| Hmm. Interesting. So then what would keep those same exhaust gasses from blowing back through exhaust valves when the piston finishes pushing the spent fuel out but the valves are still open...in fact open long enough that they overlap with the intake valves. Nothing does, and what you are referring to is called exhaust gas reversion. During valve overlap at the tail end of the exhaust stroke when the intake valves begin to open, the higher exhaust pressure WILL force exhaust gas back into the combustion chamber and even into the intake runner, if the pressure is great enough. This is also the reason why our engines have VTC, which allows us to retard the intake camshafts at the point where the reversion becomes detrimental to torque production. Can you give me some specific examples of where people were having richer mixtures due to a leaky EGR valve? I've personally never heard of such a case. Even so, I would link that back to some additional problem with the car. What I'm still confused about is how exhaust gasses entering the plenum through the EGR can lean out a mixture when the gasses are inert? What you aren't recognizing is that when the exhaust gases recirculate, the engine isn't breathing as much air through the throttlebody as some of its intake air is being fed through the EGR pipes that join to the underside of the plenum. Part of the exhaust air is being recirculated back into the plenum. This causes less air to be drawn in on the intake stroke through the throttlebody, which is connected to the MAS, which in turn, tells the ECU that there is less air coming in, which it in turn will lower the fuel delivery and advance ignition timing. I know of about a dozen cases so far from customer's of mine that have taken their cars in for california inspections without the EGR in place and none of them had issues passing either the test or the visual. Perhaps lucky, but then again, most of these techs apparently aren't doing their jobs 100%. But for cali folks, it might not be a bad idea to just put the EGR valve on the back of the engine so that if someone looks, it will be there, although completely disfunctional.
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