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Q: Maintainence for my car

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Hello, I have a 2000 Vauxhall Astra and the car has got over 850,500 miles on it it has a folder full of service history and I have checked the whole car over and I can happily at theres not one single issue however!,today I went to start the car for to take it for its for its annual 6thousand mile service and I noticed it is using quite a bit more fuel than usual. Now, I have had it in the garage and they have checked and confirmed that there is absolutely no fuel leaks or anything wrong with the engine or fuel at all and there is not a single problem with the car they said except from a bald tyre ( I bought a new one today) what could be the issue I used to get about 460miles from a tank of fuel and now I'm getting 340miles and I drive the same way I always drive what's the issue? Thank you

If the engine has high miles, I would first consider losses in cylinder compression as a possible cause of the degraded mileage. Other possibilities are a malfunctioning oxygen sensor (check output on an automotive scope) and a stuck open cooling system thermostat keeping the engine from reaching an efficient operating temperature. The engine should be scoped to look at secondary ignition voltage firing patterns to determine if it is running rich or whether there are ignition system deficiencies. Running rich can be caused by injector malfunctions and/or fuel pressure regulator problems. You could also have underinflated tires, dragging brakes and transmission problems that could cause degradation in fuel economy. Clearly, there is a degradation in mileage so the mechanics are mistaken in their finding that "there is nothing wrong". There is something amiss and I am sure you will find it beginning with the aforementioned list of possibilities. If you have additional concerns or questions, please let us know.

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