pressure relief question

[js5278]

I've removed the water cover on my 1984 40hp Evinrude electric. Above the thermostat is a red plastic thing on a spring that the manual has labeled as a relief valve. Can someone explain how that functions?<br />My understanding is that it functions when the engine is run at higher speeds. I can understand how pressure from behind this valve will compress the spring against the water cover and cause it to open, but what exerts this pressure, and what does that accomplish? I believe the end result is that some water will shoot out the exhaust ports just below the powerhead on the back of the engine.<br />I have searched the site and can't seem to find a good explanation. It's probably simple and I'm just not getting it.

 

[ob]

Re: pressure relief question

js5278,You've got it pretty close.The waterpump on your outboard is one of positive displacement,meaning that with increased rpms it will continue to discharge higher pressure and volume.This increased water pressure needs to be relieved some where besides through the thermostat assy.If you were to completely block off this port it would lead to overpressuring of cooling jackets resulting in an increased probability of leaking head gaskets and pump back-pressure.

 

[Kenny Bush]

Re: pressure relief question

JS, I believe that water valve opens at a certain static pressure, thus allowing the extra water to vent out..... The reason for extra pressure is due to the water pump impeller mounted on the lower unit driveshaft spinning at higher RPMs and water pushes into the lower unit easier at higher water speeds. This will keep your engine more at operating temperature...

 

[js5278]

Re: pressure relief question

A mechanic told me to think of it as a thermostat bypass. Fine. Water flows out of the water cover through the thermostat to be dumped out the prop hub. If the pressure valve is opened, some water will dump out of there also. My confusion is the source of the pressure that opens the valve. It can't be in the water cover, because pressure there would keep it closed. So it must come from behind the valve, but if it's water pressure from behind the valve then that would only put more water in the water cover. Is it exhaust pressure from behind the valve that causes it to open, thus allowing extra water to escape through the valve? I apologize for being dense. I'd just really like to understand how this works.

 

[clanton]

Re: pressure relief question

Water pressure opens the valve.