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Discussion Starter #1
just rebuilt raw water pump. replaced it (after 1 month, waiting for parts). now engine overheats at startup. raw water is shipping over the side, so pump appears to be A-ok. not sure if it could be thermostat? header tank coolant seems to be moving, but cant be sure. tested the discharge of raw water and it is quite cool (i am in lower windward islands of caribbean). heat exchanger feels hot. no raw water thru oil cooler. that is not plugged i had it off and chkd it all out. any ideas, or should i assume the thermostat? tks in advance. this forum has been very helpful in the past. walt
 

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"no raw water through the oil cooler", "Air bubble" somewhere?

Dabnis
I'm with Dabnis - if there is an air pocket trapped somewhere, you could have raw water flow, but still insufficient cooling. Go simple first - try to vent your heat exchanger(s) before you tear into things too far.
 

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Water heater have a heat exchanger? They get air bound when engine cooling is worked on.
 

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Discussion Starter #7
heres latest update. removed thermostat, chkd it works fine, still out tho. still overheating. leak doesnt start til overheated, and very small drip. removed heat exchanger this am. looks pretty cruddy, but, i can blow thru fresh water hose conns easily. also, can blow thru raw water hose conns easily. looks pretty bad however. any fresh ideas? i'm at wits end. cant believe it would be the fresh water pump, it worked fine when i repaired the raw water pump. must be something i've done tho??!!
 

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Depends on how your cooling system is set up. On many boats the cooling system is NOT like a car. In a car, the thermostat opens or closes the flow of the coolant. On most boats, the thermostat is a diverter valve, i.e. a "Y" valve. The raw water pump is always pulling water in the raw water intake and pushing it out the exhaust. Only when the thermostat opens, is the water diverted INTO the cooling passages in the engine.

So, if the thermostat fails to open, the engine gets no water.

It is easy enough to check this. Pull the thermostat, put it in a pot of water on the stove. Tur on the burner and see if the thermostat opens by the time the water is bubbling. (A thermometer is even better.) If the thermostat doesn't open...replace it.

We had that problem once. Had to move the boat "TODAY" and replacement thermostats were backordered a week or two. Stuck a rock in the thermostat housing (yes, a rock) which blocked about half the flow and forced the rest through the engine. The temperature wasn't perfect...but hey, it did the job.

Extreme low tech, fixing an engine with a rock. (G)
 

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When I changed out the antifreeze in my 4-108 a couple of years ago, I had the same problem with the engine overheating and yet had good flow of raw water out of the stern. It turned out that I had an air lock in the primary coolant side. In the past I've been able to berp the system letting air bubbles come to the surface of the radiator cap, but this time it was not sufficient. Ended up taking the hose that goes from the engine to the hot water tank off at one end....holding it above the engine and putting a funnel and antifreeze into that connection forcing the air bubble out of the system.
 

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Discussion Starter #10
tks for your replies. i am getting in deep with this small problem. air lock sounds like a possibility. will try to chk that.
 

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Walt,

Not sure whether or not you removed the fresh water circulating pump or not. If so, be advised that on the 4-108 there's a thin steel plate which sits between the pump mounting face and the engine. This plate has a gasket on each side of it, and they are NOT IDENTICAL. An island mechanic once replaced my circulating pump and used identical gaskets, thereby blocking an important port and causing overheating.

I wound up flying a good friend/expert mechanic down from the states, who quickly identified this and several other problems causing the overheating.

Good luck.

Bill
 

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Discussion Starter #13
new question. when i blow thru fresh water hose conn, it runs free thru the heat exchanger. also, same thing with the raw water (tubes) side. can i assume the exchanger is working correctly?
 

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new question. when i blow thru fresh water hose conn, it runs free thru the heat exchanger. also, same thing with the raw water (tubes) side. can i assume the exchanger is working correctly?
No!
1: You have to pressure test to see that there are no leakage between the two sections (salt-fresh water). Block the outlet, pump up about 25PSI. It should stay and not drop!!
Do this on both fresh and saltwater side.
2: If sludge/salt-build-up in the tubes the capacity of the heat exchange may have been reduced too much.
 

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Why not follow the procedure in the perkins manual for bleeding the air out of the engine? 2 plugs on the top of the head, One at the back, behind the injectors, and the other on the opposite front corner, and lastly the one on the header tank. Fill header tank until coolant appears at the rear plug, replace plug, add more coolant till the coolant is at the top of the front plug, and finally close off the header tank vent, when coolant reaches it. Oh and don't forget to put the thermostat back in, it is there for a reason. The thermostat should start to open at 156f and fully opens at 188f, and should be .281 when fully open. If you decide to pressurize the thing to 25 psi, you could generate a new problem since the rad cap releases at 15 psi, or less, so I doubt it will like 25 psi. :)
 

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Discussion Starter #16
the results are in!! tks to everybody so much for their collective help. i removed a hose that once led to a water heater. installed two plugs and replaced the whole shebang. turns out the exchanger is fine, as is the oil cooler and thermostat. so....;looks, at this point as tho the culprit was air lock in that hose. tks again everyone!!!
 

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what about if when you put the heat exchanger back on you switched the hoses and have the water going through the wrong way. if you could find somone else with the 108 you could compare?
 

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Discussion Starter #18
e37 duff. tks for the input. found, what i now believe, was the problem all along. forget about the hose i wrote about, i chkd the the brand new rebuilt raw water pump and the brand new impeller was toast. replaced, ran the engine and it runs at 180 degs for an hour. have no idea why the impeller would have mashed up after only one hour run time. had water all along. anyway, tks all for your help
 

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have no idea why the impeller would have mashed up after only one hour run time. had water all along. anyway, tks all for your help
Have a look at your exhaust back pressure. I used to have that problem on my SO37 and Yanmar.
Two things caused it:
Jeanneau had mounted the sea water strainer on the suction side, and it turned out it had a leak due to a too hard tightened hose clamp. This made the exhaust back pressure 'shot off' the impeller wings at start-up before the pump/system got filled with water. The change from the std Yanmar 6 wing impeller to a Jabsco 10 wing really solved the problem.
 
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