The next time you utilize a drive belt, timing belt, or timing chain, you’ll likely have to 
loosen a tensioner pulley to remove it. Following these general recommendations and specific guidelines from your own owners manual or restoration manual, your belt or chain will function for the life span of your car.
Toyota and additional timing belt tensioners are loosened by simply removing them from the engine. You need to gradually compress them in a bench vice and lock them with a pull-pin before reinstallation.
Hydraulic (not hydraulic-damped) tensioners are nearly always positioned in the timing case, mostly upon automobiles with timing chains, while some are used with timing belts. Hydraulic tensioners are run by oil pressure from the engine essential oil pump and could press on a tensioner pulley (timing belts) or tension slipper (timing chain). You will likely need the year, make, and model details, and you may have to use special equipment for this sort of tensioner pulley.
Car Pulley Belt Typically, a hydraulic tensioner needs to be “reset” and locked after removing it from the engine. Remove the lock only after the tensioner, pulley, or slipper, and timing belt or timing chain are set up and aligned.
The spring maintains tension, while the hydraulic damper keeps it from bouncing under load changes. This prevents timing belts and timing chains slapping and jumping tooth and retains drive belts from slipping and producing sound. To loosen a drive belt spring tensioner pulley, refer 1st to the repair manual or owners manual’s specific season, make, and model information.
You might need a special tool, but many spring tensioners have a square hole, for a 3/8” or 1/2” breaker bar, or a hex or square protrusion for a wrench or socket. Using the correct tool, release tension on the belt. You’ll need to hold some spring tensioners while slipping on a fresh belt. Others may possess a locking mechanism, like a hole for a locking pin or hex crucial.
To loosen an NAI tensioner, loosen the locking nut or bolt, then back off the tensioner screw. Push the pulley toward the various other pulleys or add-ons, loosening the belt.
Spring tensioner pulleys, since the name implies, make use of a springtime to hold tension on the belt. Most, if not all, spring tensioner pulleys are NAI tensioners and include a hydraulic damper. They are more technical and expensive but don’t require modifications and are less susceptible to user error.