Rhythmic vibration/resonance under light acceleration

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Dantor
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Rhythmic vibration/resonance under light acceleration

Post by Dantor » Wed Aug 10, 2016 6:58 pm

My Z4 (35i with DKG) produces very strong vibrations under acceleration or driving up-hill. This only happens with a low RPM of the engine, so for example driving 90 km/h on the highway and then accelerating to 110 km/h on the cruise control.

The vibrations are rhythmic and cause a strong buzz throughout the car (comparable to driving over road markings while changing lanes on a highway). The vibration only happens during light/medium acceleration or driving up-hill on the cruise control, mostly when in the D7 gear. It immediately disappears if I shift down manually or accelerate harder to cause a down-shift. There are no vibrations when driving under normal circumstances (e.g. city traffic or cruise control on the highway).

What do you think it could be? I've had the hardy-disc (flex disc) checked visually and looks fine. Also no errors when reading out the diagnostic log of the car. Further, the rear tires are almost gone but still have about 2-3mm left. I've been to a BMW shop and they did a test drive, confirmed the problem, but have no immediate idea. Before spending way too much on diagnosing, I'd like to see if someone here has an idea.

Could it be:
- worn clutch plates of the DKG?
- wheel bearings?
- rear diff?
- something inside the gearbox itself (as it's only in the 7th gear)?
- bearings of the drife shaft?
- worn rear tires? (but wouldn't that then be a constant vibration, not just when accelerating lightly?)

Many thanks for your expertise here!

Cheers,

Maniac
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Rhythmic vibration/resonance under light acceleration

Post by Maniac » Wed Aug 10, 2016 7:44 pm

Imho if this is only 7th gear related it's an issue with the gear. Not clutches or anything else. That's an IMHO as I'm not able to test the car.

Take it to a specialist for diagnostics.

Dantor
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Rhythmic vibration/resonance under light acceleration

Post by Dantor » Wed Aug 10, 2016 8:07 pm

Hi Maniac, thanks a lot for the reply. I've been to a BMW dealer and they did a test drive, confirmed the problem, but have no immediate idea what it could be. They have proposed to keep the car and take it 'home' with them so they can drive it for an extended time and try to isolate the vibrations. I'm guessing this will be an expensive quest to find the problem and start (randomly?) replacing parts... :cry:
That's why I'm trying to gather some ideas before hand and see if the problem can be scoped down based on other Z4 owners' experience. Thanks again! :thumbsup:

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Rhythmic vibration/resonance under light acceleration

Post by Maniac » Wed Aug 10, 2016 8:31 pm

Don't let them fiddle with the DCT, if they think it's that then have the entire thing replaced.

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Rhythmic vibration/resonance under light acceleration

Post by Ozie » Wed Nov 22, 2017 1:02 pm

I got this also (like everything else). Does anyone know what seems to be the problem?

I have HPFP and injectors replaced in last 6 months.

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R.E92
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Rhythmic vibration/resonance under light acceleration

Post by R.E92 » Wed Nov 22, 2017 1:20 pm

It's because you are putting huge strain on the engine. High load, low rpm and a high gear is the worst thing you can do.

It wouldn't expect that to happen on a stock 35i though, unless your car has been tuned?
If the car is stock then I would also be tempted to investigate the ignition system. A bad spark plug or coil would cause issues like that.

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Rhythmic vibration/resonance under light acceleration

Post by Ozie » Wed Nov 22, 2017 2:10 pm

R.E92 wrote:
Wed Nov 22, 2017 1:20 pm
It's because you are putting huge strain on the engine. High load, low rpm and a high gear is the worst thing you can do.

It wouldn't expect that to happen on a stock 35i though, unless your car has been tuned?
If the car is stock then I would also be tempted to investigate the ignition system. A bad spark plug or coil would cause issues like that.
It's stock 2010 35is.

Well coils haven't (yet) been replaced.

