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Mercruiser Thermostat Housings

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Old 08-15-2016, 08:42 AM
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Default Mercruiser Thermostat Housings & Water Pressure

All three of my motors use stock Mercruiser thermostat housings. My question is, my thermostats are located/installed right at the very top, just under the first cover. Is this correct? I see in some schematics that the thermostat goes down in further and the brass sleeve with the cutouts in it goes above the t-stat. My temps are all good and even across the board but even with water pressure reliefs I still have more pressure than I want to see. Pic below is just a Google image of the housing mine use.



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Old 08-15-2016, 09:08 AM
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Raw or closed cooling?
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Old 08-15-2016, 09:22 AM
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Raw.
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Old 08-15-2016, 09:32 AM
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look up schematic by engine serial number. We ran into same question on our 500's. In our application t stat went on the top.
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Old 08-15-2016, 09:46 AM
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I swear every one of these is wrong. The schematics I look at for my center motor (originally a 1991 HP 465) and my rear motors (originally 1995 HP 500's) shows the thermostat UNDER the brass ring that has no cutouts. My question is.......................would everything still work correctly like mine are, with the tstat on top of the brass ring (all three motors stay between 150-160 degrees) but create excess water pressure? Even with pressure reliefs installed in the 1 1/4 hose between the cooler and the tstat housing I am seeing close to 30 psi at 5000 rpms and I only want 20-22. This boat has -2 Imco shorties and I have tossed a head gasket before and I have blown an intake gasket and a hose this year (before installing the pressure reliefs). I do not want to run this thing past 5000 rpms until I get the pressures down. Motors were built to spin 5800.

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Old 08-15-2016, 10:48 AM
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I just replaced t-stats in 2 of these housing units and was uncertain of this....so I am kinda tagging along here.

What I did - dropped the tstat into the top of #1 then drop in the brass liner. One thing I did differently than the pic above was to use a different gasket than what is used above. In the gasket kit I has there were 2 gaskets that fit on top of #1, I used the one where the protruding area was cut out (unlike what is shown here where that portion is solid). Maybe I used the wrong gasket myself?

While there check the temp rating on your t-stat. I had one for 160 in mine and I dropped it down to 140. Is the only way for pressure to build is to have a tstat that doesn't open?

I by accident I tried putting the t-stat above the brass ring and I didn't find muh of a difference in engine temps and I never measured pressure beyond squeezing hoses #19 & 24. I also tried this with the gasket having the "solid tab" and found this made the engines run hotter.

Having said all of this, at the time I was doing this I had 2 failing water pump impellers fading during this timeframe. One I caught before it shreaded all in the cooling system, before I could get parts to do the other engine that impeller shreaded...what a PITA to fetch all of those pieces out of the cooling system. I also found that on one of the circulation pumps I had the metal impeller in there crack...this probably ontributed to my issue as well.
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Old 08-15-2016, 10:58 AM
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These brass rings don't look like they have ever been out of these housings. I don't see what would hold the tstat in place if I stuck it in under the brass ring unless there is a little step in the housing that I can't see because the brass ring stops on it. I just want to know if this could be contributing to my excess water pressure problem (since I have owned this boat) but would still allow the motors to run cool like they are. I have the pressure relief wide open on my center motor and at 5000 rpms it still shows over 20 psi on the gauge. I even have a tee in each dry joint exhaust manifold jumper hose with a 3/8 water dump out the side of the boat (I have dry exhaust and these tails are loooonggg! All water pressure gauge lines are clear of any sand/debris, and all gauges read close to the same at all rpms.
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Old 08-15-2016, 12:43 PM
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Ok, talked to a buddy who is a great marine mechanic/Mercury tech and he told me the thermostats definitely go under the brass sleeves and that all mine are wrong, so I will correct that. These look to have been like this since I have owned this boat (4 years). What no one can tell me is if this will cause an increase in water pressure. I guess I will know soon enough as I ordered three new brass sleeves and three sets of gaskets and I will change it all this week.
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Old 08-15-2016, 06:02 PM
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check out service manual #9 page 6a-14 it shows the t-stat on top of the brass sleeve.my stock 420's in my old top gun had the same set-up.
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Old 08-15-2016, 06:34 PM
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Seems like alot of straight bottom boats with Imco lowers suffer from excess water pressure until the water intakes in the lowers are partially filled.
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