idle or erratic mileage tip

I had an occasional idle problem and each tank seemed to get different mileage though I was driving about the same route each time. I also heard a tick from the passenger side which I figured was a lifter (high miles motor). Then the truck got a little louder, took a look at the passenger side exhaust manifold and saw a crack. There was no soot or discoloration around the crack, but I figured I should change it. Idle was back to normal almost immediately, mileage has leveled to similar on each tank, and driveabilty has aimproved. Just thought you might want to know.