Overheating

[John Gardner]

"Overheating again" I hear some people say.  Well, yes, I'm definitely sorry to say it is.  I think I've read all or most of the previous discussions, and one of the main threads in them is, the way I understand it, the need to get rid of air locks in the water heater hoses.  However, looking at the engine manual, it seems that the water heater circuit is only a parallel loop, not in the direct cooling line.  And that makes sense - the design must recognize that there may not be a heater.  So what am I missing?  I just don't see where these airlocks might occur around the engine.  Even the top of the thermostat housing (and the thermostat is not installed in my engine), although it has a bleed valve doesn't look as though it could interrupt the flow.  Please educate me someone!

[Ken Juul]

As the system heats up, pressure builds.  The pressure rises to the top of the engine, pushing all the coolant toward the bottom.  Eventually the coolant pump either looses it's prime or doesn't have enough power to overcome the rising pressure.  Water flow stops and overheating occurs.

I have found the easiest way to burb the water heater is to disconnect the lower hose.  If I remember correctly the lower hose connects to the circ pump, the upper hose to the thermostat housing.  With the cap off the thermostat housing pour coolant into the thermostat housing until it runs out of the disconnected lower hose air free.  Reattach lower hose and thermostat housing.  Open the bleed screw and fill resevour  until coolant comes out the bleed screw.  Tighten bleed screw, top off coolant and run the engine.  Recheck the level. The engine manual says it make take 2-3 engine runs to get all the trapped air out.

Good luck, blew a hose a few years ago, took several trys to get all the air out.

[Ken Krawford]

John,

I had the same problem many years ago.  I finally resorted to taking the cap off the radiator and motored around for awhile.  It's important to have the engine under load.  Just idling at the dock didn't work for me.

Hope this help.

[BillR]

John,

I have had overheating issues in the past and found that the problem was the heat exchanger.  I don't know if others have had the same problem but I have found my heat exchanger to become clogged with salts over the years and it occassionally needs to be taken off and cleaned via phosphoric acid.  I have had to do it a couple of times since the boat was new.  

Bill Russell

[John Gardner]

Sorry if I misled you and you took the time to reply.  What I was trying to say is that the water heater hoses should (I think) be irrelevant to the problem.  Yes, burping the water hoses is necessary to getting heating water to the water heater, but that appears to be a parallel, non-essential circuit as far as engine cooling is concerned.

Yesterday I disconnected every hose and component I could think of, and apart from a small amount of weed in the raw water intake strainer, didn't find anything.  At the moment I am tending towards Bill's suggestion on the heat exchanger.  I had that apart too and it was a bit mucky.  By the way - a bit of useless information - did you realize that the baffles in the heat exchanger cause the raw water to make four passes through the exchanger?

Something else I did which might one day help someone somewhere.  I reached the point where I was beginning to suspect the engine coolant pump.  How to check it was working?  When the engine was starting from cool, I left the cap off.  I pushed a coat hanger down the middle of a piece of half inch clear plastic tubing (left over from fresh water re-plumbing), bent the end of the wire to a right angle so the tubing was forced to make an elbow, and then poked the whole thing through the filler cap and into the coolant return pipe inside the coolant tank.  I could tell the pump was working because it pushed a small head of coolant up the tube.

Well, it looks like I'll have to find a radiator/heat exchanger man somewhere who will promise to do a quick turn round for me, unless Bill you have a technique to do it yourself?

Thanks for your help everyone.

[Ed Ryan]

John,

Regarding your original post about an air lock in the hoses to the water heater, I had a similar problem after changing those hoses when I first bought my boat.  Tried most of the methods suggested to no avail. The solution was to open the petcock where the coolant hoses from the engine connect to the water heater.  Not much air was released, but it did the trick.

Ed.