Shaking at 50mph
 

So, I have a 2006 rx8, she’s is a beauty, however a couple weeks ago, we’ve started having some issues. It seems that no matter what, my car starts to somewhat violently shake as I cruise between 50mph and 60mph, it stops until I get to 80mph, then it starts to sound like it’s droning. Clutch in or out, engine revved up or cruising low. The entire time, the cabin and transmission feel like they’re shaking. I have just replaced a motor mount, a coil pack and got the rear(I think just the rear) tires rebalanced in an attempt to fix this. No clue what could be happening, but tonight, I just got a random misfire CEL(P0300), while trying to push it through coming home from school. It also struggles getting up to anywhere past 100 and becomes extremely unstable after about 90. I have a strange feeling it’s got to do with either the transmission, suspension, or my tires. Tires are fine, no strange pattern, no rubbing or anything. Just not sure. Any pointers would be helpful, thanks. Again, Manual Transmission, 2006, gt Shinka, Sohn adapter, K&N intake, fresh rebuilt motor with less than 20k on it, and chassis mileage at 124000.

If it's speed related I would look at wheel balance, a bad shock or a wheel bearing ( in order from most likely to least likely). If it doesn't vary with engine speed, it's not the engine.

Actually very grateful to see your username Loki haha know I’m in good hands. Anyway, I got rear balanced, been wondering about shocks, any explanation for the cel?

What's the condition of the ignition coils, spark plugs and the catalytic converter? Bad coils kill cats, bad cats kill engines. I would start there and I wouldn't drive the car very much until those are replaced or confirmed good. If you had to replace one of the coils, you probably need to do all 4. They're silent killers and need to be replaced preventatively, or upgraded to safer aftermarket options like BHR.

Also the K&N intake is often the cause of issues, though if you've always had it and it was fine, then probably not the current issue. But they're bad news.

Have you already read rx8help.com or the New/Potential owners thread here?

I have, just couldn’t find anyway dumb enough to use it😂however to fix the turbulence in the MAF I used an OEM screen to straighten the air before it even touched the MAF. As for my cats, and ignition system, the previous owner gutted the car I’d assume well over two years ago. I’ve just left the exhaust as I’m unbothered by it and don’t want to pay 6-700 dollars on a new system. I probably do need to replace my coils, however the coil wires were replaced less than 10k ago. And two spark plugs were replaced when diagnosing another issue (Accidentally bought a full pack of Trailing plugs in stead of the mix didn’t think too much of it). Starting to wonder if I’m beating down on it to hard, it’s not a track car however some days I drive it like I stole it haha. I’m starting to notice the 8 runs extremely rich, just the heavy smell of fuel burning, as well as being able to shoot flames which I did not know it could do.

meant catalytic converter

A normal rx8 runs about the same AFR as any other car. The problem is it consumes a lot of both air and fuel at high rpm, and produces the corresponding amount of exhaust. Without a cat it will stink, yes. FWIW when cold, my Forester smells about as bad :)

You don't generally want to piecemeal ignition. If the wires were done, and the plugs are now correct, get all the coils on to the same level. Any one of the 3 sets of components failing is not good, and they can fail silently.

Do beat on it. These cars suffer more from granny driving than from being wound out to self cleaning temperatures and pressures. There are no reciprocating parts like a piston engine so spinning faster isn't a problem until you get to like water pump cavitation (which you shouldn't).

So is it still vibrating then? Because that still doesn't sound engine-related.

change all you plugs. and coils and new wires.

Been a year, but had to share the fix. The rx8's driveshaft was loose at the connection to the differential. I ended up putting in new bolts and the shaking went away. However my transmission began grinding and was not making nice noises from the front(My guess is input and output shaft bearings were going out due to harsh vibrations.) Threw in a new trans, seemed to do the trick.

Having this same issue with vibrations 50 to 60 mph

Is it as easy as jacking up the car and tightening the 10mm bolts between the diff and drive shaft?

Mine went away after taking off and on each wheel to check for brake pads wear, weird , I guess previous owner didn't torque them evenly. Some lug nuts were a bit loose

^This.

Don't piecemeal it.
Do all plugs, wires and coils together about every 35,000 miles.

If you can't afford to do them all at once, you're going to have a bad time.

A good tuneup includes those, air filter, (cabin air filter also.)
Disconnect the battery, after doing the ignition components, clean the ESS and MAF sensor.
Use MAF cleaner.

Do the 20 brake pedal stomp afterwards and reset your traction control.

Now, take your car to a MECHANIC shop, not a tire shop, and get your suspension checked.
It can be more than balance or alignment, or shocks even, but weak shocks will cause old unbalanced tires to bounce.

Control arms or hub bearings could be bad.
A high school dropout tire monkey might not be able to diagnose suspension problems.