Running Rich After Injector Replacement - SOLVED

[Kwing]

My '07 Tahoe 5.3L FFV with 300k on the clock had a chronic misfire problem, which I narrowed down to the injectors (still the original injectors, which I was impressed by, LOL). Since I'm 6-months unemployed, I bought the new injectors on Amazon for $100 (for all 8).

First drive after the swap, misfire was definitely gone, but MIL came on for rich mixture on both banks. I've spent so many hours looking at telemetry data from my HP Tuner at this point and trying to reason my way through the problem that I'm pretty sure I've totally lost the plot entirely. The only consistency I can see is that the STFT and LTFT are constantly more than -18% and the LTFT spends most of the time pegged negative.

I can't find any air leaks, fuel pressure is steady as a rock at 58 psi, TPS and APS data looks clean, and given the raw fuel I can taste dumping out the pipe, I feel confident saying it's not false O2 readings. I've tried re-establishing the Delta P vs Injector Flow table based on the data, which seemed to mildly improve it, but not enough to make a difference.

Any suggestions on what to look at? Or anyone know a lot about the data than I do to try and get to the bottom of it?

 

[swathdiver]

My '07 Tahoe 5.3L FFV with 300k on the clock had a chronic misfire problem, which I narrowed down to the injectors (still the original injectors, which I was impressed by, LOL). Since I'm 6-months unemployed, I bought the new injectors on Amazon for $100 (for all 8).

First drive after the swap, misfire was definitely gone, but MIL came on for rich mixture on both banks. I've spent so many hours looking at telemetry data from my HP Tuner at this point and trying to reason my way through the problem that I'm pretty sure I've totally lost the plot entirely. The only consistency I can see is that the STFT and LTFT are constantly more than -18% and the LTFT spends most of the time pegged negative.

I can't find any air leaks, fuel pressure is steady as a rock at 58 psi, TPS and APS data looks clean, and given the raw fuel I can taste dumping out the pipe, I feel confident saying it's not false O2 readings. I've tried re-establishing the Delta P vs Injector Flow table based on the data, which seemed to mildly improve it, but not enough to make a difference.

Any suggestions on what to look at? Or anyone know a lot about the data than I do to try and get to the bottom of it?

I would put the old injectors back in 1 at a time until you get a misfire if you did not know which cylinder was misfiring in the first place.

What's your alcohol content? High negative trims at idle is often a sign of intake gasket leaks. You can try tightening the intake bolts at first before spending money on new gaskets.

 

[Fless]

I would put the old injectors back in 1 at a time until you get a misfire if you did not know which cylinder was misfiring in the first place.

What's your alcohol content? High negative trims at idle is often a sign of intake gasket leaks. You can try tightening the intake bolts at first before spending money on new gaskets.

^^ THIS. The injectors were very inexpensive and likely not worth the money spent. A $12 or $13 injector is not going to work properly or meter fuel correctly. Over-fueling is going to ruin the cats.

Verify that the alcohol content that the scanner reads is close to the fuel alcohol % that is in the tank. If not, reset the alcohol value with a scanner.

I'm curious -- how was it determined that the OE injectors were bad? Injector balance test? Scan tool looking at individual cylinder misfires? Or??

 

[Marky Dissod]

Since I'm 6-months unemployed, I bought the new injectors on Amazon for $100 (for all 8).

Here's the problem.
In Spanish: Lo barato, sale caro ... cheap turns out to be expensive.

Most cheap@$$ injectors:
are likely not actually octuplets (bad quality control, they do not behave identically)
are significantly different from what the tune expects

If the pcm gives two different injectors the same 'instructions', they will not behave the same.
If the injectors are not exactly what the pcm's expects, both initial fueling and further corrections will be skewered.

 

[Kwing]

Thanks for the replies! Some day I will remember to set the watch thread when I post, but it's anyone's guess WHEN I'll do that, LOL. I'll try to chip through the responses here...

I would put the old injectors back in 1 at a time until you get a misfire if you did not know which cylinder was misfiring in the first place.

I'm curious -- how was it determined that the OE injectors were bad? Injector balance test? Scan tool looking at individual cylinder misfires? Or??

All 8 were missing intermittently/randomly, but #6 and #4 were pretty much dead stick - based on live-read data from the HP Tuner. Having two standouts allowed me to move the plugs, coils, wires, and injectors one-by-one (from 4 to 2 and 6 to 8, then swapped 8 and 2 just to double verify) until my miss followed the moved part(s)... which happened reliably with the injectors.

What's your alcohol content?

Verify that the alcohol content that the scanner reads is close to the fuel alcohol % that is in the tank. If not, reset the alcohol value with a scanner.

The ethanol % (for this tank) reads a rock-steady 12.15% across all the data sets. The question is, how close does "close" need to be? For the next week, the blend in Arizona should be 10%, after which it should roll over to 15% until November. What the tolerances on those blends are, or on the FFV sensors/systems, or for that matter how far it can be off before it starts causing problems, are all things I don't know.

High negative trims at idle is often a sign of intake gasket leaks. You can try tightening the intake bolts at first before spending money on new gaskets.

An air leak is the first thing I thought would be at the root of it, but I can't find one. I just did a smoke leak-detection on it a couple months before I lost my job (so < 9 months ago?), trying to determine if a leak was causing the miss, and at the conclusion of fixing those leaks there were no others. This time I don't have my smoke machine, but the brake cleaner test isn't finding anything. I didn't pull the manifold to swap the injectors, FWIW.

