Hands too large...
Join Date: Feb 2009
Location: South Africa
County: Northern Cape
Having slack (from exhaust to crank) is normal by design and happens in about 1 in 3 engine shutdowns. This is caused by crankshaft kick-back due to an incomplete compression stroke, and increased resistance of camshafts to turn anticlockwise, due to the shape of camshaft lobes. The 'automatic' tensioner pulley then rather gives way and first hits the 'maximum tension' stop before forcing the camshafts to turn anticlockwise.
IMO this is the primary cause for 'premature' belt failure, or skipping of teeth on pulleys. Imagine the engine cooling off to sub-zero temperatures overnight, with the tensioner pulley at the 'maximum tension stop', and slack on the other side of the belt.
The spindle of the tensioner pulley is non lubricated, and may well become sticky under such conditions. When starting the next morning, the belt is subjected to an initial jerk by the crank pulley, causing way above normal linear forces on the belt. Due to the slack there is point-loading on the belt, where only a few teeth have to take all the punch. If the tensioner pulley does not immediately take up the slack, there is a good chance of skipping teeth.
On a new belt I normally set the pointer to hover about 8 mm above the marker. Due to belt stretch, I have found by inspection that the pointer hovers at the mark after the first 1000 km, and remains there for most of the life of the belt.
IMO there is an increased risk of skipping teeth if the belt runs slightly under-tension, due to the intermittent induced slack now becoming even more excessive. Also, the momentum build-up and hammering on the 'maximum tension stop' of the pulley becomes excessive. On my son's car (Daewoo with 2.0l GM engine) the maximum tension stop lug even broke off from the pulley-frame (due to repetitive hammering). This sharp piece of metal then fell in between the belt and crank pulley, where it took off a third of a tooth on the belt, before being spat out and ending up laying harmless inside the cam belt cover. Luckily he heard the noise, and had his car towed home, without any expensive damage done.
For these reasons i prefer the 'fixed type' tensioner pulleys, found in older type engines. The increased belt life, and peace of mind, by far outweighs the periodical re-tensioning that has to be done.