After buying a M3 gauge cluster for my 330ti, I found that the MPG gauge did not work. (Flame if you want about how useless this gauge is, I'm happy cuz I was able to make it work) It turns out that 318ti, as equipped from the factory, are not wired for MPG guages. For all those who have 6 cylinder conversions and gauge transplants with erratic, non-functioning MPG gauges, read on (318ti's, 318i's etc.)
Parts needed:
circular connector 61-13-0-007-570 (fits in X20)
bushing contact 61-13-0-005-201 (fits in X16)
approx 3.5 feet of 18 guage wire, solder, shrink tubing, wire ties,...
Using Bentley, I figured that the white/black wire coming out of the DME is the signal for fuel consuption. It runs accross the back engine bay to the X20 connector which is a big black round connector next to the fuse box. You twist to unlock and open it. You will find that the female side (top part connected to DME) has a connection for pin #24, but the male side has the hole for pin #24 empty. (You can make sure that this is indeed the white/black wire by pulling back the protective rubber boot on the female connector). You need to put the male circular connector in this slot. However, it is not as simple as just sliding it in. If you look carefully, there is a thin locking ring between the inner and outer rows if pins. This needs to be turned one click counter clockwise. Use flat jewlers screwdrivers to pry the locking tabs away while turning. It turns freely once the tabs are cleared. Now you can push the circular connector all the way in. Be sure to return the locking ring to it's original locked position.
From the newly placed male pin connector, run a length of 18 gauge wire (about 3.5 feet) onto the fuse box (open it by undoing the 4 torx screws) and through the common fire wall wire opening into the driver footwell. Thread it along with the wire bundles that go the X16 connector on the tach side of the instrument cluster.
Remove the instrument cluster and remove the x16 connector from the back of the cluster. there is a pivoting lever that swings up to release the connector. Open the X16 connector by prying off the sliding cover. A double row of female pin connectors slides out. Pin #21 is empty and needs the bushing contact to be inserted. Connect the two wire ends together to complete the connection. Tidy up the wiring, put the cluter back and there you have it. | 
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