Took out motor to replace freeze plugs then put motor back in and now doesnt want to crank

someone might have unplugged the crank sensor .

If you didn't have adequate anti-freeze in the car, it could be damaged as water expands when it freezes. In extreme cases, it can actually split the engine open. To prevent that, they put in "freeze plugs" that are designed to blow out before the engine is damaged. Hopefully that is all that has happened and your freeze plugs can be replaced. However it is possible that the radiator was damaged too.

check the wiring , replace fuel pump

Follow this diagram and trace each wire to the correct plug:

my 2000 pontiac has code 420 when on idle is ok on drive or park I step on the gas it drives like motor mounts are broken I let the gas go and it rolls good I change coils wires plugs crank shaft speed sensor I even took of converter thinking it was plug it,on stray pipe replace EGR and still have the same problem, please help lucky fly 119@ yahoo.com thanks

the price is determined by how much you have to remove in order to gain access to the freeze plug to replace it the freeze plugs themselves are just a couple of dollars but could be several hundred to replace due to gaining access to replace you also have freeze plugs sometimes that are on back of engine and require you to either remove engine or transmission to replace also keep in mind that normally if a freeze plug is leaking it is due to a neglected cooling system that is now full of rust that may have to be flushed out

Are you getting spark at all the plugs? you may have the coil pack controller disconected or you may have pinched a wire somewhere casuing it to goto ground.

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you have a dead cylinder. start by pulling off spark plug wires one at a time to hear if the motor changes rpm if it does put that wire back then do this to the rest until it doesnt make a change in the rpm. then u will have to pull that spark plug to see if its firing. whats happening is when the fuel gets put into the cylinder its not igniting and causes the back fire through the throttlebody. fixes put spark plug into wire and lay on block then have someone turn over car to see if theres a spark make sure plug is touching ground but dont touch when cranking it WILL shock not bad

check if the fuel pump is working, if you have someone near the tank and someone else crank motor you should be able to hear a bzzzzzz rom the tank from the pump. if no noise is heard then hit or thump the tank, this can make a fuel pump start to work again, if it starts, then the pump is almost dead

it sounds like when you put the "fly wheel" or harmonic balancer on you wiped out the crank position sensor. The engine ECM/computer needs the crank position sensor to know what cylinder is coming up to Top Dead Center to Inject the fuel and excite the spark plug. I had the same issue on my wifes car. when the new Balancer is installed the sensor needs to be realighned so the balancer doesnt hit the sensor and break it. I would have your local auto garage replace the sensor and alighn the sensor because a special tool is required to properly alighn the sensor with the balancer.