On Thu, 08 Apr 2021 02:05:14 -0400, "Steve W."
wrote:

Steve W. wrote:
Steve W. wrote:
micky wrote:
In rec.autos.tech, on Sat, 03 Apr 2021 02:07:24 -0400, "Steve W."
wrote:

Scott Dorsey wrote:
micky wrote:
In rec.autos.tech, on 27 Mar 2021 19:31:15 -0000,
(Scott Dorsey) wrote:

AK wrote:
I had an oxygen sensor replaced.

Prior to that I got 20 mpg in the city for my CX7.

Now I am getting 21.5 mpg.

Could that increase be due to the replacement of the O2 sensor?
If your O2 sensor was reading low and causing the computer to run the
engine too rich, then yes.
So if now I get codes P0171 and P0174, which means that both banks are
running lean, does that mean replacing the sensors will cause the
opposite of what AK had, richer mixturen and decreased mpg?
Changing the sensors will do nothing but show the same codes again.
Those codes you posted mean the sensors are working. They are seeing
that there is excess oxygen in the exhaust stream compared to what they
expect based on the incoming airflow numbers.
The issue is not them but in the engine. As both are full bank lean
codes I would look at the fuel trims and the O2 live data and check for
I did buy a code reader last year that would read the live data, but
what I should look for has been mostly a mystery since then, until the
video below. I found it a few days ago but just had time to watch it.

Cars are much more complicated than when I was a boy, or even a
teenager, and I've said for years and told quite a few people that
anyone smart enough to be a good mechanic these days could succeed in a
lot of fields.

vacuum leaks or an intake leak that is letting in unmetered air.
As you don't post any P0300 range codes I doubt it's fuel pressure as
that will commonly set misfire codes as well as the lean codes.
Thanks. I found a long video that went over at least some of the
things to do with a P0171. https://www.youtube.com/watch?v=SugtR4KMIDU

He ended up finding a leaky brake booster and a somewhat bad MAF sensor,
but more importantly he goes into some detail about what everything
means.

You and some of the other people here know everything that is in this
video. I miss the days when there were people here who knew less than I
do. They woudl benefit more from this post than you or the others here
will.
I don't know everything but I'm pretty good at diagnostic and repair
work on vehicles.

How much do you really want to know about modern engine diagnostics?

Oh where he is checking the hoses, a quick trick is to use carb cleaner
and spray it around the hoses. If the engine rpm suddenly jumps look in
the area you were spraying for a leak.


From what you posted you have a vacuum leak/pirate air issue as the
primary problem. You can use brake clean or carb cleaner, they are both
more or less paint thinners that burn ok. I use carb cleaner because the
older version of brake cleaner is chlorinated and generates toxic gas
when burned.
The thing to remember though is that in the video he is working on a car
different than yours, some things apply, others don't. For instance MAF
flow, as engine sizes go up, MAF numbers increase even at idle to feed
the larger engine.
Try this
https://support.alldata.com/sites/ma...ing_110918.pdf

Looking at the fuel trims that go up, then start coming down means that
it's a medium sized leak. As the throttle opens it goes past the point
where the pirate air is the main component and the mixture improves. If
it started improving immediately it would be a small leak.

For the sensor data you want to look at it live, and watch what it
actually reads, if the front sensors are changing voltages rapidly as
the engine runs and the rear sensors stay at a somewhat constant number
and don't vary like the fronts, the sensors are working and although
they may be worn, they are doing their job and the converters are working.
You may be surprised at where a vacuum leak can be, brake booster, PCV
system, intake manifold gaskets, a damaged port on the manifold, in the
EVAP system and a host of others, some external others internal.
OH the wuse of water for leaks is somewhat common, it's because you can
use it on a hot or cold engine or one where you have ignition sources
like open plug wires or bad coils. The amount of cleaner you will use
won't be an issue though, just be sure to start on a cold engine.
Usually something like an intake leak will get better as the engine
warms up so you want to catch it when it's cold.
Even something STOOPID like the dipstick not sealed.