2002 F150 automatic with od, 4.2L engine.. 148,250 miles. Rebuilt trans about 10000 miles ago. Truck was running and driving, however was having a odometer intermittently working, it would work sometime sometime it would not. Went to start last week and would not turn over. Thought it was starter. Replaced starter today. Connected battery back, still would not turn over. Took battery out and took to have checked. They said it was fine. Came back and still will not turn over, so took a socket and a long pry and tried to turn motor..will not turn..can turn it all the way around backward to same point, stops dead when reaches that point. Know I probably shouldnt have but yes, turned it backwards and let starter turn the engine over and boom, when reaches that point it stops dead in its tracks ?????? What could this be, drove truck that day and did fine other than the odometer comming on and off, parked in driveway, 2 days later motor seems to be locked. Any suggestions?? Thanks.

 

could be an intake gasket leaking coolant into the cylinder and locking the engine up pull the all plugs out an try to turn it over by hand an see if coolant comes out of one the cylinder nothing but air should come out

 

You might also get under the vehicle and remove the inspection cover and see if something is causing the lockup in the flywheel area.