Mercruiser 3.7 coolant in oil

[Rich1977]

I have a 1989 mercruiser 3.7 with water in the oil. i did a compression test and all cylinders where the same. i did a pressure test on the cooling system 15psi and all coolant went into the oil. i don't see any coolant coming out the the water pump weep hole. i also don't see any coolant on top of the pistons. Any ideas? i just got this boat.

 

[kenny nunez]

There is a very good chance that the weep hole is plugged up and the water pump shaft seal is leaking. Try to stick a piece of wire in the hole. I once found a screw in one.

 

[Furbird]

Head gaskets don't just fail to the compression side, they can also fail between coolant and oil passages.

https://haynes.com/en-us/tips-tutorials/7-different-ways-head-gasket-can-fail

 

[Scott Danforth]

Furbird never quote a haynes manual. thats like asking Krusty the Clown for advice

Rich1977 , first, why would you buy a 32 year old boat, much less one with a Mercruiser 470?

first, what is your compression numbers? should be about 140psi or higher

Mercruiser 470 motors will spit their head gasket with as little as a 10 degree overheat

cooling system pressure checks should be done without coolant in the water jacket.

as Kenny said, look at the water pump. most likely the seal at the cam snout is leaking. google speedi sleeve mercruiser 170

you will want to get a factory manual or view them on-line at boatinfo.no and if you need a head gasket, get a true mercruiser gasket for the motor.

 

[Rich1977]

i did shove a wire up the weep hole but not sure if it was far enough. could the seal be blocking it or anything else or if its not weeping coolant through this then its not the problem.

is there a way to test the head gasket leaking from the coolant side to the oil side.

someone gave me the boat with the option to return it if i got tired of messing with it. the interior and boat are in good shape or i wouldn't even be messing with it. is there any other way to tell if its the head gasket or cam seals without tearing it apart?

 

[Furbird]

You pressurized the coolant side and coolant went in the oil. You could always cap off the PCV valve and pressurize the crankcase but I really don't recommend that as there is a high probability it will push some other gasket out long before it pushes oil into the coolant.

Only reason I used the haynes link is because it had pretty pictures of all the way a head gasket blows. We used a similar image at the dealership (car) I used to work at. Trust me, I don't use Haynes manuals, Quaker State, or Fram oil filters.

 

[Bondo]

i did a pressure test on the cooling system 15psi and all coolant went into the oil.

Ayuh,...... Welcome Aboard,...... Yer supposed to drain it 1st, so you can hear the leak, 'n follow the sound,......

 

[kenny nunez]

While it is possible for a head gasket to blow out on a 470 I have never seen it happen. Even when he head is removed I have never actually was able to see where it was leaking at and the machine shop could not either. The problem is with the free standing cylinders that have no upper support. Probably at this point it would be a good time to pull the head, get a valve job and have it milled. I am pretty sure that the coolant is not leaking from the head gasket into the oil.
The main problem is the water pump shaft seals and the shaft will need a Speedy Sleeve also.

 

[Rich1977]

Thanks everyone I am going to try the pressure test with out coolant in the system to see if I can hear where it’s leaking. Any tricks to hear where it’s leaking from? I am hoping the head could be a winter project.

it seems like some people don’t like these 3.7 mercruisers. What is bad about them?

 

[Scott Danforth]

The problem is with the free standing cylinders that have no upper support.

agreed, floating bores (AKA open deck architecture) suck

which is why I recommend a bore stabilizing spider, AKA Deck girdle if you plan on getting longevity out of a floating bore motor

like this

or this

 

[Rich1977]

I did a pressure test without any coolant in the system and I can’t hear any leaks anywhere and it is loosing pressure. Is it still a good chance it’s Still the cam/water pump seals?