Serial Block Heater Failures

I am having repetitive failures of new, OEM block heaters installed on my M617 diesel engine (W123 300D).
My latest theory is that there must be metal in the water jacket of
the block (I may be "all wet" on this) or something else that is shorting out these heating units.
Anyone have any thoughts on this, and/or any ideas on how to verify or "flush" the block?
Thanks in advance for any info.
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Remove the block heater and inspect it and the block. I've installed a number of Benz heaters and it is a simple task once the original bung is removed from the block.
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I have inspected both failed heaters and there is no outward sign of failure.
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What is the resistance in those dead heaters?
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I do not get a reading with the ohm meter terminal-to-terminal on the one I have removed from the engine.
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