emission control causing rough idle?

slantsixdan said

Interesting test...since rich mixtures reduce the tendency to ping. Think about this logically for a moment: the idea was to clean up the exhaust. Cleaning up NOx emissions meant reducing peak combustion chamber temperature. This was done with EGR, which dilutes the air/fuel mixture -- not just the air supply, not just the fuel supply -- headed into the cylinders. If you find that logic kind of hard to follow, then go look up the jetting details of carbs used with vs. without EGR. They are in almost every case the same. There is no rejet required and the engine will not run rich by disabling the EGR.

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My understanging is that the H2O in the exhaust is what helps quench the combustion temps...that is why water injection is so effective in reducing pinging problems. The carb jets are the same in most cases. The EGR doesn't change the air/fuel ratio in the carb....it slightly leans it after the carb only when the valve is open. The only evidence I have for this is the readings I've seen on the smog machine. When the EGR was not working, the HC's read higher than when it was. Not a lot, mind you, but it was measureable. I don't run a smog testing station but a friend of mine does and the tests were done in his shop on vehicles I've had in for testing. A limited sample I'm sure and there may be other engine combos that work differently but I would think the principles would be the same for most vehicles. Any smog guys out there that can refute or verify this info?