I have a friend, that has a 2001 Eclipse, it has 130,000 miles. It wasnt running well and only got 11mpg they said, that it need o2 sensors. Well, this was a while back, I talked to them now, and they priced the car to me, for $2500, they said that the only thing wrong now, is that it needs a valve cover gasket, and spark plugs. I was just wondering, what would have caused the fuel mileage and running bad. And if it is worth 2500. thanks

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If the spark plugs are bad and causing multiple cylinder misfires, then that would decrease the gas mileage significantly. And if O2 sensors are bad that will also decrease your gas mileage.
All the things you said were wrong with the car will do that to your gas mileage. And if they're all wrong at the same time, then 11mpg is quite believable. but $2500 for a 3g with 130k miles on it is still too much especially if it still needs to have the valve cover gasket replaced. Try and see if they'll take $1000 for it, that's a more reasonable price for what still needs to be done to it.

What is the submodel? If its a GT or GTS they have to remove the intake manifold to get at the rear plugs.