Neo, provided you are a proficient DIY'er with a decent set of tools replacing the engine mounts on a 1989 300SE is a DIY project. You will need either a floor jack or a hoist to lift the engine while replacing the engine mounts. Recommend doing one engine mount at a time. Remove that engine mount to cross member bolt and leave the other one engaged by a few threads before raising the engine. It's easier to remove the entire engine mount/engine bracket assembly, then bolt the new engine mount to the engine bracket and reinstall. Repeat the sequence for the other engine mount.

For safety sake support the car on ramps or jack stands if you don't have access to a lift.

Torque values are:
Engine Bracket to Block - 18 ft/lbs
Engine Mount to Bracket - 41 ft/lbs
Engine Mount to Cross Member - 30 ft/lbs

Do a Search for the specific sequence of this job if you're unsure how to proceed.

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Fred Hoelzle