bare w me as im a little new to this. so i got a 6.0 from fb marketplace and engine span freely when purchased. i then proceeded to do a complete texas speed cam kit along w dual valve springs and ls7 lifters hardened pushrods new 10295 melling oil pump new retainer plate etc. i also installed a tbss intake w 50lb injectors. so this is where my problem starts, on friday we did the first start up on the engine w the stock 5.3 ecu that came in the truck (no tune) and everything ran good for about 15 seconds had 50 psi oil pressure when truck shut off. i assumed it was running rich because of the diffrent size injector and no tune but i then tried starting it up again but battery drained itself. i didnt have a jump box to jump batter to start up again. engine span freely by hand even after first start up but we didnt manage to find a way to jump it so we left the truck sitting for about 3 days. came back monday and now motor is completly seized and will not rotate. wondering what could have went wrong did everything according to spec and im at a lost for words. any help is appreciated thank you so much

 

So you spun it over after it ran by hand, now 3 days later it won't turn over by hand? Stuck open injector or more causing a hydrolock? Pull plugs, see if it rolls over by hand. Check the oil for both level and smell.

 

What rpturbo said . Pull the plugs and spin it over by hand

 

As mentioned, it sounds like you hydrolocked it.

I would have all the injectors checked/cleaned.

For future reference, don't install different injectors without tuning it.

 

I'm wondering why the 50lb injectors are used on a N/A 6.0 motor? My built N/A LS7 is using stock 42lb LS7 injectors at 600+ hp. Maybe for future build? As another post stated, those injectors on a stock tune would be mighty rich. What brand of injectors are they? How old? I could be there are some leaky ones, and you have a hydro lock situation. I'm not convinced it's hydro lock, but, as stated, pulling plugs will reveal an instant answer. I'd key the ignition a few times to let the fuel pump prime up, giving leaky injectors every chance to flood the cylinders. If it is hydrolocked, and you wait too long to check it out since it ran last, the offending cylinder(s) could dry out, as gas evaporates fairly quickly. My .02.

 

I have to concur with LilJay and grinder11. 50lb injectors are way too large without a tune. If you figure out the lockup issue and all is ok, Put the factory LQ9 injectors back in until you have it tuned.

 

Very good chance it hydrolocked while not running. Should rotate after you pull spark plugs out

 

I'm betting the oil pump is in a bind.

Had a guy tell me an engine I built "locked up" before he even started it. Brought it in, it was the oil pump. At least the guy was honest and copped to it.