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Roy
Roy, import/domestic specialest
Category: Chevy
Satisfied Customers: 5719
Experience:  ASE master tech for 40 years
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Low fuel pressure on 2012 Silverado. P171 and P174 codes

Customer Question

Low fuel pressure on 2012 Silverado. P171 and P174 codes lean left and lean right bank. Dealership told me the fuel pump was bad. I replaced and it had no effect Truck runs good, mileage is single digits. I felt like it might not be the pump but a regulator (didn't know my truck didn't have one at the time). I found out that it didn't have one so that was why I went ahead and replaced the pump.
JA: Has your car been stalling?
Customer: The weird thing about it is static pressure will be at least 65psi. When started, it will maintain a steady 55psi for approximately 7 seconds with the engine warm. Then it will drop to about 41psi and stay there.
JA: What is the make/model of your car?
Customer: No 2012 Chevy Silverado 5.3 Vin 7
JA: Are you fixing your Silverado yourself? What have you tried so far?
Customer: Checked fuel pressure, checked for vacuum leaks, and replace my fuel pump assembly.
JA: Anything else you want the mechanic to know before I connect you?
Customer: How does the truck maintain fuel pressure with this system? If this is a no-return system, as I see no return from the engine, then why are there 3 lines coming from the top of the fuel pump assembly?
Submitted: 8 months ago.
Category: Chevy
Expert:  Roy replied 8 months ago.

hello, my name is ***** ***** I will be glad to assist you. please allow me a couple of minutes to gather some basic info.

Roy

Expert:  Roy replied 8 months ago.

ok checking the pressure needs, it shows 45 lbs is required at idle. 41 is too low and will cause the lean codes as there is not enough fuel being delivered to the cylinders.

Roy

Customer: replied 8 months ago.
Why is my fuel pressure low?
Expert:  Roy replied 8 months ago.

most likely the pump. it is a returnless system.

what brand did you use on your replacement???

Roy

Customer: replied 8 months ago.
I checked pressure both with the old pump and with the brand new Delphi pump. They read identical.
Expert:  Roy replied 8 months ago.

ok. then try clamping off the line at the injectors to verify you do not have a leaking injector. if you did, it would add oil to the crankcase and make the oil smell like fuel. if all that is ok, try replacing with an OEM original pump.

Roy

Customer: replied 8 months ago.
Everything I've read online shows 55-62 psi specs. FWIW. That's what made me suspect the pump might actually be good before I replaced it. It's almost like it operated fine in open loop initially but switched over to closed loop and maintained a different pressure. Delphi and Delco are practically the same thing. It is described as original equipment. What line am I supposed to clamp off? The injectors are attached to the fuel rails. I watched the fuel pressure for several minutes after switching the ignition off and there was no pressure drop.
Expert:  Roy replied 8 months ago.

this pump pressure varies by load. key on engine off should be 50-100 lbs.

t idle it needs to be 45 lbs. that are the specs.

delphi and delco are close but they are not the same. they are different internally.

Roy

Customer: replied 8 months ago.
What can I check to make sure it is indeed the fuel pump? I'm not familiar with the no-return systems. Could it be related to the fuel pump driver module? Is there a fuel pressure sensor?
Expert:  Roy replied 8 months ago.

where are you checking it now???

have you checked it driving under load???

module is part of the pump

yes, but manual check is the best. the sensor if it failed will not cause the code.

Roy

Customer: replied 8 months ago.
I am checking it at the Schraeder valve on the passenger fuel rail with a quality test gage. The test gage will not reach inside the cab so I've been checking it parked. I have had an assistant revving the engine in various ways. With the exception of minor blips with near instant recovery when making rapid throttle changes, it stays steady. Maybe I can try brake torquing to at least get a minor load on it. I'd be curious to hook it up to a good scanner that can read the fuel pressure and see if it's accurate. What are your thoughts?
Expert:  Roy replied 8 months ago.

gauges are the most accurate. I never trust the sensors for an accurate reading. been fooled too many times

Roy

Customer: replied 8 months ago.
I'm wondering if the sensor might be off that it might be thinking it's regulating at a different pressure than what it is in reality. Does this truck have a "limp home" mode that would run the fuel delivery circuit in open loop?
Expert:  Roy replied 8 months ago.

no, the codes are set based on long trim readings, not by fuel sensor. as I said, the sensor is not an issue with the code.

Roy

Customer: replied 8 months ago.
Okay, any chance it could be related to the FPDM? I just want to be sure before I buy another fuel pump. And, I'm going to find a way to monitor the fuel pressure under load. And I'm going to drive it to see if the codes pop back up. And, I just thought of this or would have mentioned this at the first, but the codes only popped up while running E85, which I run primarily since it is over a dollar cheaper where I live.
Expert:  Roy replied 8 months ago.

yes, e 85 is not good. try switching back to regular fuel

e 85 will not effect fuel pressure at all.

Roy

Customer: replied 8 months ago.
I specifically bought the truck to run E85 so as a long term solution, that's not good. I was able to finagle the gage where I could see it from inside and did a road test. No load, low load, high load the fuel pressure stayed steady at 41psi. When under WOT only, the pressure would kick up to around 52psi rock steady. I tested this several times to verify and it was definitely dependent on the pedal being all the way to the floor. When I pulled back into the driveway and parked, I tested this again and WOT would send the pressure up past 50 after which the rev limit would kick in. Interesting...
Expert:  Roy replied 8 months ago.

yep

what are the long term trims??

Expert:  Roy replied 8 months ago.

please dont forget to rate me as it is the only way I get paid for my efforts

Roy

Customer: replied 8 months ago.
Long term trims are 19.5 and 18.0
Expert:  Roy replied 8 months ago.

should be around 6-8 running to lean

Customer: replied 8 months ago.
do you have any further recommendations or are you still convinced it's the fuel pump?
Expert:  Roy replied 8 months ago.

the pressures have to be correct before looking elsewhere for another area of failure

Roy

Customer: replied 8 months ago.
Okay, what does that mean? Obviously, the pump itself is capable since the pressure jumps up at WOT. The Fuel pressure read by the truck was within 1psi of the gage. What is telling the pump to only put out 41psi? Is that part of the fuel pump assembly, the driver module, ecm...?
Expert:  Roy replied 8 months ago.

it is the whole unit. contains pump and driver.

Customer: replied 8 months ago.
The driver is a separate module. You can see it next to the spare tire.
Expert:  Roy replied 8 months ago.

I hear what you are saying but without the pressure being up to and beyond minimum specs, it will show lean codes as you have.

Customer: replied 8 months ago.
I hear what you are saying as well. I want my fuel pressure where it should be. The ECM must be commanding higher fuel pressure at WOT which the pump responds to so there has to be something besides just the pump assy that controls it. I'm going to put in another pump, I was just hoping to get some better options for diagnosis.
Customer: replied 8 months ago.
It's fixed and I didn't change out the fuel pump.