I recently was given a 2001 Silverado with a 5.3 L V8. Was driven and then sat for 2 months. Then click no turn. Was looked at by someone else first. They took out spark plugs and tried to turn with crank bolt and could not. I have taken off belts, took out starter and poured Marvel Mystery Oil in the cylinders. It has been sitting a few days and still will not turn. Does anyone have any tips or tricks to try. Also if I am missing something please let me know. I have not taken off oil pan or try to turn with fly wheel yet. Any help would be greatly appreciated.

 

How much oil did you put in the cylinders? Keep in mind, you likely didn't put enough to encompass the entire piston, so you ended up only soaking the front/bottom half of the piston walls. What I would do, personally, is invest in a lot of cheap ATF. Then start filling up all the cylinders with it until the entire piston/combustion chamber is submersed in ATF. This will require that your entire oil pan is filled with it as well. My thinking is that you want to completely cover everything. I've even seen that you can mix the ATF with a bit of diesel fuel and that is a home-brew penetrant.

My guess is that the last time it ran, it was either low on oil or the oil pump wasn't working and that lead to a seized motor.

 

I have since found coolant in the oil and oil filter when dropping the oil pan. I am thinking it blew a head gasket, overheated and seized up. I am about ready to just pull it and replace with another.

 

A head gasket typically isn't directly responsible for seizing an engine, but given that you have that in addition to the seized issue...yeah, just swapping in a replacement used engine is a good solution when you start looking at cost and effort.