Good question. It's because it has to be loose when you tension the belt. This lets the tensioner spread tension evenly across the belt. If you don't, after you tension the belt and release the locks, the uneven tension will move the crank-cam-injection pump relationship.
With all the cam and injection sprockets loose, with the engine at TDC, the cam and injection pump will stay at their correct locked position after you release the tensioner. After the belt is tensioned, you tighten the cam and IP sprockets and remove the locks.
Correct. Normally most cars will have 2 timed pulley's on the belt, the crank and camshafts. But when you have 3 timed pulley's, you are going to have to have one of the pulley's loose so that the tensioner can tighten the belt. If all 3 pulley's were locked then there would be no way to remove the belt slack that would arise between the fixed pulley's.
With the camshaft sprocket loosened or even removed from the camshaft, mounting of the timing belt goes more softly, without stressing the belt. After putting the belt on its path around the crankshaft sprocket and the other sprockets, engage it by means of the removed camshaft sprocket and shift the camshaft sprocket back on the camshaft, with the belt on it, together with the loose tension sprocket.
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