EGR stands for Exhaust Gas Recirculation valve.
It allows exhaust gas through the intake passages to enter the intake manifold chamber. This mixes some exhaust gas with the intake air fuel mixture.
Exhaust gas has been burned. It cannot burn again. You can't light ashes. Nor is it oxygen or fuel. Due to these facts, it will not affect the air fuel ratio (A/F). But it will dilute it. This means instead of having 100 % of what is in the cylinder fresh air and fuel, it may be 90, 80 or even 70%, with the remainder being inert gas.
This has the effect of cooling the combustion in the cylinder. The target is to keep the cylinder combustion temp below 2500 deg F. Above that temp, the engine will create a lot of NOx emissions ( worse for you than Carbon Monoxide ) and it creates preignition and ping. Very bad for us and for the engine.
Cooling the mix prevents both of these. We don't need or want EGR at idle. There is too little air in the cylinder, and if we dilute it the car will idle poorly, if at all. The cylinder does not make enough heat to be a problem at idle.
At part load, part throttle, the cylinder does get quite hot, and needs the cooling. So EGR is allowed. At full throttle adding EGR would slow the car down. Less fresh air and fuel will result in less power. So no EGR at or near WOT (Wide Open Throttle) Under hard acelleration conditions, the fuel mix is richened up to create maximum power. Rich mixtures run cooler, so the EGR is not needed.
Lean mixtures run hotter, so an under jetted carb will create pinging. Over jetting can foul plugs and at best, cut fuel mileage.
Anything that involves combustion will be a factor in cylinder combustion temp. More timing will increase combustion temp, retarted timing will be cooler. Aluminum cylinder heads can operate more timing due to the fact that heat transfer is improved, and thus the cylinder can run cooler. There are many factors. High octane fuel burns cooler (yes, cooler) then low octane, thus preventing ping. Cooler burning high octane burns for a longer period of time, and although cooler, creates more heat due to increased burn time. Temp and heat are two different things.
EGR can be checked in a couple of ways. The large EGR valves on our cars have access to the diaphram on the bottom of the large section of the valve. First at idle, reach under it and push the diaphram up. the car should bog, idle rough, or perhaps die. If it does this the passages are clear. If not the passages are clogged.
Now with still reaching under and feeling the diaphram, rev the warmed up engine. The valve should lift up. If it passed both of these test, all is functioning and okay. If it bogged when you pushed up, but does not when you rev the engine, pull the vacuum line and check for vacuum when the engine is revved up. If there is vacuum, but the valve did not respond, the valve is bad. If there is no vacuum you have a control problem.
Control comes from a ported vacuum on the carb. This should not have vacuum at idle but will with part throttle. Full throttle makes the vacuum drop to nothing. Don't try to go full throttle. That could damage the engine with no load. If vacuum responds properly, follow the line to a Thermo Vacumm Valve on the intake manifold. This prevents vacuum from reaching the EGR when the engine is cold. When warmed up, the valve opens and allows EGR.
On the "Tail of the Dragon"
(some day, no strike that, October 2008 it turned red, still in progress!)