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Join Date: Feb 2009
Location: South Africa
County: Northern Cape
FIXED vs. AUTOMATIC Cam Belt Tensioner
I have recently changed belts and pulleys on my wife's 147. I had someone starting the engine before putting back the covers to visually check if all was well. I was shocked to see the amount of movement on the automatic cam belt tensioner during start-up, shut-down and low rpm. There is no lubrication on the spindle of the tensioner - I can imagine that it can easily get stuck in the maximum position with some slack in the opposite side of the belt, causing damage to the belt.
Half of the time during shut-down, the engine comes to standstill with one of the cylinders compressed more than 50%. This pushes the crankshaft back in reverse direction. Due to the shape of the cam lobes, and the tension of the valve springs, the camshafts are reluctant to turn in reverse direction. The tensioner spring is weaker, causing the tensioner pulley to snap to the maximum tension stop on the frame of the tensioner. On the other side of the belt I could visually see slack of about 1 cm in the belt. Upon restarting the belt is jerked, causing point load on the belt - the first tooth on the belt on the pulley has to take all the punch. This is not good for the belt, causing long-term damage that will accumulate and cause premature belt failure.
The benefit of an automatic tensioner is that it takes up slack resulting from belt stretch, and do not need to be retensioned every 30 000 or 40 000 km, as is the case with a fixed tensioner. However I am convinced that the automatic tensioner does more harm than good, and is seriously considering modifying and old tensioner into a fixed one, fitting a new bearing, and to use that in the next cam belt change.
I have three other vehicles, all with fixed tensioners. One of them has a non-interference engine. On this car the water pump is eccentric and serves as the fixed tensioner. However retensioning the belt is a major issue, since the water pump seal usually leaks if installed for more than a year and then disturbed by turning the pump. I thus decided not to retension the belt at all. When I eventually replaced the belt at 120 000 km, it was still in good condition, except for a bit of excessive slack while installed.
I would rather go for change intervals of 80 000 km, with just a retension at 40 000 km. This would also be better in line with the expected life of the variator and other pulley bearings. I am sure that with a fixed tensioner I can safely achieve that. Cam belt related costs will also decrease by 33%.
Any other opinions on this?