- 2007 LS
- 2.2 liter EcoTec engine
- auto trans
- about ~98k miles

Recent video I shot when taking a family member to work early in the morning around 6 AM (their car is in the shop):

https://drive.google.com/file/d/1--D...w?usp=drivesdk

So I've noticed that as SOON as I come to a stop - like "flipping a switch" - the long term fuel trim (LTFT) mysteriously changes to like -10%... and sometimes even "creeps down" further into the -14% to -16% range. Highest I've seen is like -17% or -19% once in a while

The behavior is as follows: I'll be rolling around and fuel trims will be mostly normal, then I will roll up to an intersection or red light (...where the car will be stationary/0 MPH for a while), then as soon as the vehicle is no longer moving, LTFT instantly changes to around -10%. You'll see it in the video a few times, vehicle is moving ---> vehicle stops at a red light or stop sign (0 MPH) ---> LTFT instantly changes negative

In the provided video, there is a perfect example of this behavior at the 16:45 mark as the car decelerates to a stop (0 MPH), then you will observe the fuel trims clearly "creep down" as the car is stopped/idling at the red light

Depending on how long the red light/traffic jam lasts and the vehicle is stationary, I notice the LTFT will sometimes "creep" further south... -11%... then -12%... then -14%... then -15%... then -16%. It seems to hang out at around -16% until the car starts moving again. Once throttle is applied/the red light changes to green and the car is finally moving again, the LTFT quickly normalizes and trends closer to 0%

It's been doing this for a while, so I was wondering what some possible causes for this might be? Why is the fuel trim "creeping negative" while the car is stopped/idling?

No "rich" code like P0172 is present, so presumably it hasn't crossed a certain threshold for setting a code yet

Thoughts?

 

Short term fuel trim is changing constantly, depending on demand.

 

I get that, but I was asking about LTFT being negative at idle

I'm just trying to brainstorm/learn some possible reasons as to why this might be happening

 

A set of dirty injectors. Dirty air filter ,
low fuel pressure at idle. And several more readings can cause this.

after a few full tankfuls of fresh fuel, it should clear up.

 

When you came to a stop did you take your foot off the accelerator pedal? That is why.
Do you know what trim is? It is the amount that the computer is adjusting the A/F in order to maintain 14.7:1. The computer has no control over the air intake only the fuel your foot controls the air.
Start with searching youtube with "fuel trims explained".

 

Yes, of course

What I'm trying to figure out is how come my LTFT is negative while other people I know have mostly 0% LTFT in the same exact scenario (stopped at a red light/idling)

Just for giggles, I borrowed my brother's 2006 Chevy Cobalt today (it has basically the same 2.2 engine as an HHR) and scanned it for the same exact parameters from the video during a morning driving session... and his car mostly hovered between -2% and +2% LTFT regardless or idling and whatnot. In fact, at almost every red light his LTFT stayed at basically 0% without very much movement at all

Meanwhile, mine creeps all the way down to -15% to -17% LTFT when stopped at red lights or idling...

 

You stopped reading after my first sentence?

 

Quote:

What could cause negative fuel trims at idle and when the car is stopped (0 MPH)?

Hey everyone, I just wanted to post a followup/update on this issue. The problem ended up being a faulty/over-estimating MAF sensor... despite there being no driveability concerns and no active codes or check engine light related to the MAF sensor

This surprised me since the equipped part (GM Genuine 2134222) is "GM Genuine" OEM-quality so I never suspected a thing when I bought it back in August 2023. I troubleshooted everything under the sun except the MAF sensor, I just assumed it was good since it was a brand-new "GM Genuine" part (doh!) so I focused my troubleshooting efforts on other things (fuel pressure, EVAP stuff, intake gaskets, etc)

Anyways, over the weekend I went ahead and ordered an aftermarket MAF sensor off Amazon (Hitachi MAF0034) for like $35 and installed it today just to see if the fuel trims would change. And they did!

I did a test drive with my scan tool plugged in to monitor the fuel trims in real time. At a stop while in drive, the fuel trims were now -3% to -4% LTFT compared to the previous MAF sensor - which sometimes had LTFT creep down to as low as -18% LTFT at a stoplight or idling, with it typically hovering around -14% LTFT. I think I might be able to get the LTFT closer to 0% after my next oil change (coming up next month), but for the moment -3% and -4% are a big improvement from where they were before

So even if you don't have active codes or driveability issues - even if the check engine light is off - you can have a part that is bad. The only other clue I had was an occasional "RPM surge" on startup (example: the RPM inexplicably surged as high as 1800-2000 RPM on startup once in a while, without me even touching the gas pedal)

So if you have this problem, I would consider simply installing a different brand of MAF sensor. Literally might fix your fuel trims in the 5-10 minutes it takes to install a new sensor

 

213-4222 is an AC Delco part, not a GMgenuine part, and depending on where you ordered it , might be counterfeit.

 

I noticed you said “No ‘rich’ code like P0172 is present,” but were/are there other codes?