Cooling System Question Chrysler 75 hp - Bypass Valve

[BrettNC]

I replaced the plastic bypass valve and spring yesterday and am now experiencing overheating problems. No water comes out of the two small ports at the rear of the engine at idle or during any other time. I have plenty of water coming out of the exhaust snout at the gearcase, but it is not even warm. The cylinder head gets extremely hot. There was nothing wrong before installing the new bypass valve. Is there a trick to installing these valves? I just figured it was time to change it out since the old one looked a little warped.<br /><br />How does the bypass valve work?<br /><br />Does it restrict the water flow to the engine if it is open or closed?<br /><br />What path does the coolant take through the motor?<br /><br />I appreciate your help.<br /><br />BrettNC

 

[12Footer]

Re: Cooling System Question Chrysler 75 hp - Bypass Valve

Welcome to iboats, Brett.<br /><br />The purpose of the bypass valve is to equalize cooling of the different pieces that make-up the powerhead. Without it, one piece could theortically expand at a different rate than the pieces mounted to it. Not good. It also acts as a water pressure regulator, so no gaskets or seals are blown from too high pressure.<br />The impellor is the weakest link. Replace it first, just out of principle. They are considered a maintainence item that should be replaced at regular intervals,(ever two seasons of normal use), regardless of condition, because they are rubber, and wear out fast.. They can also form the shape of the housing if the motor sits unused for 6 months or more.This is called "taking a set".<br /><br />Pull the plugs and inspect them. Look for a really clean-looking plug. This is an indication of a blown head gasket,"steam-cleaning" the plug.<br />There should be a thermostat on the top of the head. Remove this and re-test. It also serves the same purpose as the bypass valve.<br />Don't leave it out for more than just testing. Extended operation without a t-stat can cause major problems...But it may just be a stuck t-stat.<br />The cooling water comes-into the engine under the cavitation plate, at the 2" luevered area.<br />The impellor pushes coolant up the leg to the thermostat. The thermostat opens at a preset temperature, so no water will exit the telltale for a minute or two, until the temp opens the tstat.<br />The course the coolant takes from there is around the outside of the jugs,around the head and block, thru channels...Then, a portion of it goes out the exhaust thru a "heat-exchanger", usually just a part of the block's contruction, and mating surfaces.<br /><br />Good luck, and let us know how it's going.

 

[BrettNC]

Re: Cooling System Question Chrysler 75 hp - Bypass Valve

Thanks for the detailed reply 12Footer. <br /><br />After more thought I wonder if the new valve was improperly seated when I installed it. It took some tugging to pull the old one out and it may take a little force to push the new one in to correctly seat. I think it may be wide open, letting all of the water bypass back out into the motor leg before it reaches the block. There was plenty of water flowing out of the exhaust, but it was cold.<br /><br />I replaced the thermostat and impeller a year ago, but the motor sat for over 6 months, so that is definitely something to look into.<br /><br />How long would you expect a thermostat to last in saltwater?<br /><br />Again, thanks for the help!!

 

[12Footer]

Re: Cooling System Question Chrysler 75 hp - Bypass Valve

That could be, Brett. Especially if it circulated before, and no longer does after you replaced it.<br /><br />The impeller is probably still ok, as it is pumping thru to the exhaust. But the time to change it is before it grenades 5 miles from port.. The tstat should be changed whenever the impeller is changed.<br /><br />The service life of the tstat is contingent upon useage and lack thereof. As long as you flush the motor after every use in saltwater, it should not corode shut. If water sat behind the tstat for 6 months tho, we can assume it has corroded-shut.

 

[BrettNC]

Re: Cooling System Question Chrysler 75 hp - Bypass Valve

12Footer:<br /><br />Thanks for the information. I don't think the valve was seated properly. I reinstalled the valve and replaced the impeller today and the temperature at the cylinder head is just right. I appreciate your responses.<br /><br />Brett NC