I have an arm-chair analysis:

With the series-arrangement, all glow plugs are supposed to have equal resistance, then each plug gets equal voltage, which creates equal heat.

If one plug is weaker than another, it will have higher resistance, which will mean higher voltage, and higher heat. As the wire heats, its resistance decreases, which increases the voltage and heat further.

Therefore, replacing one or more glow-plugs will naturally create a disparity between them, so that some are getting too hot, others not hot enough (typically a new one will have lower resistance). The weakest ones are always getting more heat until they fail.

The best scenario is to replace them in sets, so that they are all the same resistance and none will have significantly higher voltage than any other. Of course buying a new set might justify simply buying the "pencil" style that work in parallel.

The newer style all are parallel, in which case all get the same voltage, regardless of resistance, therefore they don't kill the weak one and longer life should be the result.

Your set probably had a weak one (or two), and the additional voltage/heat of the new battery was what put it over the edge.

For the math behind this:

In a series arrangement, the current is constant through the entire circuit.

I=E/R or current=voltage/resistance. Since the current is constant, as the resistance increases so will the voltage (if R doubles, E would double also to maintain I as un-changed). P=IxE or Power (watts or heat) = Current x Voltage.

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Gone to the dark side

- Jeff