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Discussion Starter · #1 ·
Fellas,

Here's the issue I'm having in a nutshell: take foot off of accelerator when going downhill to maintain speed. Press accelerator once at the bottom of the hill (and beginning to go up another hill) to maintain or increase speed - at this point there is no response from the throttle until the accelerator is pressed far enough at which point the transmission upshifts and the RPM's jump. During the "lag" there is noticeable tap/ping/knock/what-have-you noise.

What I've found in the data is that during the period when my foot if off of the accelerator (coasting downhill) the engine switches to open loop and the long term fuel trims for each bank go negative. When I get back on the gas the engine switches back to closed loop and the lag begins.

From what I've gathered, in open loop the ECM is relying more on sensor data (other than O2 sensors) and in closed loop is relying more on O2 sensor data and tables. Assuming that is all correct, what could be causing such a difference in performance between open and closed loop? If everything is operating correctly I imagine there shouldn't be much difference in the way the truck runs whether in open or closed loop. What gives?
 

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When your engine is running at operating temperatures it is going to be running in a closed loop. You were pretty close in explaining the loops. Close loop is when the computer relates a good amount of data for a/f ratios from the O2 sensors. In an open loop the engine is using all of the other sensors to try to make sure the engine is running at the proper ratios until the O2 sensors are heated up and so is the engine. If the engine is at operating temperature and it trying to run in an open loop there is something wrong. Has the vehicle been tuned? There shouldn't been much difference with how the engine operates in either cycle other than the lag. There may be a little bit of lag in an open loop as the computer thinks that the engine is not at the proper ratios and is trying to keep it within parameters. Is there anything done aftermarket wise to the vehicle?
 

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Discussion Starter · #3 ·
Thanks for the reply. The truck is a completely stock 2016 with only 1300 miles on it. It was definitely up to temperature when I logged the data and there are no codes. I haven't shown the data to the dealer but I have had it in a few times for the same lack of power issue - they tell me its normal but I'm inclined to disagree.
 

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Good morning AdkLooch,

We regret to hear that you are experiencing these engine concerns in your Silverado, and apologize for any inconvenience you have experienced as a result. We would be more than happy to reach out to your dealership for clarification on the diagnosis, and see if it would be appropriate to get you back in. If this is something that interest you, please send us a private message with your VIN, full contact information and involved dealership.

All the best,

Julianne M.
GM Customer Care

AdkLooch said:
Fellas,

Here's the issue I'm having in a nutshell: take foot off of accelerator when going downhill to maintain speed. Press accelerator once at the bottom of the hill (and beginning to go up another hill) to maintain or increase speed - at this point there is no response from the throttle until the accelerator is pressed far enough at which point the transmission upshifts and the RPM's jump. During the "lag" there is noticeable tap/ping/knock/what-have-you noise.

What I've found in the data is that during the period when my foot if off of the accelerator (coasting downhill) the engine switches to open loop and the long term fuel trims for each bank go negative. When I get back on the gas the engine switches back to closed loop and the lag begins.

From what I've gathered, in open loop the ECM is relying more on sensor data (other than O2 sensors) and in closed loop is relying more on O2 sensor data and tables. Assuming that is all correct, what could be causing such a difference in performance between open and closed loop? If everything is operating correctly I imagine there shouldn't be much difference in the way the truck runs whether in open or closed loop. What gives?
 

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This is normal. It's called DFCO. The FT go negative when in this mode. If my memory is correct the LT freeze relative to the FT Cell and the ST go to -25. This is all for fuel economy. The TCC remains locked during DFCO to keep the engine spinning. When you press the accelerator the injectors then fire again to continue combustion as normal. As for the noise I have no idea.
 

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Discussion Starter · #6 ·
Most of what I've read at this point says the same thing - decel mode. Should there be such a difference in performance when switching between the two? One would think that there should be no noticeable difference in performance and power whether the is engine looking at all the closed loop sensor data versus the open loop sensor data - the big assumption being that all the tables have correct values and all of the sensors are working correctly. I would think the transition between the two operating modes should be transparent to the driver. Of course, GM could just be making a s**t product but I'm not quite there yet. I've had three loaner Silverados, all of which ran great.
 

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Well GM has been working to fix their VCM (Variable Cylinder Management) setup on their vehicles. Unfortunately they put the product on the market and then fix the bugs. VCM essentially means the same thing for different vehicles but it usually stays on track with the same pattern. Shut down cylinders to provide better fuel economy. It is great on paper but sometimes not so much in practice. One of the issues is that to keep up with everyone else sometimes GM releases a product before working out all of the bugs (as a lot of companies do unfortunately). The change in cylinder operation can sometimes be felt and sometimes it can be seamless. I would go ahead and contact the Customer Service rep above to see about getting it fixed. They may be able to use your vehicle as a guinea pig to figure out what is going on. When there is a problem that is apparent but they are unable to diagnose it, sometimes the company will send out an engineer to check it out. They will do some data-logging and try to figure it out. This could help some of the other people having the same issue.
 

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Discussion Starter · #8 ·
Thanks for taking the time again to reply. If by VCM you are referring to active fuel management, I always drive in M5 (unless on the highway which is very rare) to purposefully keep the vehicle in V8 mode.
 

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I recently hooked up my scantool from OBdwiz to look at the O2 sensor readings, STFT and LTFT readings on my 2017 Silverado crewcab. I wanted to get a baseline of good numbers to use as a reference for numbers on my 2011 Equinox. After engine was warmed up, 212 F, at idle and at 2K rpm the STFT was running normal, 3 to -3%, but the LTFT was running at -10% for both rpms. This doesn't seem normal to me. Shouldn't a new truck be reading at or around 0 for the LTFT? 5.3 V8 ecotec3.
 

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Your thought process is correct! The variables are huge. First you need to know purge valve duty cycle or you need to unplug it to eliminate it as a variable. The fuel trims only used to determine fuel injector correction, this is not the only fuel source for the engine. The evap system is used to burn fuel vapor from the tank. That will drive fuel trims both high or low depending on the density of hydrocarbons in the vapor. I know the 2.4s and 3.6s like the back of my hand. If you need info on an equinox let me know.
 

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So if I just started the truck after its been sitting a while, warmed it up, your saying excess vapor being burned from the tank could cause a rich condition therefor causing the LTFT to make an adjustment to the negative? I'm still wondering if this should be a concern or if -10% is nothing to worry about?
 

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It's normal. 10% plus or minus is not even half of what the idle cells can handle. I'm not trying to scare anyone but you should see what some intake tubes will do to fueling. The measured airflow is the only input to WOT fueling, change the habitat of the MAF meter and fueling can change more than most would think. If you would like to temporarily disable the purge valve all you need to do is unplug it. I often do this when diagnosing lean or rich conditions. The purge valve creates a variable that will skew readings. My rule of thum is for every 2% of purge valve duty cycle you will decease the long term trims by by 1%. This is for idle calculations only as manifold pressure is lowest. Other variables exist but the major one is the EVAP system. If it was an issue you would have codes. P0171 and P0172.
 
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