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exhaust leak

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does an exhaust leak cause a lean condition ?? { the WBS at the tail pipe )

yes it can do, also at low exhaust gas speeds you'll get false lean readings

To elaborate on what Chris said, i will explain the reason. What you get is a venturi effect. As the high pressure exhaust stream passes the leak zone, low pressure air (outside air) is drawn in. This of course, will introduce extra oxygen into the exhaust stream, making it appear leaner than it is.

Hope that helped!

thanks a lot my friends

but the physics said the pressure moves from high to low not in opposite way

You are exactly right, that is true.

However! Think of exhaust gas as being a series of pulses, rather than steady flow.

Between each pulse is an area of vaccum that can be lower than atmospheric pressure, so the air can suck in during this point rather than blowing out of the hole.

The actual pressure in the exhaust manifold during low load/rpm can be only very marginally above atmospheric pressure (less than 1 psi) if it's a free flowing design. So it's very much susceptible to the peaks and troughs of the exhaust pulses.

From my own experience I can confirm that an exhaust leak can definitely affect the lambda reading. This is normally more noticeable at low rpm/low exhaust mass flow than it is at WOT operation though. Regardless, if you have an exhaust leak that you know about, it's advisable to fix it before commencing tuning.

DavidV: Thats actually an even better description. I always viewed it as a stream, but you are absolutely right, there are pulses and reversions. THANKS for that! I love learning new takes on stuff!