vibration when accelerating

[obthomas]

My 03 Boxster S with 97000 miles and all new wheel bearings and suspension vibrates when accelerating. No CEL, engine runs as good as ever. After suspension replacement I got a vibration from 3rd gear to sixth gear when accelerating. If I back off the power and coast the vibration stops. If I maintain speed in any gear the vibration stops. It only vibrates when adding power and up around 3000 rpm. Both drive axles are new this last year. The transmission is smooth as silk. Last week I replaced the front motor mount with a new one. The old front motor mount seemed to be just as good as the new one. Anyway, it did not help. To me this is a drive line problem and all I can think of is to replace the transmission mounts and if that doesn't help then the dual mass flywheel. The clutch on this car feels like new. I simply do not know much about how a dual mass flywheel affects the car if it is failed or failing. I could use some advice or approval of my plan for where to go next. Thanks in advance


O.B. Thomas

[1PorscheBoxster986]

Did you do a wheel alignment and wheel balance after changing the suspension?

[obthomas]

Yes. I watched the alignment as it was done. My DIY equipment also shows that it is spot on.

[KRAM36]

Do you have a Durametric tool?

[DoninDel]

When did problem start?
After changing something, or changes are trying to cure the problem?

[Qingdao]

I know you wrote that you already replaced them, and I know its a pita to change them out... But CV axles are like the only thing I could think of.


I've never heard of a motor mount causing of vibration under accel. If the mount is bad under accel it's settled in and not shaking.

Also, never had a wheel balance issue caused by acceleration only. Those things worsen with speed. Same with a wheel bearing.


The cutch/flywheel might be, but I doubt it. Just because that would give the car a shake under an engine rev and not just acceleration.


Completely different engine and water pump design, but RX7 engines will shake on accel if the water pump is bad. They shake badly enough to make me think the drive shaft was out of balance.

[obthomas]

Some responses, yes I have a durametric tool and I am not afraid to use it. The vibration came after I refreshed the suspension earlier this month. I replaced the new right axle about a year ago so I put a few miles on it with no vibration. I put the new left axle on about two months ago so I had a few vibration free miles on it. Today I Jacked up the car there is literally no movement in the inner joints and none in the outer. Tested the new wheel bearings 3 and 9 oclock and 12 and 6 oclock and no movement whatsoever. Nevertheless, I too have come to the conclusion that it is probably one of the axles. My sense is that my problem will not be solved until I buy a new axle and start changing them out. Thanks everyone for the help.

[The Radium King]

where do you feel the vibration - seat or steering? rear suspension will squat a bit when loaded under accel - anything aftermarket in your suspension refresh? no-oem geometries (alignment, ride height, wheel offset)?

[obthomas]

Vibration is in the seat, firewall, car. The steering is solid and does not vibrate. Suspension parts are all OEM except struts are B6 not B4.
When I get to third gear about 50 MPH and give it gas I get what feels like an engine miss at 3000 rpm and goes away at 4000 rpm. I can do the same thing in 4th, 5th, and 6th. Of course the car is going faster in the higher gears. I have convinced myself its not an engine miss. No CEL.

[Qingdao]

So it dies off at different RPM ranges... now we have a clue.

I still think it could be CV axles, but if you are feeling it under rpm changes then its probably in the engine somewhere.

If it was me I'd find a quiet stretch of highway. Take the belt off and get it up to 80mph. See if your issues are gone.
^^just because that's easier than doing anything CV axle related.


I'd be willing to bet the reason you don't feel it in 1st through 4th is cause its such a low gearing.

[KRAM36]

Originally Posted by obthomas

Vibration is in the seat, firewall, car. The steering is solid and does not vibrate. Suspension parts are all OEM except struts are B6 not B4.
When I get to third gear about 50 MPH and give it gas I get what feels like an engine miss at 3000 rpm and goes away at 4000 rpm. I can do the same thing in 4th, 5th, and 6th. Of course the car is going faster in the higher gears. I have convinced myself its not an engine miss. No CEL.

Hook up the Durametric and take the car for a drive and check for cylinder misfires.

[The Radium King]

Originally Posted by KRAM36

Hook up the Durametric and take the car for a drive and check for cylinder misfires.

this - misfires don't always cel. if it feels like an engine miss then it may be a miss. also, try clutch in vs out of gear and load the engine (get it hot before hand, then go to wide open throttle) and see if you get the behavior without the transmission engaged.

[wtribeflyer]

Did the transmission mounts fix this issue? I have the exact same thing 2001 S. Definitely not an engine mis fire. Engine runs smooth as silk

[Uwe]

Ive got the same issue as well, with all new suspension, bearings, Bilsteins, trans mounts, engine mount. The car is far from smooth. I did get rid of the 3000rpm deceleration shake by doing all of this work, but ended up with a much worse condition.

[JZeke]

bumping this one as I also have this driveline vibration under acceleration but zero vibration revving the engine warm stationary.

also have a new front mount, new clutch put in when gearbox was taken out for a rebuild, new axle mounting hardware (but old axles). Alignment done last year. Diagonal control arms and end links are new but lower controls arms old. 79k 03 S. DMF was checked out when clutch replaced, they said it was good. also get a low speed clinking/rattle from driver side rear over rough roads at very low speed which may be related.

• transmission mounts
• DMF
• lower control arms
• axles

if the vibration is only present under accel and in all gears, what’s the proper way to diagnose the fix? Of these 4 potential culprits, which is more likely to cause these symptoms?

[theiceman]

wow lots of complaints of the issue , yet zero people following up with what actually cured their issue.. im sure lots of these are now fixed...

[obthomas]

I started this thread and can say that I never solved the problem until I put an LS3 in the car. About 3 years now and no vibrations.

[JZeke]

Originally Posted by obthomas

I started this thread and can say that I never solved the problem until I put an LS3 in the car. About 3 years now and no vibrations.

Did you keep the G86/20 transmission when you swapped in the LS3? How about the flywheel, is it a dual-mass one?

If so it would point to the "natural resonant frequency" of the M96 engine as the true culprit. Which would narrow the fixes to anything that could amplify frequencies like engine mount, trans mounts... even deteriorated rubber in the lower control arms. And of course the DMF.

[obthomas]

My car had all new sus,pension and still does. The flywheel, clutch, abd pressure plate are new and different. The transmission is the same. However, the vibration is gone. Still hard to pinpoint a probable cause because for a while I thought it might be in the exhaust system.

[JZeke]

Originally Posted by obthomas

My car had all new sus,pension and still does. The flywheel, clutch, abd pressure plate are new and different. The transmission is the same. However, the vibration is gone. Still hard to pinpoint a probable cause because for a while I thought it might be in the exhaust system.

seems like i need to kick myself for trusting that indy who said my DMF was fine. should’ve just got it replaced when they were in there…