Captain_Slow

After installing new Volvo mounts I had a rubbing/grinding vibration when accelerating and turning right. Initially, it happened immediately after start up and driving off and making right turns. After things warmed up I could make it happen to a lesser degree in a straight line if I made the car downshift.

I noticed my transmission mounts seemed flat and the differential had contacted the cross member (now it appears this was only due to going from going in and out of reverse). So I changed the trans mounts and expected no more grinding/vibration.

At first it seemed to be gone. It didn't happen for a few miles. But then it returned after the car was warm. I have some thoughts about this, but...not now.

I returned to jacking up the hooks to bend them (2nd time applying Stan's technique) and prying. Seemed to be plenty of clearance.

So I thought it's time to think outside the box. I started searching and I read two things I hadn't checked. 1) The transmission side-to-side clearance between mounts and trans case, and shimming the trans mounts, and 2) exhaust rubbing.

The trans mounts clearance with the trans case was good. There were no shims present when I removed the original mounts. So I moved on to checking the exhaust system.

I found nothing TOUCHING from front to rear of the exhaust. I was looking for interference caused by the Volvo mounts lifting the exhaust.

Then remembering the noise only happens when turning, accelerating, and after the driveline and mounts are warm... I tried to find places that looked like parts had come together, rubbed, then separated to rest position.

BINGO! The driver's side clamp joining the CATS to the rear pipes is the cause. Specifically, it's the fact that the clamp bolt is at the 10:00-11:00 position and was lifted into NEAR contact with the heat shield. The combinations of twisting forces during right turns and accelerating lifted the part of the clamp where the bolt passes into to contact with the shield.

Moral of the story...keep an open mind. I spent a lot of time and effort separating safety hooks that weren't rubbing. I noticed very few who've installed the Volvo mounts and adjusted the hooks were having ongoing vibration due to the hooks touching...and this lead me to take a fresh look at the problem.

I sprayed black spray paint on the heat shield where it appeared the clamp was scoring the shield. Took the car for a nice ride through all the twists that cause the vibration. The result is obvious...







The clamp is welded in place..so maybe the best solution is to use this as an excuse to get an X-pipe and new cats.

SMTCapeCod

Wow, there's several mm of flex there?
The heat shield is pretty malleable, can you press it out of the way or is it already against the floor?
No, wait, nevermind of course the Xpipe is the way to go...
;-)

granprixweiss928

I need to check that on my 88 project car. Fresh mounts I have and still some vibration. I have not bent the hooks

Captain_Slow

Steve - heat shield is up against the floor in that spot. Lots of twist in the chassis I guess. It could be that the driveline is staying relatively fixed position, and the body is flexing around it...bringing the shield down, rather than the pipe and clamps up.

gpw - Do apply Stan's technique to the hooks first. It is necessary and it does work in a very controlled manner. Won't mangle the hooks nor remove paint.

OTR18WHEELER

so, the new mounts reduced the clearance between the heat sheilds and exhaust pipes. I would just try to manipulate the affected area of the heat shield out of the way with a punch and mattot, untill the xpipe arrives. Great detective work.

Captain_Slow

Aside from the way the clamp is positioned (10:00-11:00 O'clock) I think the rubbing is the result of normal swaying of the exhaust as it is suspended from rubber hangers. I think it's mostly an inertial effect as the car accelerates in a turn (meaning there are NOT TWO, but THREE accelerations) and massive rear exhaust parts wanting to continue in a straight line. Note the clamp on the right side is in the 3:00 O'clock position and does not touch when turning left. This only happens when turning right.

I need to add that the new mounts allow the engine to rock (desirable) while the old mounts did not. When revving the engine I noted the engine rocks to the right..meaning the right side of the engine goes down and the left side (as viewed from driver's seat) goes up. Therefore, revving the engine causes a rotational acceleration (#1) that causes the rocking motion that LIFTS the exhaust on the left (driver's) side. Simultaneously, turning the steering wheel to make a right turn creates an acceleration (#2) and the exhaust "swings" left as its mass wants to continue going straight. And, I'm mashing the gas (acceleration #3) which essentially adds to the effect of acceleration #2. All combined are bringing that clamp up and to the left where the paint rubbed off.

So why, after installing new mounts, doesn't this happen until warmed up? I think it's pretty simple. The rubber mounts stiff until warmed up. Same could apply to the rubber exhaust hangers. The interference starts after the rubber is warm and stretchy/squishy.

Bertrand Daoust

Great find Jon.

If you want, I think that you could drop the exhaust, cut it in two near the clamp and re weld it at the right place.
Not the ideal solution but quite easy to do.

Also, maybe the engine will lower down a bit more with the time and everything will be good.
Maybe.

Or like you said, install new X pipe and cats and enjoy it!

Captain_Slow

Thanks Bertrand. I agree the simple thing to do would be go to an exhaust shop. The problem is how the part of the clamp with the bolt is positioned. An exhaust shop could easily rotate the clamp counterclockwise. The weld isn't joining the cats and rear pipes. Seems just the clamps are welded at the end of the rear pipes. Hard to tell. The car had an X-pipe installed for most of it's life with the second owner. He bought the car with 25K miles and removed it almost immediately. The cats have about 30K miles on them.

Captain_Slow

Yesterday I had the 928 up on LiftBars (which I like more and more with each use) doing a pre show check. Going to the Country Roads Sports Car Club car show up at Summit Point today.
http://www.countryroadscc.com/event/...pring-Car-Show

So I'm looking up at the spot where the exhaust clamp has worn away the paint and continued to cause vibration...thinking how annoying it is...wishing it wasn't welded in place so I could simply loosen it and rotate it. Then, just for a split second my flashlight glanced across the "weld" at an angle and I saw something..."Is that a finger print?...a finger print in the weld??" Instantly I realized it's just muffler patch putty! Looks just like the surrounding rusted patina of the pipes. Loosened the clamp, chipped away a bit of the putty and retightened the clamp. It is so nice to have an absolutely quiet and vibration free car again Off to the show...

Bertrand Daoust

Have fun at the car show!

Crumpler

Hey Captain, I recently installed the Volvo mounts as well on my 86.5.
Luckily, I had no issues afterwards(well let's take that back, I did have a coolant leak from the a hose shifting around to the new height).
I had it in my mind to check for any rubbing afterwards but none found. I bet I avoided all that simply by having an X pipe on already as you alluded to.
Good reason to spend some more money my friend!

17prospective buyer

Pounding heat shields out of the way, come on, we aren't hacks. Or rather, this car desserves to not be treated like a run of the mill car.

fraggle

I did a bit of pounding and wedging when I installed the X-pipe and 200cell cats on mine. seems to quiet for now, and I'm also running the volvo mounts.

SeanR

Did you miss the welded clustermess on his clamps? He doesn't have much of a choice.

the flyin' scotsman

this thread is so interesting.........after rebuilding the TT and clutch I had a noise when turning right at speed........traced to a heat shield being hit by the O2 sensor........all fixed now