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Discussion Starter · #1 · (Edited)
So I have a 2016 Malibu Limited with a 2.5l the throws a p2635 and completely fails to restart after an auto start/stop routine. We followed the TSB for replacing the in tank fuel pump and have run nothing but 93 octane. Continuity looks good at the rail pressure sensor. I plugged in the scan tool and noted a primary fuel pressure of 80 psi which grew to 95 psi as the car idled over time. Fuel rail pressue at idle was around 450 psi and 850 psi at 1500 rpm. Commanded vs Measured appeared close based on my visual analysis of the sensor feedback. Is there a secondary high pressure pump that could be failing? What else should I be looking at?

P.s. also cleaned the MAF to see if conditions improved...they did not.

Also, I just realized that like an idiot I posted this under gen 8 rather than gen 9. Is there an admin that can move this to the proper area. 🤦‍♂️
 

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You posted in the correct section. Your 2016 Limited is a hold-over Gen8 for sale when the new Gen9 2016 models arrived. GM has used Limited and Classic several times now.
 

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why do you use 93 octane with the 2.5 engine???...how many miles are currently on your 2016???...good luck with your car

Bill
 

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Discussion Starter · #4 ·
why do you use 93 octane with the 2.5 engine???...how many miles are currently on your 2016???...good luck with your car

Bill
Supplemental TSB suggests running octane levels higher than 91 as well as advising the owner to reduce long periods of idle IF the fuel pump replacement alone does not remedy the situation.

Approx 120k miles on the clock.

Thanks!
 

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The bulletin referenced is here: https://static.nhtsa.gov/odi/tsbs/2017/MC-10114082-9999.pdf

Another bulletin of interest here: https://static.nhtsa.gov/odi/tsbs/2018/MC-10147348-9999.pdf

Based on the reading of the first bulletin, it looks like GM acknowledging a habit of the fuel system and trying heat reducing workarounds. It remained a preliminary information and never became a standard bulletin. It also is a generic information that includes every car and every powertrain from the Corvette to Silverado to Tahoe to Cruze. That usually means numbers of affected cars remained low or they never figured out a certain cause and solution.
 
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Discussion Starter · #6 ·
The bulletin referenced is here: https://static.nhtsa.gov/odi/tsbs/2017/MC-10114082-9999.pdf

Another bulletin of interest here: https://static.nhtsa.gov/odi/tsbs/2018/MC-10147348-9999.pdf

Based on the reading of the first bulletin, it looks like GM acknowledging a habit of the fuel system and trying heat reducing workarounds. It remained a preliminary information and never became a standard bulletin. It also is a generic information that includes every car and every powertrain from the Corvette to Silverado to Tahoe to Cruze. That usually means numbers of affected cars remained low or they never figured out a certain cause and solution.
Yep, those are the specific ones we reference. The issue primarily started during extended periods of high ambient temperature so we started here.

After a little more driving the car has now thrown a P0506, perhaps unsurprising as the engine stumbles a bit during the auto on/off cycle. RPM seems a bit unstable when we can get it to idle as well.
I haven't rescanned after the in tank fuel pump replacement, but prior to replacement we were seeing about 80-90 psi from the tank, commanded rail pressure was around 450 psi and delivered pressure seemed to bounce between 440-480 psi. When I commanded 1500 RPM I'm seeing that commanded value pickup to about 800ish psi. I'm not very familiar with these 2.5l, do those values sound correct? The DI engines that I have worked on in the past seemed to have a much higher rail pressure.
 

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Alldata has a large page on diagnosing the P2635. Including what and if some other codes including U codes are present. I'm unable to post it here though.
Looks like a go through the manual steps diagnosis job.
 
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Discussion Starter · #8 ·
Alldata has a large page on diagnosing the P2635. Including what and if some other codes including U codes are present. I'm unable to post it here though.
Looks like a go through the manual steps diagnosis job.
Thanks for the heads up. Out of curiosity are you using the standard AllData or DIY?
 

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Thanks for the heads up. Out of curiosity are you using the standard AllData or DIY?
DIY for every vehicle in my fleet. I don't think there is a difference as DIY is very intense for the average person.
 
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Discussion Starter · #10 · (Edited)
Update: it's fixed

Condensed view of Symptoms: P2635, rough/unstable idle, occasional hesitation coming off idle, auto start/stop fails to stay in the stop mode (goes directly to off at operating temp), poor fuel economy, although the engine has not overheated the engine seems to heat up ridiculously fast (this is certainly subjective but compared to others I've worked on this was noticeable to me), plugs 1 and 3 showed signs of ash while 2 and 4 appeared "normal", fuel pump pressure running around 90+ psi at idle while DI rail pressure ran 450ish at low idle.

The final fix: replace Fuel Rail Pressure Sensor, clear DTCs, and reset fuel trims

Owner feels the car picked up considerable torque and we'll be looking for an improvement in fuel economy now that she's being fueled properly.

Thanks everyone for the help.
 
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