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I've been reading a few of the questions here just to learn a bit more about cars out of curiosity and one thing I see quite a bit is the word "surging" I searched and I'm not seeing any straight and clear definition of what surging is and how to tell when a car is surging? Can someone explain this to me?

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There is also valve spring surge, at high rpms, but that is a different matter. – ja72 Jul 14 '11 at 22:47
up vote 7 down vote accepted

Surging is when the engine revs change noticeably under constant load and constant throttle (which in theory should result in constant RPM). A very typical one is my Eagle Talon, that had a vacuum leak and at idle the revs would vary between 500 and 1500 rpm as the ECU was trying to control the idle, but didn't know about the additional air.

Another typical one is that if you have either multiple carburetors or multiple throttle bodies on an engine and they are out of sync so one of the cylinders gets a little more air than the other one(s). You can feel it as if the car was accelerating and decelerating a little as you go down the road.

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