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Intermittent Vibration at 100-120 km

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Intermittent Vibration at 100-120 km

Postby Oregato » Wed, 21 Dec 2011 9:06 +0000

Hey guys,

Car is going great so far, bar one little minor note.

On Friday evening we took a spin down to Wicklow, down the M50, all good until we went past Bray, from Bray to Enniskillen, the steering wheel started shaking quite badly, I thought it was the road surface but perhaps not.. When I got out of the car, I could smell a very strong smell of either brakes or tires..

Didn't think much of it at the time and after a lot more driving the problem didn't come back(circa 200kmls).. until last night, driving down the M50 between Blanch and Finglas, same virbration at around 100km and then when I got out of the car, same smell, seemed to be coming from the drivers side rear wheel.

Checked the tire pressures, left hand side, both at 29, back right 28, front right 30, I put them all up to 33psi. Took it for another run then from Finglas all the way out to Enniskerry and back to Firhouse, no problems or smell.

Anyone any ideas what this could be? I've read the threads on the 65mph shimmy and I guess you could call it that, but this has me stumped as it comes and goes as it pleases?

Any help would be great :) Thanks in advance.
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Re: Intermittent Vibration at 100-120 km

Postby eyore » Wed, 21 Dec 2011 9:47 +0000

Wonder if its anything to do with a sticky handbrake caliper. Next time it happens check if the car will easily roll/freewheel or does it feel stiff.
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Re: Intermittent Vibration at 100-120 km

Postby Oregato » Wed, 21 Dec 2011 10:50 +0000

Thanks for the reply Eyore, was thinking the same myself, but if it was sticky, would that mean it would be a constant problem? Where as what I have is very intermittent, most of the time, it's fine. Any ideas?
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Re: Intermittent Vibration at 100-120 km

Postby paulNA » Wed, 21 Dec 2011 11:24 +0000

something similar happend to me before, and it turned out to be the driveshaft, it was happening exactly as you described it for about 2weeks, untill one day it just went like, but i dont understand where the smell could come out of? unless its not related to the problem, but it seems it is....
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Re: Intermittent Vibration at 100-120 km

Postby Steve » Wed, 21 Dec 2011 11:39 +0000

Is it just happening in a certain speed range? All your wheel nuts tight?
Do you think it might be your engine firing on just three cylinders? If the coil pack is failing it'll most likely start arcing when engine is hot.
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Re: Intermittent Vibration at 100-120 km

Postby Oregato » Wed, 21 Dec 2011 11:41 +0000

Thanks for the reply Paul, when you say it went, what exactly happened??

I've done a lot of city driving with no smell, but the tires are new as are the brakes so the smell could be unrelated altogether, but somehow I doubt it is..

It's only happening at 100 - 120 kmls, and even then it's only happening for a short time.

I think the engine is running fine, I've done a lot of driving and it's been fine. What do you mean 'start arching when engine is hot'?

What's the best way of diagnosing the problem would you say?
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Re: Intermittent Vibration at 100-120 km

Postby paulNA » Wed, 21 Dec 2011 11:46 +0000

sorry, i should have been more clear :D yeah driveshaft just broke after two weeks, basically I was driving home one day, and there was big bang and whole car started shaking like if wheel was about to come loose,
it happened in my old honda, not the mx5, but symptoms sound the same....
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Re: Intermittent Vibration at 100-120 km

Postby paulNA » Wed, 21 Dec 2011 11:48 +0000

put the car on the lift, put it in gear, so back wheels start spinning and see for any ususal noises, vibrations, play etc...
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Re: Intermittent Vibration at 100-120 km

Postby Steve » Wed, 21 Dec 2011 12:06 +0000

"What's the best way of diagnosing the problem would you say?"

I had a look at the coil in the dark with engine running, could easily see the spark arcing out through a crack in the plastic.
No harm in pulling the wires from the coil and cleaning out the coil posts with a Q tip either while you are at it ;-)

But you say its speed related so not likely to be this.
New tyres? Very new? Start with a rebalance?
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Re: Intermittent Vibration at 100-120 km

Postby Oregato » Wed, 21 Dec 2011 12:12 +0000

The car was well maintained although the tyres are a cheaper brand. My plans are to change them anyway but I think the brakes and tires are very new, must have only been put on.

Does this sound crazy, but could there be two or more tyres that have a slight imbalance/wobble/whatever you want to call it and when they align in a certain pattern while driving this occurs? It doesn't feel down on power but when it started to shake I slowed it down a little just in case.

Like I said, it's only happened twice in the last week and twice was at speeds of 100 - 120 kmls.. I've done a hell of a lot of driving in it, even going from Finglas to Enniskerry last night with no problems at all!!
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Re: Intermittent Vibration at 100-120 km

Postby eyore » Wed, 21 Dec 2011 4:01 +0000

You generally don't really need to balance rear tyres as any imbalance isn't normally felt, unlike front imbalance, which you feel through the steering. The key to your problem is the smell, I think you need to do a bit of investigation around the rear wheels and brakes.
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Re: Intermittent Vibration at 100-120 km

Postby TOM 5252 » Wed, 21 Dec 2011 9:52 +0000

"I put them all up to 33psi" The correct tyre pressures for a mk1 or mk2 MX5 is 26psi. Some people in the know recomend 27psi.
So 33psi would be dangerous and will wear the centre of the tyres prematurely. Also since the weight of the car is being carried by the centre of the tyre face instead of over the full width of the wheel, this could contribute to a shimmy effect, like vibration. Still wouldn't explain the smell unless a tyre was actually being scrubbed against the road surface during the shimmying.
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Re: Intermittent Vibration at 100-120 km

Postby scallan » Wed, 21 Dec 2011 9:57 +0000

TOM 5252 wrote:"I put them all up to 33psi" The correct tyre pressures for a mk1 or mk2 MX5 is 26psi. Some people in the know recomend 27psi.
So 33psi would be dangerous and will wear the centre of the tyres prematurely. Also since the weight of the car is being carried by the centre of the tyre face instead of over the full width of the wheel, this could contribute to a shimmy effect, like vibration. Still wouldn't explain the smell unless a tyre was actually being scrubbed against the road surface during the shimmying.


wouldn be tire pressure anyway i run between 40 and 50psi in the rear and 26psi in the front without problems
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Re: Intermittent Vibration at 100-120 km

Postby TOM 5252 » Thu, 22 Dec 2011 12:47 +0000

It's the front that counts in terms of shimmy or judder.

Your pressure figures look fine for drifting or movie stunt driving, but for normal day to day driving I'd have thought they must be illegal considering that the manufacturers set figures are the same pressures ( 26psi ) on all four wheels. If those pressures were discovered after an accident, wouldn't your insurance become invalid.
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Re: Intermittent Vibration at 100-120 km

Postby Oregato » Thu, 22 Dec 2011 1:21 +0000

I'll set them back first thing tomorrow to 26, coming from other cars, I would have thought that 26 seemed a bit low, but if that's recommended for the mx5, that's fine.

In terms of after raising the psi to 33, it was prior to doing this that I experienced the shimmy, afterward I raised it and drove it for a while, I couldn't
replicate it.
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