I took my car few months a go do the recall, at the time the also replaced 5 mounts. The car was felt like it was brand new. Last week the car started to virbrate again I am assuming its the engine mounts again. I am just wondering why the mounts keep collaspeing? Also it there a way I can see the mount is bad with out taking it to the dealer?

 

good question, I'm having mine replaced this week.

 

The upgraded motor mount has a heat shield on it, so I'm assuming heat causes it.

You can also get a solid motor mount from mazsport instead of getting the oem ones...

 

Don't know if this is relevant to your problem, but there's a TSB on engine mounts:

http://www.finishlineperformance.com...ins_index.html
(Click on "Hesitation/Idle Vibration Due to Cracked Engine Mount".)

Apparently Mazda redesigned the mounts. The TSB describes a test you can do to see if yours need replacement: drive in 3rd gear at 3K rpm, drop your speed below 30mph and then accelerate hard—if there's hesitation or vibration, your mounts are bad. The TSB also contains instructions on installing the redesigned mounts.

 

I'd agree that it's likely heat that causes it. On my last car there was also a TSB and update for the RH side motor mount to add a heat shield. It was very close to the exhaust header and the heat from it just cooked the motor mount. On that car the shield extended the life but some people still had problems and went to solid rubber mounts instead of the OEM fluid filled ones. I'm not sure if the RX-8 has solid or fluid filled motor mounts.

 

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so, would it be possible for failing/ed motor mounts to cause the motor and exhaust manifold to hang lower and thereby rattle on the bracing near the cat converter flange?

It is driving me crazy! This rattle appeared just 2 days ago and when I look under the car and push the throttle, I swear I can see the exhaust manifold pressing on this part causing a very loud and annoying metallic rattle. I'm thinking that if the mounts went bad, the engine is sagging too low. Sucks.

 

Gravitational acceleration (G forces). Everything wears, including motor mounts (in this case compressing). The sportier the car (and driving style) the moreso.

 

odddood it could be the hanger of your supercat,

 

^^^ yeah, double check the supercat; I had the same issue recently.
Unbolt the cat, make sure it's level and tighten everything back down.

My cat was resting on the rear cross member and made a HIDEOUS rattle.
All better now.