Why would anyone want to dump exhaust back into the intake of a perfectly good
Unfortunately, the internal combustion engine is not `perfectly good.' The modern multi-valve, high compression engine can create NOx (Nitrous Oxide) during the high temperature combustion process. NOx reacts with sunlight to create atmospheric problems such as smog and so comes into the category of 'not a good idea'.
EGR is considered a `metered intake leak' and was developed to reduce the combustion temperatures to below 2,500 degrees, the threshold where NOx is created. Not unlike putting a brick in your lavatory to lower the volume of water used, the EGR valve meters a readily available inert gas (actually exhaust gas which contains a lot of very inert Carbon Dioxide) into the combustion chamber to effectively reduce the volume. Smaller effective displacement means less fire, and less heat and thus lower temperatures, thereby controlling NOx emissions.
Obviously we don't want to `reduce' the volume of the combustion chambers (effectively reduced engine displacement) during hard acceleration, so EGR is turned off when you need full power (WOT (Wide Open throttle) conditions). At idle, the engine is very sensitive to air/fuel mixture ratios and swirl in the combustion chamber, so introducing EGR at idle is not on either. However at cruise the Fuel/Air mixture is set as lean as possible for maximum economy and this in turn generates the highest temperatures, and so the EEC-V uses these conditions to inject exhaust gas into the inlet manifolds to reduce emissions. Older vehicles used vacuum devices to control the EGR valve whereas the Scorpio uses the EEC-V and it's sensors to control it.
If EGR is on during idle, stumble and even stalling will likely result. If EGR is on during hard acceleration, low power (from reduced air/fuel volume) is the result. At part load, lean mixtures for economy create high combustion chamber temperatures and without EGR, these conditions not only create Nox, they also foster pre-ignition and pinking.
EGR systems are a lot more sophisticated than they used to be. With the EEC-V, they are not only monitored, but tested routinely during each `trip.' Faults are purposefully introduced to see if, and how, the system will react. If the EGR system performs outside of predetermined boundaries, a diagnostic trouble code (DTC) will be set and the FDS 2000 will be able to read it. Certain precise conditions have to be met before a trip takes place and the EEC-V starts a test sequence of the EGR system.
My vehicle suffered from a persistent misfire under very light throttle conditions and disconnecting the EGR at it's vacuum control input restored almost normal performance. The Ford Dealer diagnosed the problem to the EGR circuit and the FDS 2000 was used to identify the exact culprit. More news later on what was fixed.
When it comes to working on these systems, you must perform certain
prescribed checks to figure out what's wrong. It can be left to the Ford Dealer
with his FDS-2000 diagnostic computer and 'trouble trees' to determine
systematically what would cause the indicated problem or faults, or an owner can
use one of the many On Board
Diagnostic readers now available to read the Trouble Codes himself.
If the engine stumbles or stalls at idle or lacks power under load, EGR may be staying on and not shutting off. On the other hand, if the engine is pinking at cruise, EGR may not be turning on to cool down combustion chamber temperatures.
Incorrect operation could be caused by a mechanical problem such as a clogged or coked EGR valve, a ruptured EGR valve diaphragm, split or improperly routed vacuum hoses, or other component failures. All sensors that control vacuum or tell the EEC-V about engine temp, throttle position, MAP, and EGR position contribute to correct EGR function, so any of these will cause problems if they go awry. On the output side, the EGR valve itself, plus associated system components such as vacuum solenoids, vacuum valves, delay valves, and associated wiring and plumbing should all be visually and functionally checked.
If you are comfortable performing some basic checks prior to resorting to the Ford Dealer then try pulling a vacuum on the EGR valve to make sure the pintle moves freely in both directions. While you're at it, make sure all vacuum hoses and system components hold a vacuum. Inspect the EGR valve visually for coking and contamination. Check EGR solenoids and the position sensor for electrical continuity. Further than that requires the WDS diagnostics at a Ford Main Dealer/well-equipped local garage, or access to an OBD Lead. to carry out the