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JFairman JFairman is offline
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Join Date: Jul 2006
Location: S. Florida
Posts: 7,274
"An opinion, or possibly fact, that idle surge is caused by a rich condition. A vacuum leak creates a lean condition at idle correct? Its unmetered air, which equals less fuel. How does that hold up to the rich surge theory?"

A rich idle mixture in a fuel injected car will always cause the rpms to oscillate up and down and thats a fact. I used to work on German cars for a living and I've seen this with CIS or K-jetronic, L-jetronic, Motronic, and the kugelfisher mechanical fuel injection used on the old BMW 2002 tii's
I have never experienced this with a carburator running rich at idle though.

The answer to the other question is more complicated.
All air leaks in a turbo cars intake system are often called vacuum leaks even though some are actually air pressure leaks because it's easier to group them all together in conversation without getting overly techanical and people that are familiar and experienced with turbo cars understand the differences.

An air leak below the throttle butterfly will be a vacuum leak of unmetered air at all times and cause a lean situation unless the turbo is boosting and the throttle is open far enough to let in more boosted air pressure than the intake ports and cylinders can swallow, then it will cause a rich situation because metered air is leaking out to atmosphere while metered fuel is being injected into the intake ports.

An air leak after the turbo and before the throttle butterfly, or blow off valve will be a pressure leak at all times other than when you first snap the throttle open before the turbo spools up.
The turbo is making pressure in the intake before the throttle body at idle and steady cruise, it's just low enough to not show on the boost gauge.

This is metered air so if any is leaking out between the turbo and the throttle body it will cause a rich mixture.

Have you tried lowering boost control pressure with your Brian Leask adjustable warmup/control pressure regulator yet?
Old 10-23-2009, 12:38 PM
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