obeeone420

Hello All,

Been sorting this RX-7 out since I bought it almost a year ago and now finally getting back on the road. Now after driving a bit, I have noticed a pretty heavy vibration that shakes the entire car. It does not feel or seem like a wheel balance issue because it will happen at varying speeds. Below are a couple of videos I shot from underneath the car when it was on stands to try and see what was going on. Appreciate if anyone has any input on the noise, especially from the transmission.

From Under Transmission

From Under Differential

Thing I have checked:
Both mounts for the diff appear to be in great shape and almost look new.
Checked both universal joints on the driveshaft and neither have play in any direction.
Checked the engine mounts and have done power braking in both 1st and reverse with little movement in the engine.
My primary concern is with the noise coming from the transmission.

FDAUTO

iTrader: (4)

it doesn't look like it but check that your ppf isn't broken. the movement in the first video says your motor mounts are jumping around but you say you checked them. the movement in the second video says your diff bushings are bad because you can see the nose of the diff torque over when you accelerate.... but you say you they look new. with stock mounts, your driveline should be in alignment. get a look at the shifter and how it sits in the hole into the cabin. it should be very clearly centered and not pitched to one side. you'll need to remove the center console and the rubber boots to see.

it really looks like motor mounts though and kind of sounds like it too. take the shield off from under the trans and point the camera and bright light at the mounts separately and see how they react when you do everything you just did.

c0rbin9

iTrader: (2)

I was getting a pretty bad vibration that increased with RPM when the PPF was installed out of alignment.

obeeone420

Thanks for the input FDAUTO, I'll get it back up in the air this weekend and check the mounts. I didn't visually get in and check the motor mounts, but just did the power braking and didn't see the engine lift too bad like I have seen in other cars. I'll definitely get a video of the motor mounts to check them as well as take a close look at the PPF.

Appreciate the help.

Edit. Adding one more video from back behind the diff.


Note: Each time the vibration happens I am spinning the car up to speed then pulling the e-brake to stop the rear end suddenly to load the drivetrain differently so I'm not sure how much movement rubber bushings will allow, but I'm assuming some is to be expected.

Sgtblue

iTrader: (16)

You said the differential bushings looked new. The stock mounts are oil filled and leak when they fail. Looks like you have build-up below them, so they may very well be new. But can you tell if they’re stock?
Aftermarket diff bushings have been known for NVH… some more than others. Probably doesn’t account for all your issue, but it might be something to check.
Some discussion of aftermarket and pics of stock bushings here if you need them:

https://www.rx7club.com/3rd-generati...hings-1128159/

obeeone420

Just looked through the thread that you linked and as far as I can tell my diff mounts would be stock. They are definitely not a poly or aftermarket solid mount, but I did get up and inspect my mounts again as best I could and there are no signs of cracking, or splitting that I can see with the limited space. Hopefully, that's one area I can rule out for now.

Thanks for the help.

dk_davis

Tire out of round or knot / lump on tire? Jack barely of the ground and rotate to test?

obeeone420

Yeah, each video is with the car up off the ground and I've actually removed the tires to eliminate that as a possibility and the car still acts the same.

DaveW

Any chance you have a bent/out-of-balance driveshaft? My 49-YO G30 Chevy Van tow vehicle has some of that.

obeeone420

That's a possibility, but I don't think the driveshaft is bent just from watching it under the car it doesn't seem to wobble at all and turns pretty true, however I'm thinking a universal joint could be locked up since the car sat for 14 years. I checked the joints and they are tight, but doesn't mean that it's free to move either.

obeeone420

OK, follow-up with the video now at the motor mounts. The drivers side is clearly caked in oil from when the oil pedestal o-rings were leaking, but not sure if that's enough movement to cause this issue? I'll likely do motor mounts this winter just as maintenance since the car is at 87k miles and update the aluminum mount arm on the drivers side, but hopefully I can sort out the vibration/noise too.


Also, don't mind the huge exhaust leak noise, I had the wideband O2 bung open as I'm installing that now as well.

FDAUTO

iTrader: (4)

99% its your driver motor mount. you can see the arm bouncing above the mount itself. the mount is stationary. the factory aluminum mount is actually bonded to the rubber part. what it looks like is they are now unbonded. your driver mount is broken. put a jack with a wood under your oil pan and lift the motor. loosen or remove the nut on the driver mount on the bottom of the subframe. you will likely be able to freely move the rubber part. if so then 99% becomes 100.

DaleClark

iTrader: (55)

Yeah that driver's side motor mount is shot. I have NEVER seen an aluminum arm mount that was good, they all go to hell.

That's the first step, fix that.

Dale

FDAUTO

iTrader: (4)

the attention to detail that mazda gave for the decision on the 2 different mounts is absolutely incredible. outside of showing the problem, that video is a really good demonstration on the engine mounts working. i want to say this idea didn't age well but i feel like it did considering we got ~25-30 years out of it.

obeeone420

Awesome guys!! Thanks for the input. Motor mounts will be the next step before putting the car back on the ground. Sure beats me thinking that something was coming loose in the transmission.

FDAUTO

iTrader: (4)

keep in mind, with removing that motor mount arm you will need to reseal the whole pan or it WILL leak... worse than it already is. go ahead and get an oil pan brace as well. banzai sells the mounts and the brace

TwinCharged RX7

iTrader: (14)

Why will it leak if it's not leaking already? Wouldn't it just stay sealed as long as you don't disrupt the pan while changing the motor mount?

FDAUTO

iTrader: (4)

The seal is disturbed with the removal of the mount. The clamping force of the mount, the natural tendency of the metal to change shape when a force is removed, the age of the rtv, the fact the seal is already compromised, the working time and effort to remove the old rtv from the motor mount holes and ultimately the REsetting of the pan flange when the new mount is installed. All of these factors contribute to a complete failure of the seal. You can expand on each of these factors as well.

You can see this in reverse when installing an oil pan. Notice how the seal continously changes shape as the hardware is tightened. Then after the mounts are installed, because of the large footprint, there's a large change in the seal disproportionate to what all the 10mm do. This effect happens with and without a brace. Don't.... but after tightening the pan, try loosening a few of the bolts and see what happens.

Is there a possibility you can do this without compromising the seal.... maybe. The odds are definitely low on that. The correct way to do this would be to pull the pan or even replace it entirely and redo the seal. Keep in mind those oil pan flanges have a number of uses to them where they just won't seal no matter what. How they are removed plays a big role in that.

TwinCharged RX7

iTrader: (14)

Thanks good to know.

Why would you say about this "hypothetical" situation. Let's say I have a rebuilt engine, came with a new oil pan installed. They forgot to install the brace, so I carefully undid the fasteners, added the brace, and bolted back down. Engine is not in the car yet. Being fresh sealant and never run, think it will be fine?

FDAUTO

iTrader: (4)

Negative.... tried that twice with a new mazda motor, same result both times. Had to do it 2 times just to confirm the suspicion. With how wet the rtv is on those new motors, I wanted to see just what would happen, maybe I would get lucky and save a few hours.

NOPE!!!

Ended up spending more time because the subframes had to come down on completed cars AND i had to warranty the work lol so, definitely not doing that again

TwinCharged RX7

iTrader: (14)

Thanks. Glad I saw this.

obeeone420

Sealing the oil pan was going to be another winter project this year, but will go along with the motor mount when I replace it. I did see that it's something that needs to be done as I was reading about replacing that mount. Just like everything else on this car whenever I touch one item 3 more need to be fixed. Haha.