3.7 woes

[oilydogg]

i have acquired a new to me lund boat with a 3.7 mercruiser. ive been reading about it for about 4 months so ive heard about all the problems with them and i am choosing to proceed with fixing this motor. i put new exhaust manifold and riser on it to correct a high speed overheat problem (supposed to be the only problem according to previous owner). the cast riser was full of crud. also replaced impeller which seemed ok anyway. so i took the boat out and ran it up and down the lake for an hour or so. no more overheating. but after it cooled off i checked the coolant to find it had oil in it. no coolant in the crankcase though. when i drained the coolant to start adressing what i believe to be a bad head gasket the drain on the side of the block let out the most oil mixed stuff....the rest of the coolant from the riser, manifold , and hoses etc. only had a little. am i looking at a cracked block? do i have to pressure test the cooling system to find out for sure?

 

[oilydogg]

Re: 3.7 woes

by the way long time reader first time poster. great forum/site!

 

[stonyloam]

Re: 3.7 woes

am i looking at a cracked block? do i have to pressure test the cooling system to find out for sure?

A cracked block is unlikely in an antifreeze filled 470 motor. I think you are probably right with your head gasket diagnosis. You could do a compression check along withe pressure test to make sure (you can get tools to do the pressure test and compression test free from Autozone). The job is not to bad, just make sure that you follow the instructions in the manual EXACTLY.Use the OEM Mercruiser head gasket, better than aftermarket. Welcome too the 470 club, you will like it when you get it running. Good luck.

 

[Bifflefan]

Re: 3.7 woes

May also be an intake gasket. Id take a good look at it before taking off the head.
Really, if it were me i'd flush the closed cooling system and refill with new coolant, then run it again and see if you still get oil in it.
Then go looking for a problem if there is one.

 

[oilydogg]

Re: 3.7 woes

thanks for the fast replies. my reasons for concern on the cracked block are because the coolant that was in it when i got it didnt test very good. was told it was only good down to about 10 degrees by the person who checked it. here in maine it got a little colder than that this winter. either way i allready have the head off it. i will replace the head gasket and intake and exhaust manifold gaskets. but see the cracked block theory didnt come to me till i was loosening the last bolt in the head. so im wondering if and how i can check the block before i put all together?

 

[Bifflefan]

Re: 3.7 woes

so im wondering if and how i can check the block before i put all together?

I saw a post once, it was about test for cracks. They used acitone and poured it in a "whatever" and then let is set for awhile and see if the level goes down. If it does there is a crack somewhere, if not then no crack.
The reasoning is that acitone is smaller molecularly and has almost no surface tension so it will find a crack that water wont. I've heard that with no pressure on the acitone it will go through a crack that takes 20 psi to push water through.

 

[oilydogg]

Re: 3.7 woes

but would a cracked block allow oil into the coolant and not coolant into the oil? same with the head gasket? ive diagnosed quite a few head gasket problems on cars in my day but oil in the coolant is something i guess i havent seen before. its allways been the other way round. where do these blocks tend to crack? oil cooler bad maybe?