Chevy 4.8L LY2 with Lean Codes
This truck runs and drives well. There are no misfires stored. When the truck is first started the fuel pressure is at 60-65 psi, after about 30 seconds the Fuel Pump Driver Module commands 50% and it sits at 43 psi all the time after that. Confirmed that the fuel pressure reading from the sensor is accurate with a manual gauge. Fuel trims are 5% & 9% at at idle as soon as I touch the throttle the jump to 14%-25% on LTFT. When release the throttle they go right back to single digits. MAF is reading 5.6 g/s at idle, A/C off, which seems a touch high but not bad enough to condemn it. This is a non-flex fuel truck, but to be certain I added half a tank of fuel and drove it home and back after work one day (50 miles total) with no change. I'm wondering if anyone know what strategies GM would have programmed to command the fuel pressure down to 43 psi? What kind of fault would cause it to do that? And I can clear the codes and the fuel pressure issue exists before they set again, as well.
Fuel pressure at idle does not help you with this. You need to see it under demand/load to verify if the pump is keeping up or not.
I didn't clarify well enough in my original post, the fuel pressure is always 43 psi; loaded, idle, A/C on, A/C off.
Jordan, by demand/load I mean WOT driving. If pressure keeps up under that condition then you do not have to worry about pump, filter, connections, or that it is a fuel supply problem.
Right on, David. We don't even need a pressure gauge reading because positive fuel trims on a marginal fuel pump would go very high at WOT.
Perform a volumetric efficiency test. MAF can read slightly high at idle and low under load if damaged or dirty. If no fuel pump codes would start there. Fuel doing what commanded.
Jordan Great information. I want to make sure I didn’t miss something. Under freeze frame for the codes were they set under cruise conditions? What RPM and load? Do you have a donor vehicle you can borrow a MAF? I’ve offered another customer a free oil change and let them know I was going to play “swaptronics” then I try different parts. Just throwing out ideas
I don't remember the freeze frame now, I do have it saved though. I had already cleaned the MAF, readings didn't really change. Did finally ask my parts manager for a MAF we could return, because I couldn't think of anything else. He got a Standard (said Delphi Technologies when I pulled it out) and the truck is fixed now. Be posting the update soon. Thanks for the help.
Hello Jordan, Check the inside of the driver side frame rail to see if it has an inline fuel filter. They were still using them on some models back then. If it has one, replace it. Then verify there are no air leaks after the MAF. Next, I recommend cleaning the MAF and then resetting the computer. From there perform a test drive sequence according to the service information and see where it is…
Fuel volume sounds likely. Could be an extremely plugged fuel filter.
No external fuel filter and fuel pressure and volume are good, just seems low for a Vortec and all the data I find says 55-60 psi.
Check the inside of the driver side frame rail to see if it has an inline fuel filter. Good advice, Glenn. A friend's 2004 Tahoe is flex fuel and does have a fuel filter wedged between the cross member and ethanol sensor.
Glenn, did you actually watch the video? I suggest giving him a break and allowing him to grow before appearing in a professional forum.
Hello Jim, I did watch the video, it was not perfectly edited but I thought it related to the subject.
Hi Glenn It wasn't a matter of poor editing. I met him at a training event some years ago. He's a good guy and needs some space to grow without exposure in professional forums. He's trying to do the YouTube “thing” and that is a lot of work in and of itself. I'm happy for him. Based on the video, he doesn't understand how the fuel system works, how to test it, got lucky, and gave bad…
Hello, Jordan. Are you absolutely sure that this truck hasn't had some E85 added to the existing fuel?
Does the command for pressure change? If so, and the pump does not deliver, well that could cause you to be lean. Reset the LTFTs and see what correction is made by the STFTs, just as a check as to whether the trims are learned bad, or there is an actual lean condition. From what you state, the trims are lean/additive even after many miles, correct? Does the STFT ever go negative? The strategy…
Resolved this one. It was a MAF, it was actually under reporting so lean makes sense. I had tried running it with the MAF unplugged and it looked like my LTFT readings were the same, but I think my test drive was just too short. Apparently the 43 psi is just how this truck is. Still confused by that because it doesn't match any data I found but my fuel trims corrected within ¼ mile of driving…
Glad you got it fixed Jordan Unplugging the MAF does not always help with these as some would think, but I believe a 3-minute test drive looking at the right data under the right conditions would have given you what you needed. I routinely did (and documented) it for decades. I also just finished a scan tool class that begins the exploration of what is possible in this area regardless of the…
I agree, that the disconnect method, is no longer the great diagnostic tool it once was…what with ETC, and the different strategies that the OEMs use when the signal disappears, or is not there on start-up. It still has some validity, but is only part of the diagnostic ‘suite’ of tools needed for diagnosing some things.
Had several GMs lately that I really wanted to key in on the MAF readings, but it so hard to find known good information. I'm starting a database of MAF, load, RPM, etc. data so I have something to compare to. I appreciate your offer and will save your info. I may reach out to you with some captures to get your feedback. I hate swapping parts, I want to be able to prove it. Which PIDs do you…