Mercruiser 3.7 (470) wont turn all the way

[Thomas1981]

Hello, my name is Thomas and since 8 years boat owner again.

I bought a speedboat with 3.7L Mercruiser 1992 that was standing still for 4-5 years, engine has 300 hour and looks in good condition (see picture).
According to the previous owner the engine was doing fine before the it was put in storage.
The engine was properly winterized, but stored outside.

I want to do a compression test before I start investing money into it.
Yesterday I took the plugs out, and inserted some oil in every cylinder.

But I can't rotate the engine in the correct direction, only opposite direction (CW) of the engine rotation (CCW). Rotating with a spanner, didn't use the starter yet.
But when I turn it 'back' (CW), I can rotate (CCW) it back to the position where was standing for 5 years.
So I can turn the engine in 2 direction each 90 degree, but in both direction eventually it gets stuck.

I already took off the drive but the problem is still the same. The valves are not stuck.
It feels like a piston is hitting a valve, but this shouldn't be possible.

Is there something I can do before I will take the cilinderhead off? The cylinders are from aluminium so I don't expect rust on it.
Is there a component to can block the engine only in a certain position?
When it's rotating I don't feel special resistance and there are no sounds.

Thanks for the tips.

 

[Grub54891]

I wouldn't take off the heads just yet. Making more work than needed. It might be fine? Turning by hand is a bit hard to do as there are many moving parts to get to move around. Why not try the starter? Disconnect and ground the coil wire and give it a crank. Putting oil in the cylinders is fine, but you will have to run it and burn off that oil before you do the compression test as its sealing the rings and will give you false readings. Since the drive is off it won't hurt the impeller unless it's the one with the seawater pump on the motor. In that case just remove the belt.

 

[Thomas1981]

I wouldn't take off the heads just yet. Making more work than needed. It might be fine? Turning by hand is a bit hard to do as there are many moving parts to get to move around. Why not try the starter? Disconnect and ground the coil wire and give it a crank. Putting oil in the cylinders is fine, but you will have to run it and burn off that oil before you do the compression test as its sealing the rings and will give you false readings. Since the drive is off it won't hurt the impeller unless it's the one with the seawater pump on the motor. In that case just remove the belt.

I was thinking of this. But what if there is some uneven surface on the cilinders due to standing for years. When I use the force of the starter this could damage the piston ring?
I can remove the cilinderhead in my garage, but replacing piston/engine is a bit too much for me.

 

[Eric 3.7LX Alpha 1]

Welcome. The block is aluminum, but the cylinders are sleeved. The rings are not riding on aluminum but iron sleeves pressed in the bores. When you work it by hand, do you get any tiny bit extra travel with each movement?

 

[nola mike]

You've taken the plugs out?
Whoops, saw you did. How do you know the valves aren't stuck?

 

[Scott Danforth]

stop turning the crank from the crank bolt. always use the starter or flywheel teeth.

since you removed the drive, you have a motor issue. if the head gasket went on the motor, you will have rust in the cylinder bores.

 

[Thomas1981]

Welcome. The block is aluminum, but the cylinders are sleeved. The rings are not riding on aluminum but iron sleeves pressed in the bores. When you work it by hand, do you get any tiny bit extra travel with each movement?

No *** goes very smoothly untill a certain point and than its stops. Feels like 2 steels parts are clashing.

 

[kenny nunez]

You may have a stuck valve. Remove the valve cover, loosen all the rocker arms and then try turning it. That will narrow down where the problem could be.
Maybe Santa will bring you some good luck!

 

[nola mike]

You may have a stuck valve. Remove the valve cover, loosen all the rocker arms and then try turning it. That will narrow down where the problem could be.
Maybe Santa will bring you some good luck!

