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Low fuel pressure

Tiny
BLAZ85
  • MEMBER
  • 2008 CHEVROLET SUBURBAN
  • 5.3L
  • V8
  • 4WD
  • AUTOMATIC
  • 150,000 MILES
My son has the above vehicle which is flex-fuel but only uses E-10 fuel. When scanned it shows the codes for "lean both banks" but there is no "check engine" light. We can clear the stored codes and they will come back but still no "check engine" light. We checked for vacuum leaks but found none. We then checked the fuel pressure using a mechanical gauge. Key on-engine off, we have 60-62 PSI. When the engine is first started, we have 60 PSI but within a few seconds, the pressure starts to decline and stabilizes at 46 PSI where it remains. We also checked the pressure with a scan toll and got 45 PSI almost identical to the mechanical gauge. The vehicle starts readily and seems to run fine with no stalling nor misfires.

I have some questions: 1. Why do we not have a "check engine" light when we have trouble codes stored in ECM memory? 2. Since this is a "return-less" system is the pressure regulator/sensor inside the tank as a part of the fuel pump module? How does it work? 3. Can the sensor be reprogrammed using the correct scan tool to bring pressure up to proper specs seeing as how the pump is capable of 60 PSI? 4. Is it just the pump getting weak? 5. What would be the long-term effects of continuing to drive it with the 46 PSI fuel pressure?

Thank you in advance for your reply. Also thanks again for all the help in the past.
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Sunday, December 5th, 2021 AT 9:53 PM

9 Replies

Tiny
KASEKENNY
  • MECHANIC
  • 18,907 POSTS
More than likely what is happening is the code is in pending status and your scan tool is just not displaying it this way. So, this way you are getting the code there when you check it, but the light is not coming on but if you let the code there it will eventually turn the light on.

Most secondary codes require multiple trip failures in order to set the light. That means you need to have the issue and then when the engine shuts down and starts again it needs to see the same issue and then it will turn the light on.

So here is where we need to start. What is the actual code that you are getting, and we can run through the testing for this code and go from there?

https://www.2carpros.com/articles/checking-a-service-engine-soon-or-check-engine-light-on-or-flashing

I am also attaching the info you requested on this system but basically it is just an electronic regulator that allows pressure to bleed off depending on what the ECM is commanding.

So, let's start with the code and then we can get the testing or a better idea as to what the issue is.

Thanks
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Monday, December 6th, 2021 AT 5:13 PM
Tiny
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Thank you for your quick reply and for the information on how this system works. I better understand it now. The codes were P0171 and P0174 denoting a lean condition on both banks. Our first thought was vacuum leaks. We checked all hoses, air ducts and even intake manifold gaskets but found no leaks. That is when we checked the fuel pressure and found what was described earlier, 46 PSI. Again, I have questions. 1. Why would the ECM command 46 psi to the control module if it is seeing a lean condition on both banks? 2. Could some other parameter be causing the ECM to command the lower pressure which in turn causes the lean conditions?

Hope this is the information you needed and will await you reply.

Thanks!
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Monday, December 6th, 2021 AT 8:47 PM
Tiny
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Okay. So, to check the vacuum leak did you use the parts cleaner method? Meaning did you spray these areas down and listen for the rise in idle?

Here are guides that will get us going on these issues:

https://www.2carpros.com/articles/repair-lean-mixture-codes-p0171-or-p0174-on-some-manufacturers

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

Low fuel pressure will cause this, but it is unlikely that the ECM will command lower fuel pressure like this especially since the ECM knows that it is lean. That means it is going to command more fuel.

So, this means we need to check two things. First, we need to check the o2 sensor data to see if they show it is lean as well which should be where the ECM is getting this info from to set these codes. We just need to confirm the o2 sensors are truly telling the ECM that it is lean.

Next, we need to check what the ECM sees as the fuel pressure.

If the ECM data shows the fuel pressure as something different than what the gauge says when you check it then that would explain why the pressure is low, but it is not compensating for it. In other words, if the ECM is being told the fuel pressure is 60 PSI, then it is not going to adjust it to compensate for the lean condition. So that would mean the fuel pressure sensor is the issue. We will need a scan tool that can monitor live data in order to know this.

