is it bad for a car to run lean?

[98outbackpdx]

my moms van has an o2 sensor is bad and i got a 17.8mpg going 75-80 WITH studded tires on. i would like to have something like that for the outback to run lean not bad o2 sensor. would it be a bad idea? with the van running lean the mileage jumped like 4-5 mpg with the studs, that would be nice to do in my Subaru. i dont want to screw anything up, but i know i will need a new engine in the future (piston slap) so any ideas, i really need to have the added fuel economy in this economy (pun intended) and no i wont slow down. i am always running late, and so speed is my friend.

[hallowpoint]

first of all dont do it. why? its making your car starve for fuel, you will lose power, its like your really thursty and you can only have a drops or water, thats not nice your motor has feelings to. im not sure of the long term affects. as for getting better mpg you drive a subaru they dont have the best econo. but its the worse . just give your self a few more mins it not hard and keep your system clean and you will be fine.

[rougeben83]

engine runs too lean, cause detonation, your engine goes boom. Just depends on how lean we're talking about. In tuning terms the general rule is that its always to tune too rich than too lean.

[ouch1011]

In most instances, if you run the engine lean past about 15:1, you actually start losing efficiency, so your fuel economy will go down.

The best thing you can do to improve your fuel economy, aside from basic maintenance, is adjusting your driving habits. Since you are unwilling to do that, there is no reason to discuss fuel economy any further.

[98outbackpdx]

well what ever is going on with the van (says bank 1 says cat failure) well what ever it is doing this i would have to say is the most efficient the van has ever ran and it feels like the power it has lost over the years has come back. i keep hearing stories on the news about people messing with the programming of their cars, for instance some guy with a mustang said he was getting 110 mpg in his suped up mustang. i just want to know how to do that if anyone knows.

and ouch1011; i have for the most part i have adjusted my driving, but yesterday i was running late for a training seminar so i had to go a little more quickly than i have been, and i have been driving at 72 ( on the freeway) or below for a while now so i really have slowed down.

[Grafton]

i can assure you that tuning a mustang will in no way shape or form get it close to 110mpg, hell i'd have my doubts about 45 mpg, hypermiling i bet would only get you to about 30 mpg in that car even modded

driving is the big one when it to MPG in our cars, i've done between 18.5 to 31.5 mpg in my legacy all due to driving style and where i was driving

[Huffer]

Get 15" wheels and tires, run to 40psi.

But don't attempt to lean out your map without proper ECU tuning.

[LunchBox04V]

From the other horizontalially opposed 4 cylinders I have run (O-235, O-360, IO-360) 50*F RICH(cooler than) of peak EGT will net you maximum power. While peak EGT will net you maximum fuel economy(both carberated and fuel injected engines). I have heard of a few people running 50*F LEAN(again cooler) of peak EGT, though it is not recomended, due to the risk of engine damage from detonation. This is also tuning for a constant RPM and no O2 information but EGT guage.

I have heard Subarus run on the rich side of safe. Personally, rather than damaging my engine, I would run higher PSI in my tires (or factory if you like, just not low) clean the air filter and run some fuel system cleaner. I drive like a granny, and my friends hate me for it, but when I get 30.5 mpg on the highway and 24 around town they kinda get it. Smooth on the throttle, coast when you can, and max endurance is about 3 mph over shifting into your final gear. (4EAT or 5MT).

[ouch1011]

A catalyst efficiency code isn't going to do anything to cause the engine to run lean/better.

[98outbackpdx]

well it is crazy, my brothers '07 impreza with a 5 speed i drove up to seattle to give the car back, on i-5 i was doing 75-80mph most of the way, then stop and go traffic in seattle and only used 2/3 of a tank and drove about 300 miles. and that impreza was at like 3700 rpm or higher, no way my car could do that.