Q I have a 2002 Citroën C5 2.0-litre diesel with a noise problem when the engine is ticking over, although it sounds OK when the engine is at a higher RPM. The noise is coming from the alternator drivebelt – which disappears for a few minutes when you put a squirt of engine oil on it.
I have put a new drivebelt on the engine, but it made no difference. I can see the belt whipping between the crankshaft pulley and the air-conditioning unit pulley. My thoughts are that there is something wrong with the aircon pump and this is causing a drag on the belt. I have tensioned the belt at all sorts of pressures, from slack to pretty tight, and there are a variety of noises, but no silent running unless one applies the squirt of oil. I haven’t applied oil to the new drivebelt as I believe it would not do a new belt any good. In any case, when I tried it with the old belt it was only silent for a few minutes and didn’t really resolve the problem.
If the problem was caused by the aircon pump, how feasible would it be to disconnect the aircon unit? Would this incur other problems? I don’t think we have ever used the aircon, so we don’t need it. John Ashley
A The reason that the noise ceases when a squirt of oil is put on the belt is because the belt will then slip. This is not an ideal situation and oil should not be applied to the belt.
The first check to make is that the air-conditioning pump pulley is spinning freely when the aircon is disengaged. If this is the case then the air-conditioning is probably not the cause of the problem. Unfortunately, if the air-conditioning pump pulley is not running freely, this will need to be rectified. Given the route the auxiliary belt takes, it does require the pump to be in the circuit.
The most probable cause of the whipping belt is the alternator clutch pulley. The pulley is designed to reduce the possibility of belt snatch when suddenly reducing speed. These pulleys do fail on a regular basis and give the symptoms you are experiencing. However, it is not easy to test the pulley and normally nothing can be felt when spinning by hand.
Another possibility is that the torsional vibration damper built into the crankshaft pulley may have failed. This is far easier to test. With the auxiliary belt disconnected, the crankshaft pulley should be inspected for any movement or any signs that the outer section has been spinning and is not fully bonded to the centre of the pulley.