In post number 13 you see one of my failed Monark glow plugs. It is heating up in the wrong place.

One year old bad glow plug

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84 300D, 82 Volvo 244Gl Diesel

My understanding is that they should be reemed prior to installation of new glow plugs?

There have been members who have had trouble starting (longer time cranking before starting) that have pulled out their plugs, reamed the holes and claimed starting was faster and easier.

Here is my guess or guesses why.

The more the glow plug tip is exposed to the air in the pre-chamber the better it heats the air up. Carbon build insulates and decreases exposure.

The thing comes from an experiment I did. I hooked up an automotive amp gauge in line with a single glow plug. Unfortunately, it only went up to 60 amps.

So, I observed what happen. When I applied the electricity, the gauge pegged out at 60ams. As the glow plug heated up the amperage dropped to about 16 amps and held there.

My conclusion from this was that the longer the glow plug was cold the longer amount of time the glow plug was going to be operating at the higher amps. I believe that spending more time at higher amps shortens the life of the glow plug.

Depending on how much carbon is in or around the glow plug it either insulates the glow plug keeping it at higher amps or if there is so much that the heat from the glow plug is conducting through the carbon to the cylinder head and onto the coolant your glow plug sending a lot heat trying to heat up your cylinder head and coolant and not as much into the air in the prechamber. At the same time that keeps the plug colder and keep the amperage higher.

Where does the carbon come from. Easy old diesel engine with the timing off, valve adjustment off and some timing chain stretch (timing chain stretch changes the valve timing and the fuel injection pump timing). The opening/pop pressure of the injectors is low and the injector nozzles are worn out and not atomizing well. Next that car is mostly driven around the city where it spends a lot of time in traffic idling. All of that is ideal to build up carbon.

So that is my opinion on it.

So in the manual it has what kills glow plugs is when things in the pre-combustion and combustion chamber are the not happening the way they are supposed to or when they are supposed to.
In particular if the injector nozzle is so bad that the spray ends up directed at a glow plug tip it will burn through it.

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84 300D, 82 Volvo 244Gl Diesel