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Old 12-04-2007, 08:58 AM   #1 (permalink)
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Default Idle shake

Hi new here and new to Subaru.

My car seems to shake at cold idle. The main dealer say it is due to load on the engine but in winter what do they expect. They have tweaked it up once already but it still shakes. It does not stall which is just as well since that would be a real pain in an auto.

Mine is a 2.0lt X all whether auto I undertsand that shake is caused by the engine revs being too low and probaly exciting a natural frequency. What I realy want to know is if it is normal or should I keep nagging the main dealer? Its on warrenty for some time still.

Appart form that the Forester is real fun. Having had a Vectra before I now know what handling means

Thanks,

Tim
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Old 12-04-2007, 09:02 AM   #2 (permalink)
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Mine shakes at warm idle...and yes, it is quite annoying. I am having it looked at now... cold idle is ok tho'
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Old 12-04-2007, 09:05 AM   #3 (permalink)
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Forgot to mention mine is very slightly modified.
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Old 12-04-2007, 09:10 AM   #4 (permalink)
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A flat four engine naturally creates a tuning moment at idel due to its configuration, this causes the shake (front of the engine moving left and right), lower idle speed is used for better fuel economy (aka CO2 emissions)....all Subaru's shake to a certain extent, and if the car is as designed there isn't a fat lot your dealer can do about it, most hot idle speeds avoid resonant frequancies, but its not always possible cold (when the mounts will be stiffer as well).

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Old 12-04-2007, 09:26 AM   #5 (permalink)
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At what idle speed does this occur?
When mine is warm , it stutters occasionally at 6/700 rpm, but a fast idle when cold is usually pretty smooth.
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Old 12-04-2007, 09:31 AM   #6 (permalink)
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Yeah, both my '04 and my '08 got the shakes. I noticed a good grounding kit helped a little on the '04 but didn't rectify the situation.
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Old 12-04-2007, 09:37 AM   #7 (permalink)
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A stutter at low speed is a misfire, so that could be anything....I'm talking of the consistant 'wobble' driven by the engines reciprocating masses....

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Old 12-04-2007, 10:16 AM   #8 (permalink)
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Quote:
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'wobble'

Simon
period of natural oscillation
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Old 12-04-2007, 11:56 AM   #9 (permalink)
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Forgot to mention mine is very slightly modified.
Very Droll Marky, "Slightly Modified" very good!

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Old 12-04-2007, 01:44 PM   #10 (permalink)
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Default Thanks

Thanks to all who have replied and so quickly.

Its fine when it fully warmed up. It sounds like it's a 'design charateristic'.

It is not a missfire just a natural resonance.

I suspect Subaru will fix it on later models since all it would need is some simple design adjustments using a little understanding of modal alignment.

But as I said... so far I am well impressed by Subaru's.

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Old 12-04-2007, 10:36 PM   #11 (permalink)
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Very Droll Marky, "Slightly Modified" very good!

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Everything has a natural oscillation frequency, hit that frequency and it will 'wobble'
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Old 12-04-2007, 11:42 PM   #12 (permalink)
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It sure will, you can't fix the inherent free forces on an engine, all you can do is try and having them driving resonant frequancies.....

When I worked at Rover we found the steering wheels natural frequancy on the Rover200 (bubble shape) nearly coincided with the idle speed of (865 versus the idle speed of 875 so lots of complaints about an 'idle instability' as it kept shaking and stopping at idle)...not good so the 25 had a large lump of cast iron bolted to the column to raise it to the equivalent of 1500rpm (so above fast idle and below where most cars are driven).

Simon
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Old 12-05-2007, 01:21 AM   #13 (permalink)
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mine shakes when its cold but only when facing down hill! when its warm its fine.
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Old 12-05-2007, 06:37 AM   #14 (permalink)
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Quote:
Originally Posted by The Rookie View Post
It sure will, you can't fix the inherent free forces on an engine, all you can do is try and having them driving resonant frequancies.....
Simon
You meant to say "not having them driving resonant frequencies".

I guess the problem is you would probally need to drop the body mode more below the idle. But the body mode is probally actually a suspension mode. Droping that would mean reducing the main spring stiffnesses which could then offect the ride and posibly handling. It's not easy to solve these problems but can be done with some good engineering. Maybe they have tried and this is the best compromise - who knows!

Alternativly change the idle speed - but as already said this will increase the fule usage - the last thing you need on one of these!
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