Once we know this info, I am sure we will figure this out. Thanks
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Tuesday, December 7th, 2021 AT 11:48 AM
Tiny
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To answer your questions, on the vacuum leaks we did use carb cleaner around hoses, etc, and we used water around the intake gaskets. One thing we did not check was to see if the purge valve was leaking but it shows no Evap codes. As for the fuel pressure, when checked with a mechanical gauge it shows 46 PSI after running for approximately 20 seconds. We did check the pressure with a scan tool, and it showed 45 PSI under the same conditions, So the two pressures are almost identical. We did not check the O2 sensors.

It may be a couple of days, vehicle is not at my location, but we will do the following, recheck fuel pressure with scan tool, get the O2 sensor data and fuel trims. Then we will get back with you and proceed.

Thanks!
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Tuesday, December 7th, 2021 AT 4:16 PM
Tiny
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Okay. That is great info. However, if when you checked the fuel pressure with the gauge and scan tool, and they were the same then that is good.

You are correct that the purge should be checked however, if this is stuck open it may not set codes. It can but I have seen many times when they don't. If they are stuck closed, then they normally set a code but that is when it would cause a lean condition. If it is stuck open, then it would be rich.

So, the fact that the ECM is commanding the 45 PSI, I suspect that is fine. It is possible to be an ECM issue commanding low pressure but that is not likely.

I suspect we have another issue causing this so you may want to use carb cleaner on the intake manifold as well just to double check.

Then if we can get the o2 data like you stated, that will at least help confirm it is actually running lean.

Let us know when you get to it. We will be here whenever you get back to us.

Thanks
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Tuesday, December 7th, 2021 AT 6:19 PM
Tiny
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I finally got around to checking some more. The vehicle is still showing the P0171 and P0174 codes as "pending". Strange since the vehicle has been driven over 500 miles (numerous starts) with these "pending" but still no "Check Engine" light. I checked to make sure the bulb was good. The light comes on when the ignition is turned on and goes out shortly after the engine starts.

I checked the live data from the O2 sensors with engine warmed up and in closed loop, The voltages were jumping around constantly. Are they supposed to be doing that?

At idle, bank 1 was.02-.62 volts, bank 2 was.08-.66 volts. At 2000 RPMs, bank 1 was.085-.760 volts. Bank 2 was.095-.785 volts. Fuel pressure with the scan tool was 45 psi (46 psi with mechanical gauge). I also checked air flow at MAF.58 lb/min at idle and 1.94 lb/min at 2000 RPMs.

Hope this is what you need and will assist in the diagnosis.

Thanks!
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Tuesday, December 14th, 2021 AT 8:17 PM
Tiny
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Yes. That helps. I am not sure I asked this because I didn't see the info but maybe I missed it.

Is there any issue with the vehicle performance wise other than these codes are there pending when you scan it?

If there are no issues with the way the vehicle operates then I suspect this is a scan tool/control module issue. Sometimes these scanners will pull a code that is not actually there.

So, all the data that you found looks perfect. The o2 data is supposed to switch rapidly because what you are seeing are the constant adjustments that the PCM is making to achieve perfect air fuel ratio.

It is not possible for the PCM to just dump the proper amount of fuel and achieve the correct ratio. So, think of it like it is a dog chasing its tail. It is constantly looking at the exhaust and the PCM makes minor adjustments to rich and lean to try and achieve that perfect ratio. So, the dog keeps chasing it try to catch it but never does but he keeps trying.

So, if it were me, and there are no issues with the way the vehicle runs, I would ignore those pending codes until they actually set but I don't think they will.
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Wednesday, December 15th, 2021 AT 1:44 PM
Tiny
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Thank you for your reply. The vehicle runs just fine. It starts immediately, is powerful, no misfires, no stalling nor hesitation. We actually found these "pending" codes while "messing around" with a new scanner. Out of curiosity, I will check it with a "plain Jane" code reader I have and see what I get. It may be a while, but I will let you know the results.

Thank you so much for your help. It is good to know we do not have a "lean" condition and I now have a much better understanding on how the systems on this vehicle work. We will just continue to drive it and see if we get an actual "Check Engine" light. Thanks again.
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Wednesday, December 15th, 2021 AT 9:09 PM
Tiny
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Sounds great. Also, just a little more info on those voltages.

.5 volts is ideal air/fuel mixture. The lower the voltage gets down to around.2 volts is lean and higher to around.8 volts is rich.

So, if the voltage stays under.5 volts, then that points to a lean condition. However, just because you see.2 volts doesn't mean it is lean. It needs to remain low but if it bounces around then that is normal.

Thanks for the update. No problem with the amount of time you need, this post will remain here, and we will get back to you then. Thanks again.
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Thursday, December 16th, 2021 AT 7:18 AM

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