The EGR valve in the smart is a mixing chamber which brings exhaust back into the air going to the engine, in order to increase temperatures, which should result in lower NOx emissions. The issue in the smart is that the soot and gunk in the exhaust solidifies inside the EGR valve, which eventually stops the flapper inside from moving. Once it stops moving, you'll get an engine light (Probably with code P0403 or P0405), and you'll be stuck in limp mode, which will reduce your power and prevent you from exceeding 3000 RPM. At that point, you need to clean out the EGR valve. Once you've done it once, you're not going to want to do it again, so you'll want to block or delete your EGR.
There are generally 3 methods for preventing the EGR from clogging up, each with it's own advantages and disadvantages.
1) EGR Emulator
This method is probably the easiest method, however, if your EGR valve isn't freshly cleaned, you may still have an issue in the future.
Essentially, you get a circuit board that plugs in in place of the EGR valve. This circuit tells the EDG that the EGR valve is responding normally, and everything is working fine. In reality, once you've plugged the emulator in, the EGR valve has no power running to it, and the flapper inside should be stuck in its default position, which is to block the exhaust input to the valve (If the EGR isn't completely clean, the flapper may not fully seat closed, resulting in exhaust still entering and slowly clogging the valve again).
The only work necessary is to unplug the wiring harness that goes to the EGR valve and connect the EGR Emulator into it. This is possible to do entirely from the engine access hatch under the carpet in the back of the car.
The EGR Emulator circuit is available to buy fully assembled from some members on this forum, or on eBay. Alternately, you can buy the components and put the circuit together yourself, using the schematics that can be found HERE (Need to actually find the link!)
From the original thread: http://clubsmartcar.com/index.php?showtopic=10247&page=1
(Need some actual instructions and pictures)
2) EGR Blocking Plate
This method requires a bit more effort to achieve, and requires removal of the back panels of the car.
Essentially, you disconnect the exhaust line that leads to the EGR valve, insert a solid plate in between the valve and the line, then reconnect everything. This way, no exhaust can enter the mixing chamber, and there is no way it can become clogged again. In this case, the flapper inside the EGR valve continues to move as it normally would, and as such, if the valve is clogged, it will need to be cleaned out. This is essentially a fully preventative measure, rather than a cure itself.
(Someone please document how to actually get in to insert the block plate! Pictures too if possible!)
(Leave this in towards end, small additional tip to the main body)
Many of the EGR pipe flanges have distorted, causing an exhaust leak. Not enough to lose power, but an annoying "whistling fart" noise and a lot of soot in the vicinity. Filing/sanding the ears of the blocking plate is an excellent idea. Fit by feel is best, hard to precisely define how much to remove. Look the mounting holes and the centre port with the circular seal area, also the metal gasket has a definite formed bead that is the seal point: start the taper about 1/4 of the seal diameter in, so the centre half is left flat then starts to thin. Remove about 15 to 20 thousandths of an inch (a scant whisker) at the outside of the ear, smooth taper out. With the bolts slightly loose you should be able to feel a slight amount of rocking motion.
3) Full EGR Delete
This is the most in-depth method, and combines the first two methods, along with some extra work. Doing this will guarantee that your EGR will never clog again!
This method involves entirely removing the EGR valve, replacing it with a straight piece of pipe, blocking off the exhaust line (so that it isn't just shooting exhaust into the engine compartment), then emulating the EGR so that the computer doesn't care that it is gone. There's probably more to it too...