car won't start after head gasket change

hey guys i just got done changing the head gaskets on my 99 buick century. now it doesn't start. it cranks, it's got spark, and i can smell the fuel. when i crank the engine, there's a loud screeching noise, but it's a low pitched screech. help would be appreciated, thanks.

Welcome to AF!!

The problem here involves starting at point a and proceeding to point z. I suggest you check all wiring connections and make sure you got all the connections put back correctly. I would check the injector wiring, the plug wires, and any other wiring that may have been disconnected duing the head gasket job. As for the noise check the belt and the tensioner to make sure everything is aligned properly since this sounds like a belt rubbing or a pulley that isn't tight.

One other thing you may want to do is perform a compression check on all cylinders. If the heads aren't properly torqued or the valves aren't opening and closing properly this could decrease the compression to the point where the engine won't fire. I hope this helps!

Does the screech seem to be coming from inside or outside the engine? If outside, as GTP Dad suggested it could be a blet driven accessory. Take the belt off and attempt to start to eliminate a belt driven pulley as the screech culprit. If it sounds like it's internal, you may have other damage such as crank or cam bearings due to coolant contamination in the oil. I would also suggest double checking the push rods to insure you didn't accidentally put an exhaust push rod where an intake should be, as they are different lengths.