Re: EGR system question...
The purpose of the EGR Valve is to lower combustion temperature
Introducing a little exhaust gas into the cylinder spaces
the molecules apart slightly, lowering temperature from
slightly above 3000 degrees where oxygen welds itself to
nitrogen, forming oxides of nitrogen.
With EGR it's down around 2900 degrees, much less NOx formation.
Today's engines run very lean, raising the temperature,
making the EGR important for the cooling.
Often when the EGR is disconnected it'll cause ping, sometimes
you can hear it, sometimes not. Even when you can't hear it
that ping raises the temperatures even higher, burning valves,
If you can, richen the engine slightly, rejet the carb (adjusting
the idle mixture screws won't do it), or cause the injection
system to run a bit richer.
It also helps to set the timing lower by about 5 degrees from
stock, lowering the temps.
The old engines in the 70's the EGR was an add-on, but later
engines EGR was designed into them, so it's more difficult to
bypass it with safe results.
The reason city driving isn't as detrimental is it's not
held at a sustained high RPM for a long time - sooner or
later you enconter a traffic light or reason to slow down,
letting the combustion chamber cool.