Mercruiser 350 Magnum cooling system design 2 (6 bibs)

[tlundblad]

I would be extremely grateful if somebody could explain how the thermostat can control engine temperature with this setup.Looking at the internals of the thermostat housing the only thermostat job is to open up warm water flow to the manifolds.I have been thinking a lot on this matter and I hope that someone can explain it to me.
BTW engine is a 350 Magnum year 1993 and outdrive is an Alpha 1 gen 2.
Thanks in advance.

 

[Don S]

Re: Mercruiser 350 Magnum cooling system design 2 (6 bibs)

It's very simple really, once you see inside the thermostat housing.
The cold water comes in from the raw water pump in your outdrive into the thermostat housing. (BLUE ARROWS)
The water then moves through the drilled (and/or cast) passages in the thermostat housing.
Water always flows to the exhaust elbows to prevent damage to the rubber exhaust hoses and keep the exhaust gases cooled down as the engine warms up.
At the same time, water is filling the engine and the manifolds with water, but the water is not circulating because the thermostat is closed.
Once the thermostat reaches it's opening temp, it opens and allows flow (RED ARROWS) from the engine out to the manifolds and out the exhaust elbows along with the cold water.
At the same time, some of the warmed water is returned to the engine which will help keep the engine at a steady temperature instead of constant opening and closing of the thermostat.

Hope that makes sense.

 

[tlundblad]

Re: Mercruiser 350 Magnum cooling system design 2 (6 bibs)

Thanks Don
The thing I do not understand is that the water from the engine passes back to the circulation pump under the thermostat so the only thermostat
action is to control the water to the manifolds!There is no thermostat control of the water going back to the circulation pump.

 

[Brentathon]

Re: Mercruiser 350 Magnum cooling system design 2 (6 bibs)

In my 1991 5.7 Alpha RWC, the only way water can get to the manifolds, is when the T-stat opens. And, manifold (exit) water is the only water that flows thru the open T-stat.

Additionally, for those of us who have the white plastic "check balls", they are to provide a little more restriction so that cool water from drive doesn't wrongfully go straight out thru the elbows, without keeping the rest of the circuit "topped-off" and allowing cool water to flow thru circ pump & block first. Low rpm overheat can be caused by significant corrosion of check ball seats.

 

[Don S]

Re: Mercruiser 350 Magnum cooling system design 2 (6 bibs)

There is no thermostat control of the water going back to the circulation pump.

It's in the design and the way passages are designed and sized. Water will always take the path of least resistance. If the manifolds are full and the water is going out the exhaust elbows as fast as it can, then the only place left for it to go is back to the circulation pump..

 

[tlundblad]

Re: Mercruiser 350 Magnum cooling system design 2 (6 bibs)

Brentathon.
This exactly as I see it.It means that the T-stat does not directly control engine temp.It probably means that when the T-stat closes all engine water is routed back to the block