When an engine mount goes out on a GMC truck (or almost any vehicle), the engine can cause vibrations throughout the truck, which makes for an uncomfortable ride. It also causes additional stress on the other mount, which could cause it to fail prematurely. Ultimately, it's important to replace an engine mount when it goes out as soon as possible. This is a moderately difficult job that requires heavy equipment. For this job, we're working on a 1997 GMC truck, but the steps are the same for many engines.
Pop the hood and bolt two ends of the chain to the engine. You want it to go across the block for maximum support. Use an existing bolt hole to hold the chain on the engine if you'd like, or locate a threaded stud and bolt it in that way.
Hook the hoist to the jack and jack the hoist up just slightly. Support the engine, but do not lift it out of the vehicle.
Unbolt the engine mount bolt that goes front to back in the vehicle using the 3/8-inch ratchet and sockets, plus an open-end wrench. This bolt secures the mount on the engine to the rubber isolator mount on the frame, which you are replacing. Remove this bolt from the mount. The hoist should now be supporting the engine.
Lift the hoist a bit more so that the engine tilts to the other side off of the other mount. There needs to be enough clearance to work, but not enough to lift up the vehicle.
Unbolt the mount from the frame, using the 3/8-inch ratchet and socket set, plus an open-end wrench. This may involve the use of the 3/8-inch extensions and universal joint, plus the open-end wrenches.
Replace the motor mount with the replacement unit. Reinstallation is the reverse order of installation.
