On my 87 SDL I needed some space for components of my vegetable oil conversion, so the fuel thermostat was sacrificed. Since I live in a relatively warm climate, and there is a coolant-filled heat exchanger right beside the diesel lines anyway, I am not too worried about gelling. I haven't had any trouble yet without it (of course I run 90% of the time on veg. oil, but the other 10% on diesel hasn't been a problem so far without the thermostat).

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Keep everything as simple as possible-but no simpler--Albert Einstein