Blow-By On Mercruiser 3.7 470 Engine

[wbelk7777]

We?re (Dad and I) having trouble with our 1988 Mercruiser 3.7 470 engine.

History With Our Engine

When we first got the boat a couple of weeks ago we did a compression test on the cylinders, the first three came back good at about 130 PSI each. Initially the forth cylinder read 0 PSI. We found a stuck valve, that bent a pushrod. We freed the valve and replaced the pushrod and turned the engine over by hand . The valve that was stuck was working up and down fine at that point. We did another compression test on the forth cylinder (where stuck valve was) and it went from 0 PSI to 50 PSI.

Today

We weren?t able to start the boat until today, because several things (impeller pump, fuel pump, etc?) needed to be fixed before the boat would run. Today we ran the engine for the first time. The engine is experiencing what appears to be blow-by. It comes out of the valve cover pretty strongly when the cap is removed and then out of the air cleaner (on top of carburetor) when the cap is placed back on the valve cover.

1. Does anyone know if this is an interference engine (pistons and valves can meet if their timing is off)?
2. Can this be caused by a sticking or poorly seating valve?

Thanks for the help

 

[Evinrude Boater]

Re: Blow-By On Mercruiser 3.7 470 Engine

If the problem is at the valve, the leakage would be in either the exhaust or intake manifold. Squirt some oil in cylinder #4 and measure the compression again. If it comes up you likely have cylinder or ring problems causing the leakage into the crank case and out the valve cover.

 

[stonyloam]

Re: Blow-By On Mercruiser 3.7 470 Engine

#4 can be problematic on a 470. You may have some water intrusion from a leaking riser (or riser gasket). Check the color of the insulator around the electrode of the sparkplug. If white rather than tan, it is an indication of water in the cylinder. A blown headgasket is also a distinct possibility. Good luck.

 

[Fordiesel69]

Re: Blow-By On Mercruiser 3.7 470 Engine

When the valve went bad, it may have hit the piston. It is my beleif that most engine are interference when the valve sticks depending on how much it stays open. Only some are immuned.

Next step is find out WHY it stuck in the first place.

 

[Fordiesel69]

Re: Blow-By On Mercruiser 3.7 470 Engine

May be a bent valve which = a less expensive repair.

If piston has a crack / hole / bad rings = MUCH more expensive repair.

 

[stonyloam]

Re: Blow-By On Mercruiser 3.7 470 Engine

If you can't get more than 50 PSi on a compression check the motor is worn out. Time to think rebuild or replacement.

Mine read 150-145-135-30. New head gasket and a valve job, it was good to go. On a 470 a low #4 is a rather common problem, probably not a worn out engine, especially if the other three show decent compression. Not to say that it might be something more serious. 470's do have their little quirks.. LOL.

 

[nola mike]

Re: Blow-By On Mercruiser 3.7 470 Engine

Maybe it's just me but if I have to pull the head off (and everything to get to it) its faster and much easier to just pull the motor and get it where I can work on it.

no way that pulling the engine is easier. just did this exact job, there's nothing hard to get to, and pulling the head is straight forward. to the OP, you almost certainly have a bad riser or gasket that allowed water to leak into your #4.

 

[wbelk7777]

Re: Blow-By On Mercruiser 3.7 470 Engine

We removed the head this weekend and inspected cylinder. It appears that water came in through the intake and caused rust to form on the upper part of the cylinder. We tried lapping the valves and honing the upper part of the cylinder, but it didn't help the compression. I next step is to replace the rings in the 4th cylinder.

Can this be done the engine in the boat or do we need to remove it

Does anyone know the specs for the piston rings? Can you use Ford 460 V8 piston rings?

Thanks for the help.

 

[wbelk7777]

Re: Blow-By On Mercruiser 3.7 470 Engine

Any thoughts?

 

[Eric 3.7LX Alpha 1]

Re: Blow-By On Mercruiser 3.7 470 Engine

Here's a link to the original merc. service manual for your engine. It will have all the specs in it, and is much more detailed than an aftermarket book:
http://www.4shared.com/account/document/zTbCbkDD/Service_Manual_08.html

drive manual:
http://www.4shared.com/account/document/zz_5pj9o/Service_Manual_06.html

The Ford rings shouldn't be an issue. The book has the specs of the bore if it's std.

I don't know if there's enough clearance to drop the pan and disassembling the rod nuts in postion. Might be an easier job to back the boat under a big ol tree and chain fall it out.

 

[mkast]

Re: Blow-By On Mercruiser 3.7 470 Engine

I next step is to replace the rings in the 4th cylinder.

Can this be done the engine in the boat or do we need to remove it

Any thoughts?

Remove the engine.
The quality of the maintenance performed under better conditions, in the garage/shop, than in the hull is significant.
Your 1988 engine needs real attention, not a band aid.
Send the head out for a valve job.
Replace rings in all four cylinders, not just one.
Inspect the crankshaft. New rod bearing would be normal at this time.
Timing chain an oil pump would be normally replaced.
Instead of looking at the project as a "Gotta get it running", look at it as "If I gotta pull the engine, I might as well assemble the engine so I don't have to do this again next year".