2012 Chevy Equinox 2.4L Rich Condition, Stalling at Stops (Fixed It!)

2012 Chevy Equinox 2.4L 66,000 Miles on Odometer

Symptoms: Hard to start after stall event, Engine Reduced Power Mode, Vehicle stalls at a stop

Codes:  ECM-P0036 (Heated Oxygen Sensor Heater Circuit Sensor 2), P0138 (Heated Oxygen Sensor Circuit High Voltage Sensor 2),
P0140 (Heated Oxygen Sensor Circuit Insufficient Activity Sensor 2), P0141 (Heated Oxygen Sensor Heated Performance Sensor 2) 
P0172 (Fuel Trim System Rich), P0191 (Fuel Rail Pressure Sensor Performance) 

EBCM - C0800 (Module Low Voltage), U0401 (Invalid Data Received from ECM)

BCM - U0164 (Lost Comm. with HVAC Module)
Other Low Voltage Codes set in other Modules

Repairs the Customer has made on their own:
Intake and Exhaust Solenoids (NOT OEM)
High Pressure Fuel Pump (Pump is NOT OEM)
Fuel Rail Pressure Sensor (NOT OEM)
Oil Change after Original HPFP Failed

Repairs/Tests I've Completed for the customer already:
Replaced Battery - Vehicle would not crank over. Performed Battery Test- Failed
Replaced Exhaust Manifold at customer's request due to noisy exhaust (Codes listed above were present before this repair and I informed the customer that an exhaust leak is not likely the cause and would need diagnosis. After Exhaust Manifold was replaced, I cleared codes and test drove vehicle. Vehicle drives amazingly under throttle until I come to a stop and Desired Fuel Rail Pressure drops to 0 and engine eventually hesitates and stalls)
Performed Parasitic Draw test (Customer Concerned about Dead Battery) - After 10 mins draw would drop to 8mA and stay
Fuel Pressure Drop Test - Key on/Engine Off, fuel pressure remained at 61 PSI no drop in pressure at all
Checked for stuck open EVAP Valve - Not Stuck Open
Monitored O2 Sensors during idle found sensor 1 is fluctuating normally between 200mV and 800mV. Sensor 2 hangs around 800-900mV steady after warmup and no test drive just idling.
After Test Driving and symptoms are replicated O2 Sensor 1 is reading rich around 900+mV and sensor 2 around 950+mV. When pressing the brake pedal rapidly, the idle increases and O2 sensors drop quickly and are reacting. LTFT -25% STFT -27%
Checked for smell of Fuel in Oil - No fuel could be detected

In this picture below is when I've come to a stoplight and the Desired Fuel Rail Pressure commands 0 PSI, and the Fuel Rail Pressure follows eventually until the engine stalls. 

 

 

In these pictures are when I completed an entire drive from home to work, with the symptoms listed above while sitting in the parking lot at work.

 

 

 

I'm looking for a good direction to go and if anyone has had this issue before. I know I've NOT checked the integrity of the O2 Sensor 2 circuit yet. I find it odd that at stops the Desired Fuel Rail Pressure is commanded to 0 PSI and then the Fuel Rail Pressure Sensor drops until about 290 PSI and engine stalls. I'm lost honestly. I appreciate any help!

Sounds like a strange problem you've got there. Do the desired and actual fuel pressure PIDs generally match before it commands fuel pressure to zero?

Hey Noah thanks for checking out my post. I've come to these results so far after further diagnostics. I've tested all fuel injectors circuits and they are good. I've also tested the Fuel Rail pressure circuit from end to end and load tested the circuit. All is good. I've also verified the integrity of the High Pressure fuel Pump Circuit from end to end and load tested the circuit. All is good. The only thing I've had trouble proving is why the HPFP is not doing what it is supposed to do. After further research I've found that the HPFP and fuel rail pressure sensor the customer installed are the brand "Standard" which is aftermarket and are not reliable. I got the  original OEM high pressure sensor back from the customer and reinstalled it. I then checked the high pressure sensor was reading accurately by commanding the in-tank fuel pump off with engine running to watch the fuel rail pressure sensor drop to zero. When I did this test the fuel rail pressure sensor dropped to 14.7PSI due to atmospheric pressure meaning the sensor is reading accurately with no fuel pressure on my scan tool. With pump enabled back on the low side pressure read 60PSI and the fuel rail pressure sensor read 58PSI further confirming there was no issue with the OEM sensor.  Moving on i then gathered more data and research and found this on Pro Demand describing how the HPFP works and is commanded by the ECM. 

The ECM is giving the correct command but I believe the HPFP is not following this command given that it is aftermarket and its internal build with the pressure regulator and relief are not up to OEM Specifications. However I do not know how to prove or test this but I have an OEM HPFP arriving tomorrow and will let you know the results as soon as possible. I've also submitted a post on a group I am in on Facebook .

This technician suggested that the HPFP is being commanded to 0PSI when coming to stop but is not following the command and is then running rich. This explains my symptoms and data collected perfectly. Whether this is the reason or not I do not know but I am replacing the HPFP with an OEM GM Genuine Pump based on my data and research.

After Replacing the High Pressure Fuel Pump the vehicle now runs good and no codes returned. The ECM is giving the command 0 PSI on the Desired fuel rail pressure and the actual fuel rail pressure drops to 70 PSI matching the low side pressure from the in-tank fuel pump. The old high pressure fuel pump would only drop to 290 PSI which explains why the engine would stall or go into limp mode. The rail pressure was so high and not dropping causing the engine to be flooded with fuel which also explains the -25% fuel trims and the O2 sensor readings of 900+mV. So there's 3 things that could have failed on the High pressure fuel pump - the inlet valve, pressure regulator, or the relief valve. Which one failed I do not know for sure but it's obvious that the high pressure fuel pump was not doing what it was commanded to do. I hope this data and information was helpful. Thanks Noah for checking out this thread and following up with me.

Thank you for the update.
I guess I was misunderstanding what the "0psi commanded fuel pressure" PID was all about. It looks like it's not really trying to make the pressure in the rail 0 psi, but rather 0% command for the high pressure fuel pump.
I'm glad to see you got it fixed, nice job.