Shaking and Deep Rumbling past 5500 RPM
 

As the title suggests, my 83' GSL 102k miles all stock RX7 seems to have a problem where the car begins to shake with a deep rumble from 5500 to red-line. This issue is independent of speed and achieves the same effects when at rest, with the clutch in or in neutral, revving the motor up to the specified range. Interestingly, the issue does not seem to indicate poor high end performance of the motor. In fact, it seems to pull just fine, and I feel no indication of weakening power when the shaking occurs. The motor feels good, although I have never driven any other comparable rx7's so maybe the motor DOES have issues..who knows. I am thinking (with my novice knowledge) that some kind of peripheral device (connected in some way either directly or not) to the motor, (and thus increasing speed along with the motor) has issues in this rpm range. It almost seems like the rumbling is coming from the center dash area, but it's really hard to tell. Maybe some kind of issue with the heating system? The AC? Anyway, I can feel the shaking through the pedals, and the seat.

Thanks all

Sam

My first thought world be to check the motor mounts?

Motor mounts. Maybe the trans mounts too.

Well to make a quick list of everything auxillary to the actual keg that rotates or is attached to something rotating when revving the engine while stationary:

1. Counterweight
2. Flywheel
3. Clutch Disk / Pressure Plate
4. Throughout Bearing / Clutch Fork [rattling perhaps?]
5. Pilot Bearing possibly warped?
6. Motor and Trans Mounts [though most simply manifest themselves as shakes and rumbles at multiple times not just a specific RPM range. though I've never seen a motor mount rumble like what your saying from simply revving it.]

When revving slow vs. fast is it like a lightswitch of smooth smooth smooth RUMBLE or does it slowly fade into a rumble?

Do you know if the car has recently been worked on concerning the clutch, transmission, motor mounts etc?

Could be a belt about to let go, or a front-engine accessory with a bearing going bad or a bent pulley, too. Or a failing fan clutch. It all rotates at the same relative speeds.

Check visually for vibration with the car parked in neutral. Look at everything that moves. Then shut it down and check for axial play in pulleys and the fan.

It is definitely is more of a light switch. The car feels butter smooth and then suddenly it gets rumbly at about 5500, and then stays pretty constant, maybe getting a little worse at it approaches 7000.

I will do my best tp check everything out as people have suggested. I looked at the motor mounts the other day and they looked fine, but I really need to check everything out again.

Thanks,
Sam

i had something similar happen on 2 different Rx's. although both you could hear the rumble all the time, and it just got worse with engine speed.

so anyways, check the water pump. ive had 2 of em where the bearings died. they didn't leak either. if you grab the fan though, both pumps had plenty of side to side play.

very easy to check!

i've also had an alternator do the same, i kind of ignored it until it actually froze up though.

i have the same exact problem. i just got a motor swap and this problem is what came with it. vibration comes from the front of the car all the way to the shifter and making its vibrate alot at that rpm ranges. anyways, first thing im going to do is check the motor/trans mounts.

ok so my first answer woulda been mounts (allready sugested) but the least likely thing i can think of if nothing else seems to be it, check the electronic clutch on the ac compressor, if there a short or anything interupting voltage (or impropper gapping, hapens with age) could be causeing the two contacts to be touching, the rpm specific situation fits this answer as the pump would be either engaged or not untill the drive end reaches enough speed to offset the electromagnets friction.

i know thats verry far fetched, but i did run into this problem with my 97 626, shook the whole car to the frame
if it does happen to be that, pm me, im an hvac tech and can help u correct the prob