I have a 1990 5.7 merc that has sat for 2 years. When hand turning the crank, it will turn a full revolution one way then get stuck, then turn it back a full revolution then get stuck.
I did have the bottom leg apart to change the water pump impeller, could I have not put it back together correct or could this be a motor issue? It ran great when I shut it down last, Thanks.

Did you pull the plugs? Sounds like you got some water in the cylinders...

remove the spark plugs and turn it by hand again,look 4 water from plugholes while turning.

I have pulled the plugs and put a little oil down each cylinder. When I crank the crank bolt with a socket it just turns one way about one full revolution then back the other way one revolution to a dead stop. I am worried if I try to start it , it may ruin something?

dont hit the key until u figure out what is wrong!

remove the outdrive and try again,if it turns free your problem is in the drive.

It could be the drive, I will pull it and see.

You didn't drop a valve did you?

I sure hope not as it did shut down fine last time I was out.

one of the valves is rusted open.... if you try to turn it or start it you will bend that valve. take the valve covers off and the rockers and see which one it is... from there you have some options.

the best solution is to pull that head and fix it correctly but i have seen people in similar circumstance flood the guide with penetrating oil ( i like corrosion x ) and tap the top of the valve w/ a soft hammer until it moves a bit and you do that for a while and it moves and returns further and further until it frees up. i am not going to say that this is the way to REPAIR anything but it has a 50 -50 chance of working and getting you going.

the bottom line here is that unless the intake and exhaust were sealed up and the motor fogged (which i have to presume isn't the case since it already rusted a valve, ) that the bores are rusty and the other open valve seats as well... so... if the motor is a beater that you will be happy to just have it start and run, then go for it... but if the motor is some valuable piece you might want to think about a freshening up to have something good when you are done.