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Weird engine surge on overrun

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Joined: Sun Mar 26, 2017 9:03 pm
Location: Christchurch

Weird engine surge on overrun

Post by Rangi246 » Mon Jan 22, 2018 7:37 am

Hi all

I'm hoping someone can help me with a problem our car has developed. Its an 86 635csi auto and for some reason it surges on overrun but only when the a/c is on.

When you lift your foot off the gas, the revs drop to about 1,000 and then they pop back up to about 1400-1500. When the a/c is off the car does this once which I understand is normal.

When the a/c is switched on though, the car will complete this cycle up to 6 or 7 times before settling down. The effect is at a slow cruising speed, the car is pulsing along the road. If you put your foot on the gas or brake, it settles down. Its slowly getting worse as it was doing it about 3 times max but now its up to 6 or 7.

When the car has stopped it idles perfectly at about 650/700 rpm.

So far I've replaced the AFM and apart from the car running a bit better with a new unit, it hasn't made any difference to the surging when the a/c is on. Maybe the ICV?

Any thoughts?


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