Mercruiser thermostat housing water flow

Describes the operation of a mercruiser water flow on fresh (raw) water systems

It flows from the impeller to the housing
At the housing it goes to the water or recirculating pump as well as to the risers
There is less flow until the thermostat opens . Water cannot circulate through the block until the thermostat opens. So the incoming water is blown out to the risers. While the water are the recirc pump / block is blocked (dead headed) so which why the pump pressure blows out to the risers.

Once the tstat is opened the hot water in the block then circulated through the thermostat back through the water pump then into the block , there is more flow from impeller to the water pump since the tstat is open and water flows to cool the exhaust

Tstat closed - no flow through block, but flow to risers
Tstat open - flow through block and exhaust

Tstat is always full open 20 f higher than rated . So mine is a 140 , other options are 160

Do not remove the check balls to the risers (not shown) otherwise you may starve the engine of water (think of flow like turning on water faucets in your house with limited water pressure)

[Paulinthewyld]

Dude, thank you.
I opened my 5.7 and the thermostat was upside down, spring side facing up. Totally threw me! I’ve had the boat for two years and it ran great with no temp issues. Never knew a thermostat could rub upside down, this explanation really helped - thank you.

[kristoffscuba]

Very useful video. I can also confirm that it is the same set up on the 4.3 mercruisers like I have. Like you say when the the thermostat is closed you have most of the raw water coming from the impeller flowing out through the risers then the exhaust. Once the thermostat opens water starts circulating through the block and leaves through the exhaust. If you want to know if your thermostat is open you can grab one of those rear most hoses. They only get warm when the thermostat is open. 👍

[scottback]

I’ve been looking into this a bit. I’ve found that somewhere in the mid 90’s they didn’t use the check balls on certain models because they changed the bypass hole diameter. Alpha’s used the check balls and bravos did not because the raw water pump was bigger for the bravo and flows enough water.
There is a service bulletin about this.

[larrywalden]

I'll have to watch this again because I still didn't understand the routes in how they work lol

[giuseppescavella]

Hi! Thanks for this video! Do you know where i can find a 16413-c thermostat housing in good conditions? Thanks!

[breezy]

I’m in the middle of replacing my thermostat. 1988 Merc 7.4. Same thermostat housing. My exhaust and riser hoses are hooked up opposite of your diagram. Wondering if previous owner hooked up wrong. or does it matter?

[jodmidgley]

i think you solved my problem. i recently repowered my merc 4.3 alpha 1 gen 1 engines with 350 vortecs. the 350s used to be fuel injected with a bravo pump. i rebuilt the blocks with 4 brl intake manifolds and used the same alpha thermostat housing from my 4.3. Engines ran fine but were "over cooling." Once engines got to temp of 175 i would open it up to plane off, the temp gauge would drop like a stone, slightly climb back up then drop again, but it would never overheat. im under the assumption that the sterndrive waterpump is pushing too much water into the block, thus needing those relief balls so the excess water is released thru the exhaust. sounds possible? im going with that!

[samg]

HI,

Thanks for this informative video.

I have a mercruiser with alpha one Gen 2, that was overheating, i replaced the impeller and that solved the problem, but my concern is that the water is coming out of the outboard is cold on one side and hot on the other side with weaker water flow.
Does any one know why ? should i be concerned?

Thanks in advance for any replies.

Sam.

[stefanosk]

SOS!! I CANNOT find this part to replace