Downside of constantly open coolant bypass?

JHZR2

Staff member
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New Jersey
Was reviewing the FSM for my 96 Cummins 5.9L. I was interested in the bypass system, and the fact that the thermostat has a dual spring setup to block the bypass when warm.

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Some aftermarket thermostats dont have the second plunger that blocks the bypass. So the bypass would then be open all the time. If that’s the case, I’d understand that cooling may be less efficient because it’s blending some coolant that’s just circulating in the block, not forcing all flow through the radiator. But is that a bad thing? Unless max cooling is essential, could this be a good thing?

Or, is an unblocked bypass going to permanently allow preferential flow in that circuit, and be problematic?

thanks!
 

wwillson

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Naperville, IL
Can you still find the original style thermostat? My preference would be to keep it the way Cummins designed it.

In reality, being that you don't pull heavy trailers, and aren't in heavy boost all the time, the aftermarket thermostat without the bypass closure is probably fine. The Cummins powered Dodge trucks we have back home pull heavy, like 30,000+ GVW, so I would want the original type thermostat in those for maximum cooling efficiency when the ambient temperatures are hot, but when it's cold outside, it won't make any difference. When the Cummins spends most of its time in boost, it rejects a LOT of heat.
 

JHZR2

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New Jersey
I’m not opposed to keeping the original design. It was more of a curiosity. I can see that forcing ALL water through the radiator should yield better cooling, and the bypass “blends” some of the hotter water back around. Or at least I think that’s the approach. This engine also has two bypass, the FSM is very explicit about a second bypass and check valve to negate the need for jiggle pins and other design attributes on the thermostat.

I anticipate 50% of my use will be with a significant payload, near max, and 50% will be unloaded. No trailering expected unless I buy a boat. So keeping heat in the engine is of interest to me.
 
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