# What causes an engine to run lean **URL:** https://forums.speedlife.net/t/what-causes-an-engine-to-run-lean/189422 **Category:** PittSpeed Automotive **Created:** [April 20, 2007, 10:53pm UTC](https://forums.speedlife.net/t/what-causes-an-engine-to-run-lean/189422 "2007-04-20T22:53:35Z") **Posts on this page:** 1 **Showing post:** 7
### Author: ![tegSimple](https://avatars.discourse-cdn.com/v4/letter/t/d6d6ee/32.png) [@tegSimple](https://forums.speedlife.net/u/tegSimple) #### Post date: [April 21, 2007, 3:22pm UTC](https://forums.speedlife.net/t/what-causes-an-engine-to-run-lean/189422/7 "2007-04-21T15:22:53Z")
> [@Microsoft Excel Help with Text Formula](https://forums.speedlife.net/t/microsoft-excel-help-with-text-formula/29632/16): > > usually the your car gets better gas mileage when you go lean… not in my case. my primary o2 sensor was bad and it was telling my car it was running lean. my car then overcompensated and started dumping fuel into the cylinders. my gas mileage went from about 28mpg to 21mpg. my car also ran like shit, rough idle, poor performance, etc… possible causes for a car running lean…bad o2 sensor, fuel pressure regulator, clogged/dirty fuel filter, dirty injectors. --- _[View the full topic](https://forums.speedlife.net/t/what-causes-an-engine-to-run-lean/189422)._