Lower Temp Thermostat with Closed Cooling System?

[Horigan]

Should a closed cooling system have a lower temperature thermostat than a raw water system? I ask because my motor is spec'd to a 160degF thermostat with a raw water cooling system. My boat has a closed system with raw water going through the oil cooler, heat exchanger, exhaust manifolds, and out the exhaust risers. When I got the boat the temps on the gauge would average about 160degF. I replaced the thermostat with a new one as part of a troubleshooting exercise (ended up replacing a manifold and riser) and it now runs 175-190degF. I installed another 160degF thermostat that I checked for proper opening at 160degF before installation and I'm still around 175-190degF. I've confirmed proper flow through the raw water system and pump operation.

I'm pretty sure that running at 175-190degF is okay, but I just wanted to check with this knowledgeable crowd. Is this temp range okay, or should closed cooling systems run with a lower temp thermostat?

 

[HT32BSX115]

Should a closed cooling system have a lower temperature thermostat than a raw water system? I ask because my motor is spec'd to a 160degF thermostat with a raw water cooling system. My boat has a closed system with raw water going through the oil cooler, heat exchanger, exhaust manifolds, and out the exhaust risers. When I got the boat the temps on the gauge would average about 160degF. I replaced the thermostat with a new one as part of a troubleshooting exercise (ended up replacing a manifold and riser) and it now runs 175-190degF. I installed another 160degF thermostat that I checked for proper opening at 160degF before installation and I'm still around 175-190degF. I've confirmed proper flow through the raw water system and pump operation.

I'm pretty sure that running at 175-190degF is okay, but I just wanted to check with this knowledgeable crowd. Is this temp range okay, or should closed cooling systems run with a lower temp thermostat?

Howdy, Rich

It's not uncommon for a closed cooled engine to run hotter than the stat temp. My 1997 454 Bravo with full closed cooling has a 160 stat and it runs at 180+ all the time.

Don't believe the gage though unless you've verified it. Get yourself a Harbor Freight (or similar) IR temp "gun" and point it at the t-stat housing to verify. (about $30)

Now, having said all that, I am assuming your heat exchanger is NOT clogged and your raw water pump is good and the risers are not plugged!


Regards,

Rick

 

[alldodge]

Now, having said all that, I am assuming your heat exchanger is NOT clogged and your raw water pump is good and the risers are not plugged!

+1 on this one

 

[Horigan]

Thanks. I've confirmed clear flow through the heat exchanger (pristine inside the raw water passages) and the impeller is in excellent shape. Overall I opened up the raw water system at various points and confirmed good flow by blowing through each section: drive to pump, pump through oil coolers to heat exchanger, and out through each manifold.

I confirmed the gauge reading with an IR gun when it was running around 160F. Will do again with the new Tstat installed. I suspect the previous owner had a 140F Tstat installed, which got me wondering if a lower temp Tstat was more appropriate.

Thanks for the replies.

 

[HT32BSX115]

I confirmed the gauge reading with an IR gun when it was running around 160F. Will do again with the new Tstat installed. I suspect the previous owner had a 140F Tstat installed, which got me wondering if a lower temp Tstat was more appropriate.

Thanks for the replies.

AFAIK San Juan Engr uses a 160 in most or all of their systems.

Is that a FULL closed system (meaning there's coolant running through the exhaust manifolds, a "blocking" riser gasket and raw water to the risers) ??

If it's a 1/2 system, you'll have raw water going up through the manifolds AND out the risers.

 

[Horigan]

It's a half system.

 

[HT32BSX115]

I suspect the previous owner had a 140F Tstat installed, which got me wondering if a lower temp Tstat was more appropriate.

Thanks for the replies.

That's only if it should have a 140 to begin with........Another thing to consider is that your engine is an EFI system (unless someone in the last 20 years switched it to a carb) .

It's sometimes very important that the engine run at the "right" temp. Running "cold" can cause you to use considerably more fuel because the ECM "thinks" the engine hasn't warmed up. I am not familiar with that ECM other than I don't think you can buy one new anymore.....

 

[Lou C]

I don't think that switching to a lower temp 'stat will have an effect on the running temps, once the engine warms up and the stat is wide open. It will allow it to run a bit cooler when there is no load on the engine, but when a load is on the engine, the temp will rise to whatever it will tend to stabilize at regardless of the rated temp of the 'stat. The stat will regulate the lowest temp it will run at, by opening and closing, but not the highest temp it will stabilize at, that will be determined by the efficiency of the cooling system and load on the engine. One thing that can cause a closed system to run a bit hotter than you'd like is trapped air in the system, you might want to try to make sure that there are no leaks esp on the antifreeze side and that all the air is out of the heat exchanger (use a radiator bleeder funnel like the one sold by Lisle, they work well).

 

[Horigan]

Good inputs. Thanks. The closed system is leak free and feel I've done a good job at bleeding it by tilting the boat/motor in such a way that the heat exchanger and thermostat are at their highest points during initial fill and bleed. After several outings the anti-freeze level hasn't changed, indicating no leakage or air burping in the system.

Thanks again all. I'm more comfortable with the current temp range of 175-190F.

 

[Horigan]

Thought I would follow up on this with my findings to share. Turned out my high operating temperatures were due to a partially clogged heat exchanger. I thought it was relatively clear when I looked at it last year, but I've since learned two things: 1) they can degrade and clog more quickly than you think, and 2) the flow through my exchanger is divided into thirds, where the first pass goes through the bottom third, then it routes back through the middle third, then back around out the top third. I originally thought it flowed through the entire unit in a single pass. The lower third passages were partially clogged. I cleared all the passages with a metal rod, then ran diluted muriatic acid through the exchanger for five minutes and now it's completely cleaned down to the brass. Boat now operates at a rock solid 160degF under all operating conditions.

 

[Lou C]

Nice follow up and good information for anyone who has a closed system or thinking of installing one on their next engine.

 

[jimmbo]

You could run a hotter stat in a closed system if you are using Glycol as coolant. However it could lead to the exchanger plugging up sooner on the raw water side due to the higher temps