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Thermostat 140 vs. 160 rehashed

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I searched through old threads, but did not see the question put this way.

Mercruiser shows three different styles of thermostat for the 87-95 350 5.7 motors. The first two use a 140 degree thermostat and the third uses a 160. For 1996 and on, they show one style (looks like the third one), using a 160.

I have the second style, but am going to be using it on a newer motor (350 5.7 vortec). So my question is.....Is it the design of the thermostat housing that determines which thermostat to use, or the year/type of the motor?


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That's a great question, I wish I had the answer, but can give my thoughts. I believe the reason boats with open cooling systems run a cooler (140 or 160) thermostat than cars (190) is that hot spots can form, maybe around the exhaust ports in the heads, and the goal is to keep the water from boiling there. A closed system in a car has antifreeze and a pressure cap, both of which raise the boiling point. I believe boats do increase the pressure in the block by restricting the amount of water that can exit the block. This could be done in the thermostat housing. There could be more "restriction" in the 160 housing, that creates a higher pressure, so a higher temperature thermostat can be used without the hot spots boiling the water.

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It's more of a scale issue; 140 for sea, 160 for fresh water. This is why your car manual requires purified water instead of tap water (with coolant) for the radiator.


"However, seawater flowing through the engine can cause scale buildup and corrosion inside the engines water jacket (the internal passages where cooling water circulates) that can ultimately destroy the engine block and cylinder walls. Freshwater boats aren't immune to this problem.

A more insidious drawback to the raw-water system is that the engine must run cooler than it could with a closed system. Above 160 F, sea salt begins to crystallize inside the cooling-system passages. This not only accelerates corrosion but also, in extreme cases, can actually block passages. For this reason, most raw-water-cooled diesels have a thermostat set at 145 F, at least 40 F cooler than a closed system."

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Someone from Mercury already called me back. The original supplier of the type I have stopped producing that model, so Mercury switched suppliers (back in the mid 90's). For some reason they recommend a 140 in the older style. Must just have something to do with the way the water flows through the system, compared to the newer style.

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For my 93 skier with mercuriser competition skier 5.7 l quadrajet carbed engine, the riser gaskets for the 140 thermostat are different than that for a 160 , i.e., instead of four open oval channels for coolant flow, two channels have only a small hole. I'm not sure there is a relationship, BUT mine (thermostat and gasket) are per OEM and she's a porche on the water. WOT last week was 47 mph and I don't think she had leveled off since I was in a bay and decided to quit while I was ahead.

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@powbmps: To your original question, it can although for the housings that all have a bypass to keep the exhaust manifolds supplied with water (very important) during warm up and tstat closed operation the same tstat will work. The key is to use a marine grade version to minimize the deterioration due to corrosion as automotive units are not designed for the marine or raw water environment. There are various tstats available that have a built in bleed to accommodate different housing designs. The Merc's pretty much all have a properly designed bypass and recirculation system which is what makes the unit large and relatively cumbersome. I would suggest new springs on the bypass rod so it works properly when you install your new mill.

IIRC you now have an aluminum intake manifold. When I did that change on my boat, although I added a lot more aluminum (heads/exhaust mainfolds) the boat required a little more warm up before it was ready for a skier pull up on cool / cold days. It would tend to accelerate then fall on its face for ~ 1-2 seconds and then be fine after that. I attributed that to the fuel not vaporizing efficiently while the aluminum was cold. Cast iron setup did not do it. For me, the gains outweighed the need to make sure I had one acceleration under the belt before pulling a skier. As an FYI, the Edelbrock Performer RPM is a knock off of the GM / Mercruiser cast iron intake manifold.

I had great customer service when talking to the Merc guys several years ago.

Edited by Woodski
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