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Rhythmic vibration/resonance under light acceleration

Post by Maniac » Wed Nov 22, 2017 9:17 pm

R.E92 wrote:
Wed Nov 22, 2017 1:20 pm
It's because you are putting huge strain on the engine. High load, low rpm and a high gear is the worst thing you can do.

It wouldn't expect that to happen on a stock 35i though, unless your car has been tuned?
If the car is stock then I would also be tempted to investigate the ignition system. A bad spark plug or coil would cause issues like that.
A bad spark could cause rhythmic vibration in 7th gear only?

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Rhythmic vibration/resonance under light acceleration

Post by R.E92 » Wed Nov 22, 2017 11:19 pm

Maniac wrote:
Wed Nov 22, 2017 9:17 pm
R.E92 wrote:
Wed Nov 22, 2017 1:20 pm
It's because you are putting huge strain on the engine. High load, low rpm and a high gear is the worst thing you can do.

It wouldn't expect that to happen on a stock 35i though, unless your car has been tuned?
If the car is stock then I would also be tempted to investigate the ignition system. A bad spark plug or coil would cause issues like that.
A bad spark could cause rhythmic vibration in 7th gear only?
I've always found that any weakness in the ignition system will show itself at high load and low RPMs. That's putting maximum stress on the engine.

The rhythmic vibration could be the engine retarding timing after sensing minor knock in a cylinder. The description of a buzzing or vibrating sensation fits with what I've experienced in the past when I've set the car into manual and floored it while in 7th at very low revs.

Using a tuning tool to take a log of the engine parameters would be the most helpful as you should see the ignition timing take a huge dump if it detects a problem.

On any other car i'd be looking at the drivetrain. Something like the DMF going bad. But with the N54 you have the big 3, Injectors/Plugs/Coils, that cause 90% of the issues.

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Rhythmic vibration/resonance under light acceleration

Post by mcbutler » Thu Nov 23, 2017 4:48 pm

Dantor wrote:
Wed Aug 10, 2016 6:58 pm
My Z4 (35i with DKG) produces very strong vibrations under acceleration or driving up-hill. This only happens with a low RPM of the engine, so for example driving 90 km/h on the highway and then accelerating to 110 km/h on the cruise control.

The vibrations are rhythmic and cause a strong buzz throughout the car (comparable to driving over road markings while changing lanes on a highway). The vibration only happens during light/medium acceleration or driving up-hill on the cruise control, mostly when in the D7 gear. It immediately disappears if I shift down manually or accelerate harder to cause a down-shift. There are no vibrations when driving under normal circumstances (e.g. city traffic or cruise control on the highway).

What do you think it could be? I've had the hardy-disc (flex disc) checked visually and looks fine. Also no errors when reading out the diagnostic log of the car. Further, the rear tires are almost gone but still have about 2-3mm left. I've been to a BMW shop and they did a test drive, confirmed the problem, but have no immediate idea. Before spending way too much on diagnosing, I'd like to see if someone here has an idea.

Could it be:
- worn clutch plates of the DKG?
- wheel bearings?
- rear diff?
- something inside the gearbox itself (as it's only in the 7th gear)?
- bearings of the drife shaft?
- worn rear tires? (but wouldn't that then be a constant vibration, not just when accelerating lightly?)

Many thanks for your expertise here!

Cheers,
Can you upload an audio recording of the problem?
Go for simple things first, check your tyres are all good, wheels not warped. I have a ford focus (barf) for work, I had a new cheapo tyre fitted and had these symptoms. as the tyre was twisted on the rim!

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Rhythmic vibration/resonance under light acceleration

Post by patriot66 » Mon Dec 11, 2017 9:52 pm

Sounds like harmonic resonance through the chassis of the car because the engine/transmission are under heavy load
That's all. It's the cars way of saying "Please change down so my revs rise and I can spin more freely!" :thumbsup:
'03 Sterling Grey 2.5i SE / SMG / Matching Hardtop / Gloss Black MV3s / Bilstein B4s / DLV Backbox / Carbon-wrapped Interior Trim / Re-Covered Steering Wheel / De-tango'd !

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