1) For starters, when the economy gauge is disconnected, it's default position is all the way to the right.
2) Also, there's a thin clear plastic vacuum tube that connects to the intake manifold, and often this cracks and breaks, which causes this problem.
3) Also, check your OVP Relay (behind the battery). The ones with a black top and only one fuse are crap, and Mercedes developed a better one with a red top and two fuses. Often when this fails, you'll have bad gas mileage, although I'm not sure that this has much of an effect on the economy gauge.
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