Q: what is an HydraulicTensioner?

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what is an HydraulicTensioner?
My car has an automatic transmission.

Hello. A "hydraulic tensioner" is simply a fluid filled device, where the fluid exerts a calibrated amount of pressure on a hardened steel pin that in turn pushes on a pulley arm to keep the timing belt taught. If the timing belt is not kept taught, under all running conditions, it could jump off of the timing/crank pulleys. So, this is a critical engine timing component. There are test protocols in shop manuals to determine if the tensioner is reusable (the tensioner has to resist a specified amount of pressure put on the protruding pin) and you can also simply evaluate a tensioner by looking for external oil leakage. There is no need to mechanically test them if you see external oil leakage past the part seal. If there is any leakage, just replace it. If no leakage, you can typically re-use them. Other parts in the timing system though are NOT re-usable and this includes any of the components that use grease filled sealed ball bearings (e.g., idler pulley or other ball bearing supported pulleys). The grease in those bearings oxidizes and dries out, due to the extreme heat of the engine, and the bearings eventually become noisy. That’s why you will typically see "timing kits" for sale by Dayco for instance that include not just the belt but also a set of replacement bearings.

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