre-heat periods compared to today's "pencil" style glow plugs.

"Pencil" style glow plugs are all encased and simply come to a smooth point or blunt end. The tip of these gets red hot rather than a loop or coil of wire. These glow plugs run at a lower voltage than the older styles and heat up much faster resulting in quick start up times and less draw off the battery for better winter starts.

Some systems have a timer that works with a temperature sensor that "cycles" the glow plugs on and off until the coolant reaches a certain temp. There are also coolant glow plugs that aid in warming the coolant faster so the engine warms up quicker to burn the fuel more efficiently.

Here are some visual images of faulty Glow Plugs (click the image to enlarge)

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FAQ Posted by j-c
Info Created: 25 April 2007
Last Updated: 07 October 2014