I have a friend who has a 99 chevy venture with a 3400. It wont start and cranks slow and erraticly. 1 Ran fine 2 Belt tensioner had to be replaced 3 then it was more than the tensioner he also had to put a new bottom balancer on it 4 then he drove it out filled with gas came home still ran fine turned off engine and wouldnt restart,it cranked as if the battery was low 5 replaced the battery and still cranked slow and wouldnt start 6 he burt the starter up trying to start it 7 replaced starter still done the same 8 changed crank sensor no change 9 changed crank sensor 10 changed crank sensor it almost cranks like crank to cam timing but i dont know how to check it for him? Any ideas Im helping him for free because he dont have money for mechanic. But were limited with testers and so on any help would be appreciated! Thanks !

How about the battery cables? Check grounds at both ends?

If whoever did the balancer didn't get it seated right it could of spun the keyway off of the lower timing gear. Pull the oil cap and make sure the rocker arms are going up and down when cranking.

...and check the coolant level too.