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It is a small oil storage device aka accumulator.
It acts in a mechanical way the same as a capacitor would act in an electronic way.
Sorry if my explanation was a bit vague. I didn't mean the diaphragm stores oil pressure.
I believe what it's for is that while there is oil pressure in the engine it displaces the diaphragm by filling the chamber with pressurised oil. If oil pressure decreases such as at shut-down, the diaphragm reacts by relaxing and consequently displacing oil which has nowhere to go other than acting as a force on the tensioner piston.
This probably is also a factor at start up when there is a volume of cold, thick (all be it without pressure) oil in there, which resists the piston being forced back by the chain guide.
I'm not sure if this is 100% correct but looking at how it's all put together and where oil goes in the system, it's what I suspect the case to be.
Last edited by Fordkoppie; 20th July 2011 at 08:44 AM.