I'm having an issue with my RX8. When driving for the first few minutes after warming the engine, I get alot of shaking at 1700 RPMs. If I bring the revs up past that, the car drives fine. This is VERY annoying when you change gears and the RPM drops to 1700 because the car shakes alot. You guys may be thinking that it's because I'm shifting too early but the car is an auto (I know...) and does the gear changes itself.

 

engine mount

 

I don't think that it's the engine mount. It doesn't stop shaking until you give it more gas and get past 1700 RPMs. An engine mount causes vibrations throughout the power band, especially on hard shifts.

I'm thinking that it's misfiring but I just want to make sure. I know that it's not detonating because I don't get a CEL. Btw, how often are you supposed to change the spark plugs on this car? I think that there is 36K miles on it (bought it used) and the spark plugs have never (to my knowledge) been replaced.

 

...
start with mounts.
then plugs, coils, wires
i still say mounts though

 

What would cause a mount to let the engine shake at ONLY 1700 RPMs? If you accelerate and get to 1700, it shakes. If you decelerate to 1700, it shakes.

 

check neutral

 

the mounts could be damaged in a way that the engine mounts and engine @ 1700 vibrate on a sympathetic frequency causing a lot of vibration at only that specific rpm.

 

^^ Very true. I had torn engine mounts on my 99 Miata and it vibrated a lot at idle. Anything above idle and it felt normal.

 

motor mounts/ maybe coils and plugs but i would not think so i would check engine mounts

 

Try sitting at a stop in first gear in manual mode. Then shift it to second gear, still at a stop. Do you hear or feel a difference in the engine. I had a bad mount and if you had a wall next to you you could hear a difference in the sound of the engine.
I have an AT also. Been through three sets of engine mounts in 60,000 miles.