P0172 Fuel Rich - Chevy Impala Forums
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post #1 of (permalink) Old 01-17-2019 Thread Starter
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P0172 Fuel Rich

Hi all.

I have a 2009 Impala that has been running P0172 for quite awhile.
My mechanic changed plugs, fuel filter, and MAF (the MAF was actually cracked) the first time.

The code continued to come up. I had him try the ethanol reset trick since this is a flex fuel, but it didn't seem to have worked. The ethanol shows 75% and I've never run ethanol fuel.

Now that it is cold, it starts very hard if it hasn't run for awhile. It acts like it isn't getting fuel and will fire and and then stall. It now takes 2 to 3 tries. If I try to drive off too quick after a first start of the day, it stalls out. I must let it warm up.

I bought new O2 sensors to continue this troubleshooting.

Once the car is running, it's rock solid. Zero performance issues.

I want him to do more testing than just swap out the sensors.

Has anyone else run into similar issues? Any suggestions?

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post #2 of (permalink) Old 05-13-2019
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Yes, I have the exact same symptoms. Now that its warmer, the car will start without extra accelerator, but the P0172 still sets.

Some discussion and info in a separate thread here: https://www.impalaforums.com/automot...619-po172.html

Other threads with additonal info, including the text of the GM P0172 DTC: https://www.impalaforums.com/mechani...h-p0172-3.html

What I have confirmed:
> fuel pressure is ok and sustained
> spark plugs are ok. Issue remains when they are changed.
> MAF sensor is ok. Issue remains when MAF sensor is swapped
> MAP sensor readings are in range
> visual/audible inspection indicates no leaking hoses.
> no airflow obstructions through air filter/duct/throttle body
> H02 sensors appear to be reporting as expected.

I have not had the fuel composition % value reset. It appears some folks have some success doing this, while others do not.

However, when I create a leak (after the MAF sensor), the engine at idle does not respond as expected. On a different car ('08 Allure/Lacrosse), creating the leak immediately caused fuel trims to shift to TOO LEAN. On my '08 Impala creating the same leak there was no noticeable change in fuel trims. I'm not sure what to make of this. Is it because the fuel mixture is so rich that the extra air made no difference? Yet, when changing from idle to 2500 RPM changes do take place in the fuel trims.

Thoughts, updates?
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post #3 of (permalink) Old 05-19-2019
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Can be a vacuum leak as well.
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post #4 of (permalink) Old 05-20-2019
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^ Vacuum leaks usually trigger lean codes (p0171/174).
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