160° Thermostats in a 6B: What is normal behavior?

Hi Francis, thanks for the reply. I appreciate the back & forth and please don’t take this response as less than cordial.

With that said, I think it is you who is misunderstanding what’s going on. Think about it…you start your ‘cold’ engine and it starts producing heat. This heat warms the pistons, block, oil, coolant, etc etc. Even when tied to the dock the temp starts rising. 100, 110, 120, etc. Hotter and hotter if you were to let it sit there and idle even with absolutely no load. After a good while, when it gets to 140…it just stops getting hotter. Why? I mean the engine is still producing heat, of course. Where is the heat going? To the air in the engine room? Only a tiny fraction. So why does it heat up, up, up and then just magically stop at 140 (just like mine!)? Because something changes at 140…the TEMP stabilizes or becomes STATIC (THERMO-STAT). At 140 your thermostat is OPENING and allowing the hot (140°F) coolant flow through the heat exchanger and transfer heat through the bundle and into the cool seawater and then the heat goes out the exhaust with this slightly warmed seawater. THAT is why you idle at 140–the same as me! If you put thermostats that didn’t start opening until 180, guess what your steady-state idle temperature would be? 180!!!! Even at the dock with no load!!! Yes it might take a good while to get there but it would get there for sure. Practically speaking there is nowhere else for the heat to go. Now what if you replaced your thermostat with a solid piece of metal that blocked all flow? You really think you’d still idle steady-state at 140?! Of course not! The 140 steady-state temp is a function of the thermostat behavior.

I don’t get why when I asked about idling at a steady-state 140°F you told me to check my gauges? Seems like the better response would have been “I have the 160 t-stats and idle steady state at 140°F as well so it seems like it might be typical.” You eventually shared that info which I find very informative so thanks for that.

Obviously I’m very pleased by the idea that I can run at full load/WOT/3000+rpm and not touch even 180°F. Cruising at 160°F (or below) is going to give me great peace of mind and getting the engine temps down at cruise was the whole point–so overall I’m very happy with what I see so far.

I just like to understand things as completely as possible and the 140°F idling temps have me scratching my head a little. I have only heard the idling temps of 2 boats with these stats in them (yours and mine) and they both idle steady-state at 140. Magical coincidence? No, definitely not. I would love to hear from Tony if this is by design or not…some detail on what the intended operation of the t-stat is. I have noticed that these rambling threads (yes I’m very guilty) don’t get the involvement so if we don’t get an answer I will post a different, much simpler thread to try to get the answer to the question “What is the typical steady-state temp of a 6BTA with a SBMAR 160° t-stat at idle, cruise, and WOT under load?”

Francis Valerio said “If the engine is overheating when idling then the cooling system is not working properly.”

I totally agree with you and this contradicts your earlier statement that the engines do not even produce enough heat to run at 160 at idle. This just isn’t true. If your engine idles ‘steady state’ at 140 then coolant is getting by/through the thermostat at that temperature, as it is in mine, or you’re boating in the arctic circle. I don’t follow you at all when you say you can’t get an engine to operating temperature at idle…of course you can.

Gene Fuller said “The usual definition for the “temperature rating” of a thermostat is the point at which it *starts* to open, plus or minus a couple of degrees. The thermostat will be fully open 15 to 20 degrees higher than the rated temperature.

* The thermostat is faulty.”

I totally agree with how you describe the ‘usual’ operation of a thermostat which is why I’m surprised that it climbs to 140 at idle…and stops (although I would say the transition typically happens over ~10°F rather than 20°F). I remember boiling thermostats in the past to make sure they were opening and at what temperature and it happened over a pretty tight temp range although this is dependent on how the t-stat is designed so I’m sure there are plenty of different profiles.

I also understand your logic behind the 3 conditions but I am not sure that if the t-stat starts to open at 140 the t-stat is ‘faulty’ but rather could be considered ‘unconventionally labeled’. SB markets these as ‘high flow’ t-stats. Perhaps we could hear what exactly that means but I think perhaps it means they open in a slightly different way than a ‘conventional’ t-stat which could include starting to open at 140 and be fully open by 160?? That is the info I’m after. At least one other person here reports idling at 140 with these thermostats…curious to hear other reports. Also want to be clear I’m not complaining but rather just curious to know if this is typical behavior for these t-stats.

As I mentioned, with the 180F t-stats, the engines always got to 180 idling, even when the seawater side was fresh. These engines produce plenty of heat at idle to even overheat (let alone get to 160) if the t-stat prevents coolant flow, although it would take some time.

I’ve confirmed accuracy of the gauges w/an IR thermometer–the gauges are not the issue.

Really asking about the behavior of the 160 t-stats compared to the 180’s. I noticed that the 160’s have a larger ‘pass through’ hole but can’t imagine that is the difference. I’m guessing that they start opening at 140 and completely open by 160 but not sure. Hopefully someone who knows can fill us in.

Good thought on the shut-off (I believe it would actually have to be a bypass) for the heater although I will have to study the plumbing to see how that would work, although truthfully if I go to that trouble I will likely just delete all of it rather than making it even more complex.