2018 5.3 fueling issue

[lsxwidopn]

I have an issue that I feel like some of you fellows on here would be knowledgeable about. I'm working on a 2018 Tahoe fueling issue and its left me scratching my head. It started off as a rough idle, then a day or two later got some codes for P2635, U0074, and P0300. After hours of trouble shooting, I have found that the low pressure fuel pump is suppling 80's-90 psi on the fuel line even when the PCM is commanding a normal pressure. The FPCM and the fuel pressure sensor on the fuel line have both been replaced. I have verified that the GMLAN circuits have continuity and ohm out correctly. From my understanding the FPCM controls the fuel pump by pwm. What could cause the pump to be basically running constant instead of being pulsed by the FPCM? I have HP Tuners and I'm able to read the fuel pressure, and can see the desired pressure from the PCM. Regardless of what pressure the PCM is commanding, the pressure stays in the upper 80's to 90 psi all the time. I just can't figure this one out. Any help on the matter is greatly appreciated! 

[garagerog]

Be easy on me here as I'm an old timer more comfortable with a timing light and dwell meter than I am with a code reader. Have you tried swapping out the fuel pump relay with a known good one? If the fuel pump is supposed to pulse on and off, I would surmise that a relay stuck on might possibly be an issue to address?

[lsxwidopn]

I took your advice that checked that out. The only relay I can find related to the fuel pump is a "fuel pump prime" relay. I swapped it out and it made no change. That was a very good thought! I believe this fuel pump is mainly controlled by the FPCM. Although the FPCM is new, I'm wondering at this point if it could still be the culprit. Something is making the pump run full steam. 

[newdude]

GM has a PI on P2635 and U0074:

 

MC-10156913-9999.pdf (nhtsa.gov)

 

They want the U0074 diagnosed first.  

 

DTC U0074 - Control Module Communication Powertrain Expansion CAN Bus Off Malfunction

 

Some other TSBs on possible wire open locations:

 

18-NA-144 1..4 (nhtsa.gov)

 

MC-10160659-9999.pdf (nhtsa.gov)

 

[lsxwidopn]

That is some great info. Thanks so much. It describes the exact issue I’m having. As I trouble shoot I will post my findings in case it could ever help someone else.

[lsxwidopn]

I have been trouble shooting and still trying to figure this one out. I have checked the ohms on the GMLAN data lines from the ECM to the FPCM. The data lines are showing the proper ohms on the obd port also.  I have yet to find a break in any wire. I have checked resistance on the data lines and also checked them to ground. I still haven't been able to track this one down. Its a head scratcher for sure. 

[rlcole321]

time to go back to "Old School", use a DVM to check power at the power wire going to the fuel pump, use DC first to see if it is a solid 12 vdc, then check on AC (unless you have a dwell setting on your DVM) , if PWM then it should read an AC voltage, if no PWM then it will read 0 v. Then you can go from there, may have a short to power on the FP wire.

[lsxwidopn]

After more trouble shooting it turned out to be a bad ECM. On this model, the Data lines run directly from the ecm to the fuel pump control module. Installed a new ecm and everything is back to working properly. Thanks for the help guys, and hopefully if anybody ever has a similar issue it will be beneficial for them also.