My block heater makes a hissing noise kinda like an electric kettle just seconds after it's plugged in (if the engine is cold). The top rad hose is warm after an hour or so . I've heard never to use 'starting aids' on the MB diesel because of the pre-chamber design. The battery has to be in great condition once the temps start to drop. I have cranked mine for up to 30 seconds to get it going when my GP relay was hooped. I usually use the block heater for 3-4 hours before starting, have it on a timer.

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