some more interesting info regarding the diesel egr:
EGR implementations
Usually, an engine recirculates exhaust gas by piping it from the exhaust manifold to the inlet manifold. This design is called external EGR. A control valve (EGR Valve) within the circuit regulates and times the gas flow. Some engine designs perform EGR by trapping exhaust gas within the cylinder by not fully expelling it during the exhaust stroke, which is called internal EGR. A form of internal EGR is used in the rotary Atkinson cycle engine.[citation needed]
EGR can also be implemented by using a variable geometry turbocharger (VGT) which uses variable inlet guide vanes to build sufficient backpressure in the exhaust manifold. For EGR to flow, a pressure difference is required across the intake and exhaust manifold and this is created by the VGT.[citation needed]
Another method that has been experimented with, is using a throttle in a turbocharged diesel engine to decrease the intake pressure, thereby initiating EGR flow.[citation needed]
Early (1970s) EGR systems were unsophisticated, utilizing manifold vacuum as the only input to an on/off EGR valve; reduced performance and/or drivability were common side effects. Slightly later (mid 1970s to carbureted 1980s) systems included a coolant temperature sensor which didn't enable the EGR system until the engine had achieved normal operating temperature (presumably off the choke valve and therefore less likely to block the EGR passages with carbon buildups, and a lot less likely to stall due to a cold engine). Many added systems like "EGR timers" to disable EGR for a few seconds after a full-throttle acceleration. Vacuum reservoirs and "vacuum amplifiers" were sometimes used, adding to the maze of vacuum hoses under the hood. All vacuum-operated systems, especially the EGR due to vacuum lines necessarily in close proximity to the hot exhaust manifold, were highly prone to vacuum leaks caused by cracked hoses; a condition that plagued early 1970s EGR-equipped cars with bizarre reliability problems (stalling when warm or cold, stalling or misfiring under partial throttle, etc.). Hoses in these vehicles would be checked by doing a vacuum leak test or pressure smoke test, with a professional smoke generator. When testing, smoke escapes from the hose being tested or the vacuum test gauge indicates a particular hose is leaking.[citation needed]
Modern systems utilizing electronic engine control computers, multiple control inputs, and servo-driven EGR valves typically improve performance/efficiency with no impact on drivability.[citation needed]
In the past, a fair number of car owners disconnected their EGR systems in an attempt for better performance and some still do. The belief is either EGR reduces power output, causes a build-up in the intake manifold, or believe that the environmental impact of EGR outweighs the Nitrous Oxide emission reductions. Disconnecting an EGR system is usually as simple as unplugging an electrically operated valve or inserting a ball bearing into the vacuum line in a vacuum-operated EGR valve. In most modern engines, disabling the EGR system will cause the computer to display a check engine light. In most cases, a disabled EGR system will cause the car to fail an emissions test