Hi everyone,
My moms car, 2016 impala 3.6, auto, 32k miles.
Yesterday morning, after driving about 1 mile from a cold start, it stumbled and stalled on my father. Started back up with no problem and ran fine. It did the same thing to my mother this morning. A mechanic at my fathers old work said it has something to do with morning dew (around 60F in the mornings here) on the throttle body, or that maybe the throttle body is bad.
It doesnt sound like there's a CEL but Ill check it out when she gets home today. I have a decent scanner that can look at some live data.
Anyone ever experience something like this? Or would an italian tune up potentially help this car? It does mostly short, local trips so I may blow it out on the highway later anyway.
Thank you,
-Nick
dew on the throttle body?? Sounds a bit far fetched. Why would that matter? Did he explain?
And why would that be a problem after a mile down the road, and not right when it starts. And then be OK when you start it back up.
it stumbled and stalled on my father
under load or at idle
I have a decent scanner that can look at some live data.
if you can reproduce the stumble, read the misfire counter on all cylinders, and note the fuel pressure and trims.
You could try cleaning the throttle body and MAF sensor.
https://www.youtube.com/watch?v=NWCKvU2FmDc
(hopefully) bad maf.
Hopefully it's just dirty, not failed completely. Replacing the MAF is probably the most expensive option
Fuel rail pressure (gauge): 303kPa [this seems normal for idle]
No, that seems low. I think it should be closer to 400kPa
But don't guess. GM service data is only $30 for three days for unlimited data. You can download the entire service manual if you want. And I hear that if you order it on a weekend you get an extra two days.
It's also not enough to just measure idle pressure:
- Ignition ON, engine OFF, command the fuel pump relay ON with a scan tool and observe the fuel pressure gage while the fuel pump is operating. Verify the fuel pressure is (your spec).
- Verify that the fuel pressure does not decrease more than 34 kPa (5 psi) in 1 minute.
- Relieve the fuel pressure to 69 kPa (10 psi). Verify that the fuel pressure does not decrease more than 14 kPa (2 psi) in 5 minutes.
- Operate the vehicle within the conditions of the customer's concern while monitoring the fuel pressure. The fuel pressure should not drop off during acceleration, cruise or hard cornering.
Fuel rail pressure: 2070 kPa [not 100% sure what the difference is, I understand P_atm vs P_gauge concept]
The 3.6L LFX engine uses high pressure direct-injection
https://en.wikipedia.org/wiki/GM_High_Feature_engine#LFX
There is a second, exhaust camshaft driven fuel pump which delivers fuel to the rails for injection directly into the combustion chamber, which as you can imagine is at very high pressures.
MAF reading: 2.5 g/s <------------------ I think this is the issue.
It does look low. Try cleaning the MAF and looking for vacuum leaks (Scotty has videos about using a smoke machine for this.)
But keep in mind... It may not be the only issue. You could have more than one problem simultaneously.
therefore injecting less fuel than it really needs, which triggers a lean condition.
Is this logic sound?
It's one possibility.
dew on the throttle body
Agreed, didnt sound right to me either.
under load or at idle
When starting from a stop, light turned green.
f you can reproduce the stumble
I actually drove to my moms work and picked her up, trying to reproduce it on the way home, but nothing.
Ill try running it again in the morning.
why would that be a problem after a mile down the road
It happened at the same intersection for both of them, the stop/start fix almost makes it sound like some type of computer glitch to me. I was also checking the throttle percentage to make sure that wasn't wonky either. I'll have to record relevant data and drive it around tomorrow morning.
Picking this post back up @imperator
So I couldnt reproduce the stumble. However, a CEL came on with P0171 & 174 yesterday, both banks too lean.
I put my thinking cap on, it's probably either a fuel pump/ filter issue, vacuum leak, or (hopefully) bad maf.
Found out my scanner has freeze frame access, here were the conditions when it tripped the code:
STFT 1: 11.7%
LTFT 1: 35.9%
STFT 2: 11.7%
LTFT 2: 38.3% [ Obviously these 4 things tripped the codes]
Fuel rail pressure (gauge): 303kPa [this seems normal for idle]
Fuel rail pressure: 2070 kPa [not 100% sure what the difference is, I understand P_atm vs P_gauge concept]
Intake pressure: 36kPa
RPM: 652
Vehicle Speed: 0
MAF reading: 2.5 g/s <------------------ I think this is the issue.
Some other data there that I dont think is important, voltages & evap data seems appropriate. The engine was fully up to operating temp.
My thinking is the MAF is on the fritz, sometimes it shows around 3.8 at idle, and sometimes it drops to the mid 2's. I think my next step is to drive the car and compare the MAF data to the fuel trims. If the MAF is reporting much less airflow than what actually going in the engine, then the computer injects fuel amount based on the MAF reading (which is low), therefore injecting less fuel than it really needs, which triggers a lean condition.
Is this logic sound?
Hopefully, Chuck's response to cleaning MAF is all it needs.
Thank you,