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.

BINGO!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! !!!!!!!!!!!!!!!!!!!!!!!!!!!!!

That fits my symptoms to a T.

Since the purchase in 2005 winters on the lake have been mild except the last. I used the 496HO automatic sea water drain valve system every year thinking it was failsafe.

How do you now prevent the HE from freezing as you described even after draining per instructions?

Have you tried to have the HE repaired by a local radiator repair shop? Results?

Been doing some time-killing surfing in the archives prepping for winterization, and was curious what you found out on this problem? Was it the exchanger after all? Or head gasket?

if you run the boat for a extended time and keep it on the rev limiter it will blow the head gaskets. had a customer lend his boat to his nephew and he held it wide open for over a hour and it blew the head gaskets. he also had it over trimmed causing it to hit the limiter. spoke with mercury tech support and they told me that is what will happen. the rev limiter is not designed to be run for that long. to many thing going on like retarding the timing and injector pulse width changes.

Looks like though this guy has (had) a problem with increased water volume and pressure inside the closed loop of the block itself causing an overflow event in the coolant bottle. I'm guessing the two probable causes are head gasket blown allowing more water volume into the water jackets (though not in the cylinders/oil pan)?.....or the heat exchanger has either frozen or rusted thru in one of its' internal tubes allowing external raw water pressure to "enter" the jacket loop pushing the existing coolant out thru the reservoir cap?

This is an old post (4/2007) I was just curious to see which problem it was should it ever happen to me or someone I know.

It was the latter. Freeze damaged HE.

Ouch! That's what I figured most likely was the cause. What kind of temps did you experience last winter, and how far did you go winterizing? Just curious to see what the lesson learned was and how far is not enough. I'm in a moderate climate area, so we don't really get the sustained cold stuff like a Michigan, or Indiana, etc. does. But I don't want to be caught off guard by not doing enough with regards to the Heat Exchanger.

The 496HO air pressure purging system is good enough to get your through a couple overnight dips below freezing without doing a full winterization.

HOWEVER, a full winterization of the system with traditional antifreeze filling the sea water side of the system is MANDATORY. This prevents any issues, and also includes a rust inhibitor. If you just push all the water out via air, you are exposing all of the internal passages to air and introducing potential for rust.

Rage, I am glad you nailed your problem down. Its not just the H.E. that is vulnerable to freezing issues, but also the oil cooler and exhaust.

Yes, this was a total screwup on my part. The boat is on a Hydrahoist year round on Table Rock Lake. Three seasons/mild winters using the Merc drain out system only with no problems (also the lake never does freeze and acts as a heat source to moderate the air temperatures near the water). The fourth season on a warm and sunny Sunday day in Fall I was in a hurry just before sunset and just before my four hour drive back to St Louis to make some A/F data runs for ECU recalibration. I was planning on being back the next week end and had also not looked at the weather forcast. Well we got a brutal cold snap all that next week and I had not drained or winterized anything. I eventually dissassembled and pressure tested everything, manifolds, cool fuel, PS oil cooler, HE, etc. The HE was the only fatality.

Good point about the rust inhibitors of nothing else. Typically whenever we get a warm spell (such as lest year's entire January) I will fire up the motors and let them lubricate themselves (mine is on a lift out back).

Thru reading some crafty posts on here abouth using buckets, hoses, spickets, etc. for flushing anti-freeze into the raw water system I have decided to make my own variation. Since I now have the flsuh kit installed it should be easier at the lift vice trying to do it with flush ears.