I have a pair of 1990 model 150 Mercs on my boat. I shut the engines down while waiting for my trailer at the ramp. One engine will not turn over now. I tried to turn over by hand and it seems locked up. I tried backwards by hand and it will turn over. All pistons are moving. There is a grinding sound that seems to be coming from under the fly wheel. I did not have the proper tools to remove the flywheel but will attempt next weekend. Any ideas from anyone would be appreciated.