Coolant expansion tank overflow QSC

[Mike Mason]

Last night during our annual holiday parade I noticed my coolant expansion bottle was full and overflowing into the bilge. Boat was only run at 1000 rpm or less and temp never got over 165. Have never had this happen. Any ideas and where to start for correcting the issue.

Never had an overheat. 1800 flawless hours.

Thanks

Mike

Topic

[Clark Leighs]

JMO but when I have work to do on the coolant side I drain all of it. That way no surprises.
Depending upon the engine and installation you may still have a small quantity escape.

Actually I do the same on the raw water side although that is not quite as clear cut.

[Mike Mason]

Update! Leak Found

I put the HX under 10 psi of pressure and found a leak where the coolant inlet pipe meets the HX. There was a steady drip from here with 10 psi. I found that was the source for the majority of the coolant in my bilge. The overflow bottle was completely full but the majority of the coolant came from the inlet pipe drip at the HX. Looks like I need to drain down the coolant below the inlet pipe to replace o ring. Anything else I need to know? Is this pipe full of coolant? I am assuming the t stat is on the other end of the rigid pipe? How much coolant do i need to drain?

Thanks in advance.

Mike

[Mike Mason]

Thanks for the replies. I ran the motor today in the slip and it runs fine. I did notice very small bubbling in the coolant tank running with the cap off. Not sure if this is air in the system as there was coolant loss or a bigger issue.

The coolant was sucked back into the engine the night this happened. The overflow bottle is about 3/4 full

Any added info appreciated

Mike

[William Walter]

Overflow

Mike I’ll second what Gene said about the heat exchanger. I had a merc 454 with the same symptoms and it turned out the heat exchanger was leaking and overflow tank was flowing over. How old is heat exchanger.Depending on maintenance they have a certain life.Bill

[Gene Fuller]

There are two generic causes for overfilling the expansion bottle.

First is the failure to return coolant to the engine after operation, when the engine cools down. I have seen that happen a couple of times. It is not all at once, but a little bit each time the engine is operated. Since the QSB 5.9 has a level sensor that scenario is not too likely without an alarm indication.

The other cause is that the volume of coolant has been increased. Overheating can cause that effect, but you noted there was no overheating. The more likely cause is a leak in the heat exchanger, allowing some raw water into the coolant system. Various causes of gas introduction into the coolant system would also cause an increase in volume, but it is not clear how that would force a lot of coolant into the expansion tank. Instead the gas would escape into the tank where it would not be noticed.

If reasonable I would run a test to carefully measure the levels before, during and after operation to try to pin down when the excess volume occurs.

Gene

[Author]

Replies