Do I need to drain the coolant to replace a thermostat on an '85 300TD?

-Erik

Its not necessary, but if you have not drained the coolant in a while, it might be a good idea while you're in there.

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2001 Dodge Ram 2500 Cummins Turbo Diesel - 4x4, auto, 3.54 gears, long bed
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'92 300D 2.5 Turbodiesel - sold
'83 300D Turbodiesel - 4 speed manual/2.88 diff - sold
'87 300D Turbodiesel - sold
'82 300D Turbodiesel - sold

You will spill some coolant if you don't drain at least 1/2 gallon out first. You don't need to remove all of it, just get the level down below the stat. Be gentle with the plastic housing and remember that you will need a new o-ring seal.