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Old 11-16-2019, 06:04 PM   #1
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Rough Running Under Very Specific Conditions

Hi all,

I've owned a '98 986 for a bit over a year at this point as a daily driver, putting 20K+ miles on it in that time. Other than a water pump bearing, it's been very dependable and well-behaved. It does have a behavior that disturbs me a little, and with winter returning, it's back and rustling my jimmies.

Basically, any time the outside air temperature is cool, say, below 50F, and I have the car at partial throttle, under load, between 2000-2400rpm, the engine shudders and vibrates a ton. Like an old Type 4 trying to idle in the freezing cold level of vibration. Let off the gas, or floor it, put in the clutch, or move out of this rpm range, and it feels perfectly fine. Idle's fine, gets decent fuel economy, doesn't smell or act rich/lean.

I can't find anything online about anything quite like this. Has anyone else experienced it? I'm wondering if it's an anti-knock issue, or maybe just plugs that are in need of replacement, or something like that.

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Old 11-17-2019, 02:33 AM   #2
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Sounds like a bad motor mount, if the RPM is stable. They tend to be most noticeable in the 2-3k RPM range, and the cold weather would harden the rubber, accentuating the issue when it's cold.
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Old 11-17-2019, 05:59 PM   #3
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Originally Posted by rick3000 View Post
Sounds like a bad motor mount, if the RPM is stable. They tend to be most noticeable in the 2-3k RPM range, and the cold weather would harden the rubber, accentuating the issue when it's cold.
Can also check the tranny mounts -- because they essentially act as the other side of the engine mounts.

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Old 11-19-2019, 02:17 PM   #4
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I think you guys are right. Opened up the engine bay today and revved it a bit, and pulled the car back and forth. The engine moves around quite a bit, and the mount bushings look pretty deteriorated. Guess it's time for those.
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Old 11-21-2019, 07:33 AM   #5
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Buy a new Bosch MAF Mass Air flow sensor. Install it to see if it runs better. If not just keep it you will need it someday.

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