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Q: Bad gas mileage, rough idling, rear end noise

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I've been getting very bad gas mileage on my car. It used to be in the 18-24 MPG range, depending on whether I drive it on a highway or in the city traffic. It's now dropped to 10-13 MPG. Also, my car started idling roughly a bit. Besides this, the rear end of the car, on it's right side, makes a thumping noise. I figured that something must be loose so I went to the shop and they had the shocks checked. They are fine. Are these problems connected? What should I do?

Hi there, thank you for writing in about your 1997 Jaguar XJ6. The poor mileage and a rough idle is potentially due to a bad coolant temperature sensor or a broken wire to this sensor (the coolant temp sensor that is connected to the fuel injection). A broken wire in this circuit sends a false "cold" signal to the engine causing excess fuel consumption.

If the thumping noise you mention occurs while driving, you may have a failing constant velocity (CV) joint on the right side axle. If you would like to have this looked at, a certified technician from YourMechanic can come to your car’s location to diagnose the rough idling issue, thumping noise, and decreased mileage in order to have this properly corrected.

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