- Apr 26, 2012
- 404
My son has a 2013 5.3L Silverado LTZ. It has always run perfectly, except when starting a cold engine at very cold temperatures (like, below -25 C). No codes thrown. I have access to a good scan tool, so I thought that I would try to look at Fuel Trims etc. when trying to do a cold start at such a temperature. Vehicle did start for me, but these are some typical readings right after start:
Intake Manifold Pressure 9 psi
Long Term Trim Bank 1: -16%
Long Term Bank 2: -19%
Short Term Bank 1: -2%
Air Flow Calculated: 11.84 g/s
MAF Sensor: 14.27 g/s
Also, after driving around (warm engine):
Idle: Long Term -12%,-14%
Short Term : -3%
IMP: 9 psi
Air Flow: 5.93 g/s
MAF: 6.74 g/s
1500 RPM
Long Term -16, -19
Short Term -2
IMP 10psi
2500 RPM
Long Term -5, -10
Short term -11
MAF 21.40 g/s
I am new to this technical a diagnosis, but it seems to me that such high negative values indicate a problem which is leading to his difficulty starting; when it is so cold. I realize that these values are just snapshots, and not a graph, but it seems like the PCM is trying to lean out the mixture. However, I thought that fuel trim isn't used when the vehicle starts (not until closed loop operation).
1) If the engine is really cold and the air is more dense, shouldn't it still start okay if the mixture really is too rich all the time?
2) And aren't those high negative values really strange? Or is that because it is so cold here.
3) Why do I have negative short term and negative long term in a snapshot? That doesn't make sense to me, probably because I don't understand enough.
Can anyone get me a little smarter on what is happening here? Do I need to graph a longer drive?
Intake Manifold Pressure 9 psi
Long Term Trim Bank 1: -16%
Long Term Bank 2: -19%
Short Term Bank 1: -2%
Air Flow Calculated: 11.84 g/s
MAF Sensor: 14.27 g/s
Also, after driving around (warm engine):
Idle: Long Term -12%,-14%
Short Term : -3%
IMP: 9 psi
Air Flow: 5.93 g/s
MAF: 6.74 g/s
1500 RPM
Long Term -16, -19
Short Term -2
IMP 10psi
2500 RPM
Long Term -5, -10
Short term -11
MAF 21.40 g/s
I am new to this technical a diagnosis, but it seems to me that such high negative values indicate a problem which is leading to his difficulty starting; when it is so cold. I realize that these values are just snapshots, and not a graph, but it seems like the PCM is trying to lean out the mixture. However, I thought that fuel trim isn't used when the vehicle starts (not until closed loop operation).
1) If the engine is really cold and the air is more dense, shouldn't it still start okay if the mixture really is too rich all the time?
2) And aren't those high negative values really strange? Or is that because it is so cold here.
3) Why do I have negative short term and negative long term in a snapshot? That doesn't make sense to me, probably because I don't understand enough.
Can anyone get me a little smarter on what is happening here? Do I need to graph a longer drive?