I was tracking a steady decline in gas mileage for over a half year, about 1MPG decrease per month. As we got near 12MPG, repeated trips to the service department were still turning up nothing except blame on my roof rack and my 'oversize' 225/70 tires.
As we eventually touched single-digit(!) MPG, one cylinder started to miss. This was the first time there was noticable rough running. The service department still couldn't find anything, as the on-board computer detected nothing wrong and was still logging no errors.
The Tech then did a temperature measurement and found abnormally hot temparatures near one rear cylinder. He pulled the spark plug, the one hidden at the right rear of the engine (the one I couldn't easily reach and check a couple months earlier). It had cracked it's ceramic insulation, and the crack was progressively widening, as the spark apparently burned its way out. The overheating had melted the coil o-ring and allowed water in to rust the plug badly. And the coil unit was heat damaged. From the amount of distruction there, we figure it had cracked all those months ago, or went in cracked from the factory, and was responsible for the slow MPG decline.
The Libby is now back up in mileage where it used to be, and holding steady and reasonable for a couple months now. I got a chance at the engineering roundtable at Camp Jeep to suggest making the on-board computer a little more comprehensive.
