Exhaust Leak Cause Lean or Rich?

Does it depend on the leak size and location or do pre O2 leaks generally create the same condition?

Does a manifold gasket leak cause the same condition as a leak at say a flex pipe?

I can't wrap my head around the theory of it, whether it's sucking air in or pushing g exhaust out and how that impacts the sensor reading.

Thanks

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If I understand the theory correctly, I believe it's not only pushing out exhaust gas, but also sucking in air. :silly: It's definitely not an intuitive concept.

To be honest, I very rarely see fuel trim issues caused by exhaust leaks. If I do, then it has to be on the right vehicle, and in the right location. The Nissan P2A00 code is a perfect example, from the newest Premium series:

www.scannerdanner.com/scannerdanner-prem...sensor-1-part-1.html

I've found that if an exhaust leak is gonna impact fuel trims, then it usually needs to be huge. Like F-150's with 5.4's - their exhaust manifold gaskets pop studs and leak all the time! :lol: But I've observed trims before and after, and fixing the leaks made little to no difference.

The theory - and reality - is that the cylinder is in a negative pressure state at the moment when the exhaust valve opens. This causes a backflow of exhaust gases into the cylinder from the exhaust system. If there was a leak present in the exhaust before the front O2 sensor then fresh air is sucked in and the O2 content of the exhaust gases is altered, making the O2 sensor see a leaner mixture than it actually is.

The true effect depends on a variety of factors like exhaust system design, flow characteristics, leak size. Often times I have not seen noticeable effects on trims with even considerable leaks while on fewer occasions pin hole leaks caused lean runnjng codes and positive trims. Not able to establish a rule for it....

To get an idea of how the pressure behaves/changes through a complete 4 stroke engine cycle have a look at in cylinder pressure wave forms, there is so much information into it, from timing over valve seating to general mechanical condition of the engine.

PRESSURE WAVEFORMS RULE!