so my 00 V6 is getting pretty bad gas mileage compared to what it used to get. in fact, about 4 mpg less on the highway than usual, and about 3-4 mpg city less. i've recently fixed my exhaust leak (replaced flex pipes prior to cats) and my tire pressure is good. i know i have an oil leak which i need to fix, but oil leaks haven't affected my mpg before. any ideas on what i should check?
also, is it possible for the gas to be burning rich still even after fixing the exhaust leak? maybe i need to reset some settings or what-not?
Since the cost of two upstream O2 sensors is about the cost of a VAG, now might be the time to invest. I put it off for over 8 years, but finally bit the bullet 6 months ago.
If I were attacking it like an older car, I'd clean the MAF and change the upstream O2 sensors. A lot of sources recommend replacing cars' upstream sensors around 100k even if they're not acting up. To me, this is just needlessly throwing money at the car.
I'd also at least inspect the plugs. The stock ones are supposed to be good for 40k (or more).
I would start by changing your plugs, air filter and oil/filter and then add a bottle of "good" fuel injector treatment to your next tank of gas. At least then you can rule out the obvious.
One issue with a leaking exhaust manifold is that it fools the o2 sensors into thinking that the engine is running lean, so it will dump in more fuel. Depending on the location and severity of the leak, this could mess up the fuel economy for sure.
The other advice is also spot on -- do a basic tune up with plugs, air and fuel filters, fuel injector cleaner (Lucas stuff works well) and maybe a MAF cleaning to get the basics out of the way. Also you may want to average a few tanks of fuel before deciding what gas mileage you're actually getting. Higher ethanol content will cause your fuel economy to drop, too.
A stuck open thermostat will hurt MPGs, too. The ECU will dump extra fuel to get the coolant up to temp, so you will see 190* on the dial. This, of course, makes you think everything's fine. I believe a bad CTS can cause a similar problem, too.
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