496 Merc Coolant Reservoir Over Flow when Engine Running
 

My 2005 496HO coolant reservoir over flows when the engine is running. The reservoir stays at the over flow fluid level when the engine is turned off. About 170 hours on the engine. First time this has occured.

Any body else experience this symptom? If yes what caused this and how fixed.

i would just say it is over full.

I neglected to stay that before the engine started the reservoir was barely showing a fluid level. After a short run of about 30 minutes the bilge had at least a gallon or more coolant in it and the reservoir was showing full to the brim. I have never seen that much coolant expansion.

I wonder if the heat exchanger is leaking internally.

The sea pump circuit could be pushing water into the fresh water circuit through a hole in the heat exchanger.

Whats the coolant look like in the reservoir ? still
the same color, or is it starting to look like the
water you boat on ?

Wow. Mine will go from the full cold mark (at the tank seam) to where the coolant is touching the top of the tank at the tank's rear but still have some capacity towards the front of the tank after a hard run. Never had it puke out though.

Rage:
Refill the collant tank to half tank cold and then put a Stant pressure pump tester on the top of the coolant tank cap opening . Remove all the spark plugs and then pump up the tester to 13-15 lbs. Let it sit for a while and see if the pressure holds. I suspect it won't. You are probably going to find water in a cylinder from a compression leak. If not then look to the thermostat not opening or sticking, that can also cause this condtion. Let us know what you find.

Regards,
Ray @ Raylar

Thanks Ray.

Actually as noted "the reservoir was showing full to the brim" after the overflow" the reservoir did not appear to have had the coolant pushed out by a gas because the reservoir ended up having more fluid in it when I shut the engine off (showing full) and setting for about an hour than it did before I started the run (barely showing fluid in the reservoir) when the overflow occured. Then there is the additional gallon plus coolant in the bilge as well.

When a thermostat sticks or does not open would the reservoir remain showing overfull after pumping out a gallon plus of coolant?

I will advise results.

Where can I find the Stant pressure pump tester you mention?

If it's not a stuck thermostat, I think there are only two other ways a "half system" like the 496HO closed cooling system can overflow like that.

First would be a heat exchanger failure introducing sea water into the closed system. When you pressure test the system, pull the end caps off the heat exchanger to look/listen for leaks.

The other way you could have an overflow would be due to a hot water heater failure (obviously only applys to an engine that is connected to a hot water heater).

Dave

I had a car many years ago that did the same thing, and it was a blown cylinder head gasket. I guess if that is the case compression from the engine enters the cooling system.

I have gone through 3 heat exchangers. If you are located in an area that gets below freezing, and you just drain the system, water can stay in the heat exchanger and freeze. Then when your sea pumps pressure is over 18 psi, it will fill your res tank. PS, heat exchangers are not cheep. Good luck.