Waving idle speed in BMW – what is the cause and how to fix it?

BMW driver question

When the suction is turned off, the engine at idle runs erratically – wavers. I do not know how to deal with this problem. The valves have been adjusted, I replaced the lambda probe, spark plugs and ignition cables. I have cleaned the dome of the ignition apparatus, stepper motor and purged the air flow meter with air. Please advise.

The problem was with a 1989 BMW 320i E30, but it may also occur in other models.

BMW 3 E30 | By OSX (Own work) [Public domain], via Wikimedia Commons

Response from BM Cars service

A very common fault that causes idle speed ripples in gasoline engines is a leaking engine air supply system. In BMW engines in both the E30 and its successor E36 and subsequent models, the air ducts were made of plastic. Particularly vulnerable to breakage and damage was the piece connecting behind the flow meter with its characteristic “accordion”.

After many years of operation in harsh temperature conditions, the elasticity of this element rapidly decreased, resulting in the formation of micro-cracks and gaps supplying the so-called “left air. A very common cause of damage to this element is also alterations during the installation of LPG supply.

We recommend doing a thorough diagnosis of the power system and other engine components.

BMW 3 E36 | By OSX (Own work) [Public domain], via Wikimedia Commons

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BMW 3 E30 | By Bajuvar (Own work)[GFDL or CC-BY-SA-3.0], via Wikimedia Commons

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