1998 3L Mercruiser - strange coolant loss

Hey guys, question for you: After driving at 3000 rpm for 30 minutes, I stopped at the dock and turned the engine off. All of a sudden coolant started rushing into the overflow reservoir, and squirting out the spigot at the top. It continued doing this until there was a few liters of coolant in the bilge. Then it stopped, and what was remaining in the reservoir got sucked back into the engine, until the reservoir was empty.

The coolant reservoir level is always kept at the correct mark (cold line when cold), and it was at the hot line when I was driving it.

I'm thinking it might be a head gasket issue, and the suddenly expanding coolant might be from exhaust being forced into the coolant system?

I should mention: I replaced the freshwater heat exchanger with a brand new one about two months ago, and it's worked great.

Re: 1998 3L Mercruiser - strange coolant loss

and what was the temp reading? when was the impeller replaced ?

Re: 1998 3L Mercruiser - strange coolant loss

Temp guage is old and with few markings on it, but the needle was at where it normally is when underway. Sorry I don't have an actual reading for you.. I need to replace it, I know.
The impeller is probably one to two years old.

The boat is an old Sangstercraft, likely 40 years old. The engine's a '98 though.

Re: 1998 3L Mercruiser - strange coolant loss

Bought a MityVac and pressurized cooling system to 17 psi. No leaks anywhere, and the needle stayed at 17 for longer than 2 minutes.

Fired up engine and watched for needle vibration in the MityVac. The needle barely moved at all, except for a gradual increase in pressure which I had to let off to keep it at 17 psi.

Pulled spark plugs and the three aft plugs are blackened while the one fore plug is clean. No residue was on it. This would indicate a leak in the gasket near this cylinder, right?

No coolant in the oil.

It was too windy to take the boat out to do the test to see if bubbles are leaving the engine in the overflow hose to the overflow reservoir. Maybe in the next few days I'll get to it.

Right now it looks like a coolant leak through head gasket to me.. but the engine runs fine, so I can't imagine coolant getting into the cylinder? Any ideas?

Re: 1998 3L Mercruiser - strange coolant loss

Could the radiator( Coolant Resevoir) cap be bad. If the cap won't hold 17lbs like its supposed to or if it is the wrong cap it would do this also. If exhaust gases were leaking into the engine it would do it while running not after it was turned off.

Re: 1998 3L Mercruiser - strange coolant loss

First of all, you can't do a pressure test of the cooling system with coolant in it. You test with air only. BUT, since you left it in and ran it and the pressure kept increasing, I would venture to say you have a compression leak into the coolant system. Head gasket, cracked head, or block would be my thought. You could do a leakdown test on the cylinders with the heat exchanger cap off and watch for bubbles from the problem cylinder.

Re: 1998 3L Mercruiser - strange coolant loss

Don S said:

Head gasket, cracked head, or block would be my thought.

Click to expand...

Thanks Don, yes it looks that way. I decided to take it to the marine mechanic to do the work. I'll try to come back and post the eventual outcome here. Thanks guys.

Re: 1998 3L Mercruiser - strange coolant loss

Well the mechanic finally got to it. Yes the head gasket had a leak between 1&2 cylinders. Apparently it looked burnt in that spot on the gasket. Not sure how the gasket would get burnt there, but at least the problems fixed.