Developed vibration at slow speeds
 

My wife has an 04 RX8 automatic. We drive it hard, and it has been very good to us, always there when the 93, 94, and 95 RX7s let us down. Recently there has been a nasty and seemingly increasing vibration when partially applying the brakes at low speeds. A good example is pulling out of the gas station and gently applying the brakes to give me more time to look for oncoming cars. Nasty vibration. My first thought was warped brake rotors. The car is far from stock although I don't think ths problem can be blamed on any of the modifications. My other suspected possible problems are:

driveshaft u-joint
motor mounts
tranny mounts
diff. mounts.

Please help us locate the source of this vibration, Like I said it only occurs at low speeds when patially applying the brakes.

Here are my mods:

RE Ame. headers
RX7.com midpipe
Magnaflow catback
RB Ultra plug wires
K&N cold- air intake
Royal PP synthetic tranny and diff fluid
RX7.com oil filter magnet
oil & filter changed every 1000 miles

It ran 16.8 @ 83 mph stock and it is still slow as hell with the mods. Only the noise is unbearable now.... :) actually it sounds good'n'rotary.

Anyway please help and no flames please

My Miata's warped rotors announced themselves exactly as you're describing... vibration during low-speed braking. That would be the first thing I'd check.

The motor mounts on my 8 were destroyed when I was hit really hard in the rear by a truck. The vibration then could be felt even when the car was sitting still, in neutral. As soon as you'd rev the engine even a little, the car would vibrate. So if yours is only vibrating during braking, I'd suspect brake rotors.

I'd check the passenger side motor mount for damage just to be safe. With headers, you may have heat-damaged the rubber & caused a crack. It's easy to see with the wheel off, just pull back or remove the rubber splash shield & have a look. You may have to jack the engine up slightly on that side to see damage.