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Matt, Mechanical Engineer
Category: Jaguar
Satisfied Customers: 22750
Experience:  BEng hons Mech engineering, in auto industry 22 years
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X type 2.5 auto 2003 petrol. suddenly doing about 14 mpg. no

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X type 2.5 auto 2003 petrol. suddenly doing about 14 mpg. no fuel leaks, engine management light on. No loss of power, auto gear box fine. Checked out by garage, had new airflow meter replaced. Problem not resolved, engine management light back on and mechanic at a loss. Suggested the cat or sensors on the cat defective, tried to remove the 2nd bank 1st lamda sensor, but was too difficult to remove without damaging the cat which would then cost over £600 to replace with the cost of the work. The computer reading does shoe this lamda sensor to be below normal operation.
Do you have any suggestion please
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Customer: replied 1 year ago.
No I have not found an answer. The problem is still there. The engine seems to be running fine otherwise the car is running at about 14 miles to the gallon, it should be about 20-22 in urban areas where I drive mostly. it is a significant difference. I have not yet driven it on long journeys or motorway as I am worried it may breakdown. I would like to confirm there is no fuel (petrol) leaking and the engine management warning light is permanently on. Today the warning message 'cruise control not available' came on if that is connected, though I don't think so. The engine has missed a little on accelerating once from low speed if that tells anything. Thanks Pete Boy


if the check engine light is on then you should be able to pull a fault code off it which will then point to the issue

1st though check for exhaust leaks before the O2 sensor as if this has been experiencing an exhaust leak then it will have been running very rich as the sensor will see the extra air pulled in as fresh / lean fuelling and be adding fuel to compensate.

this can lead to catalyst failure as the extra fuel can re-light inside and melt the catalyst

The best way to find an exhaust leak is to run the engine from cold and feel around the exhaust joints with your bare hands - you'll have about 30 seconds before it all gets too hot and you should feel any escaping gas blowing over your fingers

it could also be incorrect fuelling due to a faulty water temp sensor. If this is the problem then the ECU will be leaving the car constantly ' on choke' and not reducing the fuelling as the engine warms - so unplug the sensor and clean the contacts on both sides with some switch / electrical cleaner (not WD40) and inspect the wiring for any signs of damage to the insulation especially where it runs over other components and through the bulkhead- if this doesn't help consider replacing the sensor as they're not expensive


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Matt, Mechanical Engineer
Category: Jaguar
Satisfied Customers: 22750
Experience: BEng hons Mech engineering, in auto industry 22 years
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