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Discussion Starter · #1 ·
So I've been trying to learn more about my truck (07 sierra 5.3l NNBS 68K miles) and how to use my scan tool( ELM327 wifi scanner with iOBD2 App).
I was looking through the stuff I can read and saw fuel trim and it seems mine is out of spec. I'm not getting any codes and I don't see any symptoms of a problem, but I'm wondering if this is something to be concerned with and what to do next. I recently got the truck and in the past week have replaced the evap vent solenoid/valve, fuel tank pressure sensor, plugs, and wires. I was having an issue with hard starting after it sat for a while and the fuel pressure was dropping slowly with the key on engine off and pressure is stable when running, but I ran seafoam through the tank and the vacuum line to clean out the engine and hopefully clear what I thought might be some leaky injectors. The starting problem seemed to go away for now(hopefully forever), but I haven't had the chance to check my fuel pressure again. Now all i'm seeing is this fuel trim data and wondering what could be wrong and how to go about getting it back to where it needs to be. The attached pictures are the fuel trim and the o2 sensor voltage after the truck has warmed up. Any help is greatly appreciated.

EDIT: Also recently cleaned the MAF and Throttle body
Is it wrong to assume that since both banks are reading the same the issue is likely something that effects all cylinders and not something like a bad injector?
 

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First off, are these fuel trim readings at idle or while driving? But the -17 indicates that you are running rich, -25% is max and ideally you want to be close to 0% at all times. You mentioned that you were losing fuel pressure with the key on and the engine off. Leaky injectors could cause you to run rich for sure but there are many other possibilities as well. Your fuel pressure shouldn't be decreasing like that and I would start troubleshooting that first. I agree with what you said about it affecting both banks but you never know you might have more then one leaky injector. Currently I am dealing with running lean in my truck (+25% fuel trims) so i have been doing a lot of research on fuel trims haha.

I would look up the codes PO172 and PO175 and take a look at the common causes. I don't believe these codes would come on until you hit the max of -25% but you are getting close and the same would apply.
 

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Discussion Starter · #3 ·
chevra said:
First off, are these fuel trim readings at idle or while driving? But the -17 indicates that you are running rich, -25% is max and ideally you want to be close to 0% at all times. You mentioned that you were losing fuel pressure with the key on and the engine off. Leaky injectors could cause you to run rich for sure but there are many other possibilities as well. Your fuel pressure shouldn't be decreasing like that and I would start troubleshooting that first. I agree with what you said about it affecting both banks but you never know you might have more then one leaky injector. Currently I am dealing with running lean in my truck (+25% fuel trims) so i have been doing a lot of research on fuel trims haha.
Those readouts are at idle and I plan on getting a pressure tester on it again soon to verify my fuel pressure.
 

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You could try clearing the keep alive memory by disconnecting the battery and see if they go back to the same readings. The high trims could be a result of a dirty MAF and it hasn't had time to correct after you cleaned it.

With the trims being almost identical it is something affecting all cylinders, so an injector problem is unlikely. Same with the fuel pump bleeding down, that may give you starting problems but won't cause your trim problem. Fuel pressure too high could though, there may be a problem with the fuel rail pressure sensor or fuel pump control module.

You might want to look at the purge valve and see if it is leaking slightly, it should be completely sealed until the PCM commands it open. If it leaks at all the engine can pull fuel vapors from the tank and cause a rich condition. Disconnect the EVAP line and put your finger over the purge valve port at idle, there should be no vacuum.

One voltage reading from an upstream O2 sensor is useless. You need to look at them in graph mode, they should both swing from near .1V to .a bit over .8V. You can ignore the downstream ones for the purpose of figuring out your trims. If they don't go low enough or high enough, they could be getting lazy and cause the high trims.

The STFT should swing back and forth between + and -. If it stays more - than +, then there is a problem. Easier to watch on a graph as well. Pull a vacuum line off to create a vacuum leak and see where the O2 sensor goes to, it should go under .1V. Then do a snap throttle a few times to dump some fuel to it and it should go over .8V.

Start with that and see what happens.
 

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Discussion Starter · #6 ·
GMT604 said:
You could try clearing the keep alive memory by disconnecting the battery and see if they go back to the same readings. The high trims could be a result of a dirty MAF and it hasn't had time to correct after you cleaned it.

With the trims being almost identical it is something affecting all cylinders, so an injector problem is unlikely. Same with the fuel pump bleeding down, that may give you starting problems but won't cause your trim problem. Fuel pressure too high could though, there may be a problem with the fuel rail pressure sensor or fuel pump control module.

