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The fuel mileage on my 2000 Jaguar XJ8 mileage has dropped…

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The fuel mileage on my...
The fuel mileage on my 2000 Jaguar XJ8 mileage has dropped from 18 to 14 mpg. There are no engine codes, and it still seems to run smoothly. I have noticed no exhaust odor, smoking, or knocking. I use Shell Premium 93 octane except for occasional ethanol-free 90 plus a half-bottle of octane booster. Also putting in Chevron Techron every 3rd tank. The spark plugs were replaced with Bosch double-platinums about a year or 6K miles ago, along with the battery. Earlier this year, the Jaguar dealer's star mechanic replaced the rear main seal, the timing chain cover gasket, and all four chain tensioners. (Also an oil pressure switch that went bad and an oxygen sensor that he accidentally broke.) There is a slight oil leak from the cam cover gaskets, possibly getting into the spark plug tubes, but I have not checked that yet, and it's been going on for awhile. This mpg problem seems new. I confirmed that the parking brake is fully disengaged. The brakes have not needed service since buying it 2 years ago, when the brake fluid was changed and the master cylinder cover was replaced. I'm planning to get the cam cover gaskets and maybe spark plugs replaced, but is there anything else I should do first?
Submitted: 1 year ago.Category: Jaguar
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Answered in 6 minutes by:
9/29/2016
Jaguar Mechanic: Richard, Jaguar Technician replied 1 year ago
Richard
Richard, Jaguar Technician
Category: Jaguar
Satisfied Customers: 15,741
Experience: 12 years at a Ford/Lincoln Mercury and Jaguar dealer as technician and shop foreman.
Verified
We could have a problem with the fuel pressure regulator which would slightly increase the pressure possibly using more fuel. An out of calibration mass air flow can also cause this. I recommend we first disconnect the negative battery terminal cable for about half an hour to reset the fueling adaptations. Let me know.
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Customer reply replied 1 year ago
Thanks, I'll try that and get back to you on the results. I'm just a bit concerned about losing other settings (besides radio -- not sure if I have the code) that the battery maintains, such as the security/alarm system....
Jaguar Mechanic: Richard, Jaguar Technician replied 1 year ago
As long as the vehicle is unlocked the security system is in its disarmed state so that should not be a problem.
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Customer reply replied 1 year ago
OK, thanks!
Jaguar Mechanic: Richard, Jaguar Technician replied 1 year ago

You are very welcome.

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Richard
Richard, Jaguar Technician
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