For those with 871’s

[Traveler 45C]

“N’s” preferably.

When you next run your engines, at idle, would you please take IR temp readings on your exhaust manifolds, 1 inch from the head and post the readings? This is for comparison purposes to mine.

On mine I read:

Port eng, Left Bank- 177-184F
Port eng, Right Bank- 177-180F
Tstat housing- 167

Stbd eng, Left Bank- 177-184F
Stbd eng, Right Bank- All four 200F
Tstat housing- 173

It’s the 200F I’m concerned about.

If the T-stat on that side were acting up, would it cause that side to run hotter as indicated?

[Genesis]

Check the rack first. Most likely you're slightly out of balance on that side.

Other possibility is a partial obstruction on the coolant passages in the manifold, but I'd check the rack first as that doesn't require removing anything other than the valve covers.

[Traveler 45C]

I thought about the rack being out. But what are the chances of just one side of the engine going out of tolerance?

I figured the T-stat on that side would be the most likely cause of failure. Perhaps it is not opening all the way. That would partially block that side, wouldn’t it? If so, would I be seeing a temp increase on that side only?

So, why do we have 2 T-stats in these engines?

[Genesis]

Look at how the system works and it will make sense (follow the hoses and pipes)

Its not "one T-stat per side" - its a bypass system where when closed there is no coolant circulating through the heat exchanger, and when open it all does. There is a valve (thermostat) on both ends.

[Traveler 45C]

Right, not a “one T-stat per side” system, got that. But it is a 2 T-stat system. If one of those were only partially open it certainly would block “something”.

[MikeP]

2 Tstats does seem an unnecessary complication but, at the same time, it probably serves to ensure more consistent overall engine temperature. With 2 stats, the temps of each side/each head can be regulated. A single stat has to be centrally located which means that it senses an "average" temp of the entire engine.

A water pump will not deliver exactly the same flow to both outputs so there is the potential for one "side" of the engine to run at a different temp than the other. 2 stats provides some ability to regulate this individually. Whether it is really useful or just theoretically usefull...I couldn't say. Obviously, DD decided it was really useful.

It would be interesting to put a 160 in one side and a 180 in the other and see what happens with an IR temp gun. Interesting, but not worth the trouble!

[Traveler 45C]

Being that the outputs of both exhaust manifolds are recombined after the Tstats, I would think that if one of them were acting up, it would affect the overall engine temp, not just one side. I guess I just answered my original question: One bad Tstat won’t cause just one side to run hotter; it will affect the entire engine.

Perhaps this analysis (or over analysis) points to a rack or ex. valve out of specs as Genesis has indicated.

I think I got it…

But I still don’t understand the need for dual Tstats. Perhaps redundancy: If one fails to open, the other will and still regulate temps good enough to operate???

Whoops, forgot to mention... Those temps in my original post when taken at idle, warmed up as much as I could get them. I've edited it accordingly.

[Nonchalant1]

If you had only one thermostat, the coolant could get cooled a little by convection currents in the output side pipes to the cooler "stagnant" coolant in the exchanger and therefore cooling the operating coolant running in the engines. Like if a cold wind blows through your house from the front door to the back door, would you just close the front door to keep the house warm, and leave the back door open?

Doug