That's what I'm thinking. More likely than rings that hang only at a certain point imo... Also common failure point to leak from the reservoir back into the #4 exhaust valve

 

[Bt Doctur]

When you loosen and remove the push rods KEEP THEN IN THE EXACT ORDER OF REMOVAL for reinstallation. Valve are adjusted by the length of the push rod

 

[QBhoy]

Worth taking the outdrive off it ?

 

[nola mike]

Worth taking the outdrive off it ?

Already done

 

[QBhoy]

Hi. Read through the whole thing just there. Are you aware of the infamous issues with these engines ? If there is any doubt about its mechanical health, throw a good used 3.0 in there instead. If that 3.7 isn’t broken now, it will be soon…then again and again after that. Disaster of a thing.

 

[nola mike]

Hi. Read through the whole thing just there. Are you aware of the infamous issues with these engines ? If there is any doubt about its mechanical health, throw a good used 3.0 in there instead. If that 3.7 isn’t broken now, it will be soon…then again and again after that. Disaster of a thing.

Horrible advice. The engines, if maintained, are fine. And the power is closer to a V6 than a 3.0

 

[Thomas1981]

OK, I will check the valves. I just noticed that valves are moving during this 180 degree free rotating. When a valve is stuck there should be space between the valve and the rocker arm? I didn't noticed that.

 

[QBhoy]

Horrible advice. The engines, if maintained, are fine. And the power is closer to a V6 than a 3.0

I think we all know it’s the unfortunate truth of it. Only guys that insist on keeping these things running…are guys that spend more time with fixing the thing, than using the thing.
As for the power being closer to a v6…not with the head warped and pissing steam all over the place, they don’t. To 99% of boaters, these things are just grim and not worth the hassle. The idea that proper maintaining solves all…you’d need to redefine the definition of the word, to cover the extent of what that involves, over a normal marine engine.
Stick a 3.0 in and be done with it, I’d say. Reason many do this, is because they fit straight in with no modifications to mounts etc. A V6 wouldn’t.

 

[Thomas1981]

I think we all know it’s the unfortunate truth of it. Only guys that insist on keeping these things running…are guys that spend more time with fixing the thing, than using the thing.
As for the power being closer to a v6…not with the head warped and pissing steam all over the place, they don’t. To 99% of boaters, these things are just grim and not worth the hassle. The idea that proper maintaining solves all…you’d need to redefine the definition of the word, to cover the extent of what that involves, over a normal marine engine.
Stick a 3.0 in and be done with it, I’d say. Reason many do this, is because they fit straight in with no modifications to mounts etc. A V6 wouldn’t.

What I've read is that the engine will get damaged when overheated, and the original alternator sucks. For the rest, how bad can such and old-fashioned engine be? My engine ran 300 hours, and I will use it 50 hours per year. That is similar to driving 15.000 miles. Shouldn't be a problem....

 

[QBhoy]

What I've read is that the engine will get damaged when overheated, and the original alternator sucks. For the rest, how bad can such and old-fashioned engine be? My engine ran 300 hours, and I will use it 50 hours per year. That is similar to driving 15.000 miles. Shouldn't be a problem....

I hope it’s ok for you. Really do. But they are famous around the world for their issues. Even mercury abandoned them after a few years of disasters.
If you are keeping it, I fitted an alternator kit to one years ago (before it blew up the second time) much better than the old stator system. Keep and eye on that water pump. But once the head goes, you’ll be chasing it for ever after. All the best with it anyway abs happy Xmas

 

[Scott06]

What I've read is that the engine will get damaged when overheated, and the original alternator sucks. For the rest, how bad can such and old-fashioned engine be? My engine ran 300 hours, and I will use it 50 hours per year. That is similar to driving 15.000 miles. Shouldn't be a problem....

The other pain point on these is cam to water pump seal ends up putting AF in oil.

 

[Scott Danforth]

An overheat causes the headgasket to leak into the cylinder walls because an iron head and open-deck aluminum block is not the smartest design

Only being able to turn 90 degrees would have me pulling the motor and putting it on a stand regardless of the motor