Could this be lowering my gas mileage?

Hello Dopers,

I have a fairly involved question about my car. My car seems to be equipped with a mass airflow sensor, as well as two oxygen sensors (one upstream of the catalytic converter, one downstream). Recently I have discovered a sizable crack in the boot connecting the MAS to the throttle assembly, and I’ve already ordered a replacement. However, this is making me wonder how my car operates.

  1. The check engine light is NOT on
  2. My gas mileage seems a bit lower than it used to be
  3. My engine temperature seems normal

I guess my question boils down to:

a) What is the chance that the engine is actually running significantly leaner than it should be without illuminating the check engine light? Is it possible the ECU only uses the input from MAS for mixture control and uses the oxygen sensors only for cat sanity checking?

b) Would running leaner actually decrease gas mileage noticeably?



The MAF sensor essentially tells the ECU how much air is entering the engine, which is a significant factor for calculating how much fuel should be injected. Unmetered air, such as from the crack in the intake boot, will not register, thus not get proportionate fuel, thus cause lean running. Often, however, a crack in the boot doesn’t allow much air past it except during hard acceleration, when the engine’s torquing over pulls on the boot and temporarily enlarges the gap at the crack. So it may be that most of the time it’s a relatively minor effect.

The upstream O2 sensor, in measuring exhaust oxygen levels, is telling the ECU how rich or lean the engine is running. This also is a significant factor, and the ECU is constantly trimming the amount of fuel injected up or down in response. In order to set a lean code, typically the engine must be running lean by a certain amount for a certain length of time.

The downstream O2 sensor mostly helps the ECU evaulate catalyst efficiency.

Presumably in your case the degree and duration of lean running has not yet reached the threshhold to set a code. Nevertheless, it is probably running leaner than the stoichometric ratio that provides the most efficient combustion, so it could affect mileage.

So in brief:

a (1) -
I don’t know the “chance,” and I wouldn’t use the word “significantly,” but I find it plausible; (2) No, the MAF and upstream O2 sensors are major players in fuel mixture, while the downstream O2 sensor is for cat checking.

b - Yes, it could. In more extreme cases there can be lean misfire (i.e., misfire caused by too lean a mixture), which really affects mileage. But in between “perfect” and misfire, there’s “okay but not the best.”