OK, let me be clear. WITHOUT the EHA you will have no deceleration shutoff and your fuel mileage will be WORSE. You will not improve your mileage by disconnecting the EHA.

You WANT to feel it drag, as you described.

The first thing I would check is that all linkage is moving freely, the decel microswitch is good and the throttle switch is good, in that order.

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2012 E350
2006 Callaway SC560