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I was digging around under the front end, and noticed the passenger-side mount's bottom nut was backed off to the end of the bolt, with more than a 1/4" of threads visible. I had sort've suspected something like this, as I've had some bad 2Krpm bucking the past few months. I installed a Racing Beat dual-pipe exhaust kit around that time, and the bucking started after that (quasi-useless info). Anyhoo, back to the topic. I tried to tighten the nut (the front end is on stands at the frame rails), but the rubber part of the mount just twisted along with the bolt. So, I'm curious if that bolt is supposed to be solid all the way through the mount, or if there is supposed to be two embedded bolts at either end of the rubber mount. I know the fuel pump isolators are the latter type, so I was confused. I didn't think a chunk of rubber was the smartest thing to suspend the engine on. Then again, that's what they did with the entire exhaust system, so there's no accounting for engineering. So, do I need a new mount, or just keep ganking on the nut 'till it turns or the rubber breaks?
Just a side note, but suspending the exhaust from all those rubber hangers. . . what were they thinking? All but one of mine were broken when I replaced the exhaust.
I think the mount is broken when it breaks into two pieces when you take it out of the car. I know one of my mounts were bad that way. The mounts are made with the rubber sandwiched between the two metal pieces. They're actually "glued" together.
Yeah, the exhaust hangers are pretty old for our cars, I had to replace a few of them myself. They're not cheap either. $9 IIRC from the dealer.
As stated above, a broken motor mount will lead to strange sensations during idle conditions as the car finds it's dynamic resonance frequency - i.e., that certain rpm that results in the car shaking violently enough to rattle your mirrors.
The mount itself is two metal plates sandwiched with a rubber disk in the middle. The bolts and nuts on each end are not connected through the center, as this would kinda kill the whole shock dampening effect. When they go bad, you can pull the rubber out of the center and pull the whole thing apart. If your front engine mounts are bad, also check your rear transmission mount - same design, and usually goes out at about the same time.
Good mounts all around will help to maintain a smooth idle that's rock solid from inside the car. Definitely worth it for any sports car enthusiast to install.
On the exhaust system, Mazda used the rubber 'donuts' to isolate the vibration and exhaust note from the cabin, which is a good idea. Otherwise, if it were hard mounted to the frame, you'd hear every 'ping' and 'ting' in the exhaust note and it would quickly get annoying. If you swap these out as they go bad, it will keep things quiet and also keep your piping from hanging down too low and scraping on speed bumps. HTH,
Well, that clears things up nicely for me. Thanks for the replies.
"The bolts and nuts on each end are not connected through the center, as this would kinda kill the whole shock dampening effect."
Not necessarily, if you assumed the metal plates on either end represented the quiescent state of the shock absorption mechanism, while the interior rubber donut provided shock dampening in one direction of travel. The bolt and nut would have to be quite strong to withstand the rebound force, though.
As for the exhaust, I was thinking metal eyelets with rubber donuts inside of them surrounding the exhaust could provide shock and vibration dampening, while still providing a fail-safe hold on the exhaust system. Or, just embed the existing hangers with steel cable.
Hehe. Just noticed LongDuck's title. Spank you, Helpy Helperton.
After several rounds of PB Blaster, I finally managed to break the nut loose. It was sort've amusing, as I've never broken a nut loose to tighten it. Anyhoo, I tightened the mount up, and drove it for an hour, with no low-RPM bucking. Mission Accomplished.