Gord,<P>There's a lot of lore surrounding thermostats. They are very simple things whose purpose is to be closed upon startup to create an artificially small water jacket that warms up quickly to shorten the warmup cycle. Once operating temp is achieve, they open to bring the radiator into the circulation (circulation was restricted to the engine block alone).This pays dividends in reduced wear, and the emissions systems and engine computer of modern vehicles also rely upon them.<P>Putting a "colder" thermostat in an engine simply prolongs the warmup period slightly more than the factory intended. This is less an issue with diesels, as they don't have timing maps and the like in an engine computer. But I don't think it would have any impact on keeping a hard working engine cooler. Once it's fully open at high load operating temps, it's open and already providing max cooling potential for the engine. Whether it overheats or not after that point is up to the integrity and adequacy of the cooling system.<P>Regarding buildup of blockages in the radiator, I've never seen them benefit from a flush. Generally, once they're blocked with scale or other crud the 'flush' can't get in there and any thing strong enough to dissolve that stuff might also hurt the engine block itself or the many water jacket sensors. I find it best to physically remove the crud to ensure full radiator circulation.<P>DougM<p>[ 16 December 2001: Message edited by: DougM ]