I am helping my friend with a '96 Tahoe 5.7 vortec. I just replaced the fuel pressure regulator. Now it won't start. When I try to start it, it makes a single horrible clanking sound and then will not turn over again. The belts rotate about a quarter turn with the clanking sound. I can smell a little bit of fuel so there is probably a fuel leak. Would a fuel leak cause this loud clank? I do not think that it is a backfire. I may have run down my battery by trying to start it.

Any thoughts or suggestions would be greatly appreciated as I am stranded without a vehicle now. Thanks.

 

Check for a broken/cracked flexplate.

Might try asking here too:
http://www.pacificp.com/forum/index.php

 

Turns out that the fuel pressure regulator was loose and was just spraying fuel into the intake, flooding one the cylinders with gas. This caused the piston to be unable to achieve TDC since gas is not compressible, so the starter (I assume) was making the horrible noise because it could not turn the flywheel so was backing off and skipping gears. We pulled the spark plugs and gas leaked out. Towed it to a mechanic so we didn't blow ourselves or the engine up.