Diesel Thermostat

I have an '85 K5 Blazer with a 6.2L diesel. I pull a boat with this and when I go up hills, it gets warmer than I would like to see. So therefore I was
thinking of throwing in a colder thermostat. Before doing this I asked a mechanic friend of mine about it. He said that you do not want to mess with the thermostat setting on a diesel for various reasons. Could anyone confirm this and provide more info? Thanks and hope to hear more soon.
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Can anyone here respond to this? This group has helped me in the past with issues on this particular diesel. Thanks.

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Sorry but a lower temp thermostat will not help lower your running temp in this case (under load). The temperature rating is just the level at which the thermostat will be at full-flow. If you are overheating with a stock thermostat, then you will overheat with a lower temp thermostat as well, it just might take you a few extra minutes to get to that point. All else being equal, both will flow the same amount at wide-open, it's just that the lower temp thermostat will get to wide-open sooner than stock.
By using a lower temp thermostat under normal operating conditions you just may prevent the engine from achieving its proper operating temperature, which can be bad for performance, mileage and emissions. Proper combustion depends not only on compression, but temperature as well.
What you may need to consider are a few simple modifications, including an external oil cooler, an auxillary fan for the radiator, or perhaps a new radiator if yours is getting clogged and not flowing well. A higher capacity water pump may help, as well as simply downshifting and driving slower up the hills. Remove anything from the grille that restricts the airflow through your radiator, make sure you have a proper fan shroud to help your fan push air only thru the radiator and not blow it over the top and bottom, and check the condition of your fan clutch (if it's slipping it could cause your fan to turn slower than needed). A thorough cooling system flush couldn't hurt as well.
Cheers - Jonathan
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Jonathan A. Race
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Thank you very much for that thorough explanation Jonathon. I will start with other items then.

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