Have a '79 RX-7, only 70,000 miles on it. Pretty good shape. But now experiencing vibration (whole car) at speeds of 75 mph or more.
If you're on the California freeways, you'll know why I'm driving 75 mph or more...
Anyway, I have a service scheduled at the Mazda dealer this week.
Will ask them to double-check the following:

- Check UV joints
- Check Driveshaft, mounting bolts (loose), bent driveshaft
- Check Rear axle – see if it’s bent
- Check Control / Rocker Arms
- Balance all 4 wheels – see if any bent
- Check rear and front wheel bearings

Is there anything else that could cause such vibration that I should get checked?

Also, if it's the driveshaft, from a cost-of-repair standpoint, would that be considered a terminal illness for a 1979 SA? (i.e. no transplant parts?)

 

List looks good, I would add engine and tranny mounts to it. For a wheel or the rear end to be bent, it would take an impact. One usually notices those incidents. The control arms and watts like will bend before the axle housing does.

Not sure if I understand your last question. Iirc, the u-joints should be replaceable in that early of a model. If not, a new driveshaft can be found for under 300.

 

That shaft is rebuildable.

 

Thanks Guys.

I just checked my car repair log.
In May 2005 I had all my UV joints replaced, my front tie rod ends replaced, and my bearings checked, and rear end checked (alignment).

No faults founds at that time.
And I have been driving this car like an early bird grandma on her way to church on Sunday.

It must be the dam drivetrain , which makes me worried in terms of expenditure.

But I love this car!!!

 

I had the same problem. I got my tires rebalanced and made sure they had proper air pressure afterwards. Problem solved.

 

Went to the dealer for tuneup + oil change:

- Checked UV joints = all OK
- Check Driveshaft, mounting bolts (loose), bent driveshaft = all OK
- Check Rear axle – see if it’s bent = OK
- Check Control / Rocker Arms = OK
- Check rear and front wheel bearings = OK
- Balance all 4 wheels – see if any bent = couldn't be balanced!

The mechanic told me that your wheels are "anything but round, especially the rear ones".

It's very likely that one or both rims are bent.

He used a new wheel balancer that also takes into consideration when the wheel is turning on a surface.

So I need new wheels.
Now one of the guys from Service Specialties, a wheel provider in Irvine, CA said he could source some 15 x 7 Konig Rewinds with 205/50/R15's Kumho for $895 all in.
Here's the thing - the wheels are with zero offset, 4 x 110 bolt pattern. He said that these would fit.

Is he full of s*it? I keep reading that zero offset will rub. But I have zero offset wheels now (13 x 5.5).

 

Unless something severe has happened since this issue started (crash, etc.), then I would guess that your wheels are fine. Either the tires are old and out of round due to sitting for long periods, a belt slipped, or you lost a balance weight. I would go by a tire shop and get a second opinion at least...

 

Vibration at 75 is almost always the rear U-joint. If the vibration goes away in 4th, then the front countershaft bearing is dying (the preferred failure mode for every trans I've had...) but the U-joint is much easier to deal with.

Tire related concerns usually show up in the 45-up range and get pregressively worse. I've never had a vibration come in from the rear, the weight of the axle seems to damp out any tire imbalance effects. Even when I had a 2-ounce weight fling off of one of the wheels, no vibeys. In the front, a quarter ounce will be detectable on a smooth enough road...

 

Rear UJoint and transmission input shafts are possibilities. But some guys have solved the problem simply by replacing the transmission support mount, which tends to deteriorate from stray oil.

 

got any friends with stock rims and tires you can borrow for a couple of hours? you can find out easily if the mechanic was right about your rims being bent or not without spending money. YOu can see if the vibration goes away. Good luck.

 

I would say the u joint to. But ty another set of wheels and see if it goes away.

 

I had a vibration before at 36mph and it ended up one wheel having a slight bent to it, had it rebalenced and now its hardly noticible, start from there

 

I had Vibrations at 60 and I went to a local tire shop and had my tires rebalanced. It fixed the problem and now goes 80+ and has no vibration at all. Check the tires first because it would probably be cheaper to get your tires rebalanced at a tire shop than at a dealer

 

what speed rateing are your tires? Wouldnt a lower grade distort at higher speeds, thus creating vibration?

 

It would but with the stock size our cars require, the speed rating isn't going to be that low. Once I got my tires rebalanced, I was able to go 80+ with no vibration.

Small tires, found on economy cars, will have very low speed ratings. Cars like ours and their very "hard to find" size nets them as performance tires.