You might want to look at the purge valve and see if it is leaking slightly, it should be completely sealed until the PCM commands it open. If it leaks at all the engine can pull fuel vapors from the tank and cause a rich condition. Disconnect the EVAP line and put your finger over the purge valve port at idle, there should be no vacuum.

One voltage reading from an upstream O2 sensor is useless. You need to look at them in graph mode, they should both swing from near .1V to .a bit over .8V. You can ignore the downstream ones for the purpose of figuring out your trims. If they don't go low enough or high enough, they could be getting lazy and cause the high trims.

The STFT should swing back and forth between + and -. If it stays more - than +, then there is a problem. Easier to watch on a graph as well. Pull a vacuum line off to create a vacuum leak and see where the O2 sensor goes to, it should go under .1V. Then do a snap throttle a few times to dump some fuel to it and it should go over .8V.

Start with that and see what happens.
Cool, thanks for the tips I will try that stuff this week and see what happens. The STFT seems switch between +/- so hopefully it will just be a matter of resetting the memory. How long does it normally take to see a significant change in LTFT?
 

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GMT604 said:
If you clear the memory, idling for about ten minutes should tell you.
Should short term fuel trims and long term fuel trims be close to the same numbers? I haven't been able to find an answer to this question but aren't the long term fuel trims just an average over a longer period of times and the short term is real time data? Or am I thinking of this completely wrong? Its not making sense that my short term trims are bouncing around from 0 to +7 while my long term trims are +20 to +25. Maybe a reset is in order? I will fully understand these trims someday haha.
 

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Discussion Starter · #10 ·
chevra said:
GMT604 said:
If you clear the memory, idling for about ten minutes should tell you.
Should short term fuel trims and long term fuel trims be close to the same numbers? I haven't been able to find an answer to this question but aren't the long term fuel trims just an average over a longer period of times and the short term is real time data? Or am I thinking of this completely wrong? Its not making sense that my short term trims are bouncing around from 0 to +7 while my long term trims are +20 to +25. Maybe a reset is in order? I will fully understand these trims someday haha.
I don't completely understand them yet either, but what I got from what I've read is the long term trims are staying high one way or the either to keep the STFT at that low number to compensate for whatever the issue may be, but if everything is working correctly then they should all be close to zero.
 

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Discussion Starter · #13 ·
So I watched the videos and they were very informative but I wish he would've done more about running rich instead of primarily running lean. This morning I started it and took a look at the readings again. Once warmed up the trims went to about -18 and rose slightly if I got on the throttle. If I tapped it quick they would drop to -10 then go back to -18 but steady on the throttle at about 2k rpm they rise to about -18.8. Any thoughts? The fuel pressure at idle when I checked it last week was around 60 psi at idle.

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Try and see if ya can find any for your situation. I ran across these a while ago it does help but it does take some concentration :D Did you try and disconnect battery to reset fuel trims to get it to relearn?
 

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Discussion Starter · #15 ·
So I have been doing a little more research and found that the flex fuel cars sometimes have an issue with the fuel alcohol percentage reading high causing the fuel trim to run rich. I checked mine today with my scanner and it was showing 34.5% and I only run normal 87 octane with up to 10% ethanol so I need to reset the percentage, but my scanner can't do it. I tried asking a few places to reset it or allow me to and they were all saying they would have to do and charge for diagnostics to do a <5 minute job and some didn't know what I was talking about. Does anyone know of another way to reset it? Will disconnecting the battery for a given period cause it to relearn the percentage or is that stored?
 

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You can try a battery disconnect, I'm not sure if it is stored in volatile memory (erased on battery disconnect). If that doesn't work you need a Tech2 or other uplevel scanner to reset it.

Good work finding that, but you should have mentioned it was a flex fuel. It is probably the source of your high fuel trim readings.
 

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Discussion Starter · #17 ·
GMT604 said:
You can try a battery disconnect, I'm not sure if it is stored in volatile memory (erased on battery disconnect). If that doesn't work you need a Tech2 or other uplevel scanner to reset it.

Good work finding that, but you should have mentioned it was a flex fuel. It is probably the source of your high fuel trim readings.
thanks I'll try the disconnect. I didn't realize the flex fuel thing mattered until doing the research earlier today it really never even came across my mind. I'll definitely include it if I have issues in the future though.

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To be honest it slipped my mind too, and I should remember that because I have repaired a few with that problem. :lol:

Just update your signature that way it will always be there. Should add NNBS to it as well.
 
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