Motor mounts are highly overrated, since the engine probably won't fall out of your car without them. Well, not "just" the engine, anyway. You'll probably lose your transmission, steering, front suspension and control linkages, too. OK, motor mounts are important. There's also a pretty good chance it'll ruin your day when that 800-pound hunk of metal lodges under the chassis and turns into the largest speed-bump you've ever hit. Even if that doesn't happen, there's still a fair chance that your mount-less ride will rattle and flex its way right into the junkyard.
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Vibration and the Domino Effect
Motors mounts typically wear out before they break, exhibiting a number of symptoms in the process. The first thing you'll notice is slight to heavy shaking or vibration, which will feel like a side-to-side rocking if your car is rear-wheel-drive and back-and-forth if it's front-drive. While it is irritating, the primary problem with such vibration is that it places more shear force on the other motor mounts. This results in a sort of cascade effect where one bad motor mount takes out the others, then the other bad motor mounts kill off what was left of the first one.
Engine movement is the single biggest visual cue where motor mounts are concerned. Pop the bonnet and you may note a bit of shake at idle, depending upon which mount is broken, but you'll definitely notice movement when briefly hitting the throttle. The engine will rock two inches or more from side-to-side or front-to-back. A certain amount of engine movement is normal -- that's why the mounts exist in the first place -- but excess movement can break lines and hoses and cause the engine to hit other objects in the engine bay.
Broken Fasteners and Brackets
Engines create a lot of torque when they move; without functioning mounts, all of the engine's torque, as well as tiny vibrations, transfer directly to the nuts, bolts and fasteners that secure the engine mounts to the engine and chassis. Any nuts or bolts not secured by lock washers or locking compound may vibrate loose and allow the fasteners to back out. This holds true as much for the chassis as the engine, since all those major and minor vibrations will hammer their way through the frame and into anything it touches. Problems like this can manifest as anything from a rattling boot lid to misaligned panels to leaking weatherstripping.
Honk Twice if Engine Falls Out
Major failures like this are more likely to happen with front-wheel-drive cars than rear-drives, since the front motor mounts support the engine and transmission. Odds are good the front cross-member will catch the engine and transaxle before it falls out, but freak accidents do happen. For instance, if the rubber in a rear-drive's mounts fails completely, the engine will rest solidly on the frame or cross-member. If the engine torques the transmission but doesn't move itself, it could easily split the transmission's aluminium case in two.
One oft-overlooked consideration where motor mounts are concerned is that most cars utilise the engine and transmission as a structural (stressed) member of the chassis. That 800-pound chunk of metal does more than motivate the car; it also serves to tie the body together and keep it from twisting under cornering stresses and over road imperfections. This is especially true of unibody (frameless) cars, where the engine and transaxle effectively serve to triangulate the chassis and strut towers. Aside from causing sloppy handling, leaks, rattles and misaligned panels, excess chassis flex can stretch metal and permanently damage welded seams in the body. Allow bad motor mounts to go for too long and the car may never regain its former condition, even after you've replaced them.
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