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I do recall that post from last year, and indeed it was very informative. But in that case, the NOx was over the limit at low speed (above idle for sure, but maybe 1500 rpm max) but *not* at 25 mph (where rpm may have been marginally higher due to being in a higher gear, but clearly with larger throttle opening and attendantly higher gas flow and gas temperature).
Again, my point is that at higher rpm and throttle openings the exhaust path is full of high-energy gas trying to find its way to atmosphere, and it just doesn't seem likely that air at ambient pressure would be drawn into that stream (barring any special venturi gas path geometry). [Stream of consciousness thought as I write this... pipe geometry may play a part in this.]
In the case at hand, we don’t know what the situation is at idle since NOx was apparently not measured, but at road speed the NOx is off the chart. For thoroughness, it would certainly be prudent to check the complete gas path upstream of the cat for leaks in light of your well-documented discovery of last year. But I would really be surprised if an exhaust path leak was the cause of the high NOx. Let’s hope the cause is identified and “2volvos” posts the findings.
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Thank goodness we don't get all the government we pay for. -- Will Rogers
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