Quote Originally Posted by KILNA View Post
So in summary the three states the EGR system can be in:

1. Normal operation... a small amount of exhaust is routed back into the system, as controlled by the computer, based on criteria to keep the temperature, pressure, fuel and oxygen at a sweet spot. Fuel economy is at an optimum, while not effecting performance.

2. Exhaust gasses blocked... the normal amount of exhaust cannot be routed back to the engine for one reason or another... blocked EGR tubes, EGR valve failed in closed position, or someone intentionally blocked the EGR. The fuel, instead of burning smoothly over the entire stroke of the piston, will burn quicker at the beginning because of the extra oxygen. Early burn also means that, because fuel is injected over the course of the stroke, not all of the fuel will get burned (the later fuel is wasted). It means less efficient transfer of energy from the explosion in the piston in mechanical terms. The engine runs hotter because more of the energy is transferred thermally as opposed to mechanically. Performance is generally not effected much, but fuel economy is.

3. Too much exhaust gasses... if the valve is stuck in open position. This can produce misfires and partial burns. The engine will be cooler (there's less oxygen), you'll have bad fuel economy because there's not enough oxygen in the cylinder to mix with the fuel for the explosion. The explosions happen later in the stroke, which leads to bad transfer into mechanical energy. Sometimes they don't happen fully or at all, and the engine will sputter and misfire. As a result, both performance and fuel economy are poor.

Most people with EGR problems have state #2... but you sound like you have #3 with it occasionally going back to #1. If you switch to state #2, know that it has it's own trade-offs. I'd just fix or replace the valve as soon as it were feasible.
That is a nice synopsis. Someone should post this as a sticky thread in the repairs section under EGR troubleshooting.