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If my 99 ford f350 with a 7.3 diesel is not plugged in it will not start under 50 degrees what should I check?

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When you say, "Won't start" you mean it won't crank or it won't fire when you are turning it over? More than likely your glow plugs are not working optimally, but based on your question, I'm not sure. – Pᴀᴜʟsᴛᴇʀ2 Mar 5 '14 at 16:08
won't fire cranks good glow plugs or glow plug relay – JACK D Mar 6 '14 at 17:38
it was glow plugs – JACK D Mar 17 '14 at 15:14
can't be sure but here's an old trick I learnt in the Victorian high country (Australia) during icy conditions when diesel won't start.Pour hot water over manifold.Will not solve your problem but good to know. mike western australia – mike Apr 7 '14 at 7:37

From your comment, I'd suggest you take a look at the glow plugs and/or the glow plug relay. Here is a YouTube video which should explain how to check the relay:

Here is the link to the Google search I used to find it, which may even provide better results.

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