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

asked by on August 24, 2016

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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