I had a recent vacuum leak from the hose that goes from the throttle body to the leak detection pump, I think thats what its called, the leak detection pump I am talking about sits under the fuse/relay block, I found the leak and replaced the hose, problem is now that the truck will not start unless I remove the hose then re-attach it when the truck is running, truck runs great when the line is removed then re-attached, just will not start with the line attached..

2002 Dodge Dakota 3.9 Club Cab

 

If you have crank case vent problem, That maybe the only reason why the engine will start with the vacuum hose off, so the engine could breath.