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My '90 E30 with the six cylinder M20 engine idles unevenly, especially when the engine is cold. A local mechanic said the mixture seems to be too fat but had no concrete solution saying I have to live with it.

But I think a too fat mixture kills the catalytic converter over time.

Is the mixture being fat the only reason for an uneven idle? Couldn't it be a leak on the intake?

How would you diagnose such a problem?

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It's most likely a bad engine temperature sensor. (common problem)

When the car is cold the computer uses a cold start fuel map. If the engine temperature sensor is bad the computer won't know the car is cold and will not activate the cold start fuel map.

More information: http://blog.bavauto.com/8178/bmw-e30-e28-engine-temperature-sensor-m20-engine/

It think it's the sensor under the blue connector in this picture. http://farm4.staticflickr.com/3308/3600906900_1b748f09da.jpg ...but I'm not 100% sure, I haven't worked on an M20 in quite a while.

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I've changed a couple of vacuum hoses this weekend and now I think it's better. I'll change the temp sensor when I get the chance and see if it'll fix it. –  Engin Kurutepe Apr 23 '12 at 8:19
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