Also, FWIW, the negative trims aren't just at idle. The bars are negative under all conditions except for when the accelerator is first pushed to rev it up, at which point it approaches zero trim for a second or two, then starts climbing back negative until the pedal is let up, at which point it pegs hard negative for another second or two before leveling off back at it's -18%+.

Other than a smoke machine and brake cleaner (and obviously visual inspection), what are some other ways I could try to track down an air leak?

 

[swathdiver]

The ethanol % (for this tank) reads a rock-steady 12.15% across all the data sets. The question is, how close does "close" need to be? For the next week, the blend in Arizona should be 10%, after which it should roll over to 15% until November. What the tolerances on those blends are, or on the FFV sensors/systems, or for that matter how far it can be off before it starts causing problems, are all things I don't know.


Also, FWIW, the negative trims aren't just at idle. The bars are negative under all conditions except for when the accelerator is first pushed to rev it up, at which point it approaches zero trim for a second or two, then starts climbing back negative until the pedal is let up, at which point it pegs hard negative for another second or two before leveling off back at it's -18%+.

Other than a smoke machine and brake cleaner (and obviously visual inspection), what are some other ways I could try to track down an air leak?

Smoke didn't get past my intake gaskets but the trims cleared up whenever I tightened them and after we replaced them.

Ethanol content at gas stations can vary wildly, it's almost never at 10%. The 5.3s seem to get roughly the same gas mileage from 0% to 30% and then about 23% less between 40% and 80%.

Oxygen sensors best calculate the percentage when they are less than 100K miles old and are GM Original Equipment. Aftermarket sensors, even the Denso, are known to usually not calculate the alcohol percentage accurately. That will mess up the trims too.

 

[Kwing]

Money and time slowed me down, but I finally wrapped this pig up. Just got home after the fix.

It turns out the answer was all-of-the-above.

The crappy Amazon injectors were definitely garbage, but in pulling the intake (which I did not do the first time), I discovered ALL of the manifold bolts were bare bolts. The seals had totally disintegrated and all of the bolts were loose.

A new set of NAPA injectors, Felpro intake manifold gaskets, and a set of manifold bolts, and she's running smooth as silk.

Appreciate all the help! You guys are always the best!

 

[JamisMG]

Money and time slowed me down, but I finally wrapped this pig up. Just got home after the fix.

It turns out the answer was all-of-the-above.

The crappy Amazon injectors were definitely garbage, but in pulling the intake (which I did not do the first time), I discovered ALL of the manifold bolts were bare bolts. The seals had totally disintegrated and all of the bolts were loose.

A new set of NAPA injectors, Felpro intake manifold gaskets, and a set of manifold bolts, and she's running smooth as silk.

Appreciate all the help! You guys are always the best!

So, I read through this whole thread because I’m having the same issue with my 07 Tahoe Flex Fuel running rich and getting a P0172, P0175, and P0300 (bank 1 & 2 rich and random missfire). I thought the pressure regulator crapped out because my fuel pressure is around 60 psi, similar to what you reported. So, I put an OEM GM sending unit in the other week, but that didn’t fix the issue. All the brand new Denso injectors Ohm out good (around 12.5 ish). I replaced a slew of other parts too, including the MAF which caused my fuel trims to stabilize at around -9% at operating temp. - which they previously would stay at -25%. After reading your response I’ve order a Fel-Pro intake gasket and I’m banking on that.

However, I’m very curious if your fuel pressure went back down after the injector swap and intake gasket replacement? I’ve read that regardless of regular 5.3 or Flex Fuel, anything over ~58 psi is too high.

 

[Fless]

So, I read through this whole thread because I’m having the same issue with my 07 Tahoe Flex Fuel running rich and getting a P0172, P0175, and P0300 (bank 1 & 2 rich and random missfire). I thought the pressure regulator crapped out because my fuel pressure is around 60 psi, similar to what you reported. So, I put an OEM GM sending unit in the other week, but that didn’t fix the issue. All the brand new Denso injectors Ohm out good (around 12.5 ish). I replaced a slew of other parts too, including the MAF which caused my fuel trims to stabilize at around -9% at operating temp. - which they previously would stay at -25%. After reading your response I’ve order a Fel-Pro intake gasket and I’m banking on that.

However, I’m very curious if your fuel pressure went back down after the injector swap and intake gasket replacement? I’ve read that regardless of regular 5.3 or Flex Fuel, anything over ~58 psi is too high.

Have you checked the fuel alcohol content with a capable scanner? If you're running E15 or less, the PCM alcohol content should read low. If you're running E85 it should also track but the alcohol value should be high.

If you're running E10 or so but the PCM thinks it's E85, then it's going to dump more fuel into the cylinders, causing a rich condition.

Consider putting your vehicle info into your signature so it's always there for us.

 

[JamisMG]

Have you checked the fuel alcohol content with a capable scanner? If you're running E15 or less, the PCM alcohol content should read low. If you're running E85 it should also track but the alcohol value should be high.

If you're running E10 or so but the PCM thinks it's E85, then it's going to dump more fuel into the cylinders, causing a rich condition.

Consider putting your vehicle info into your signature so it's always there for us.

Alcohol content was reading at 14% when I bought the vehicle but I also don’t know what the previous owner was running. So I ran the fuel down, reset the value, put regular gas in it, and now it’s reading at ~3% and still having the issue.

 

[Kwing]

@JamisMG, my fuel pressure ran a rock steady at 58 psi before, during, and after my nightmare. I think the biggest variation window i saw from dataset to dataset was 0.2 psi. It was little enough to call it zero.

Where are you reading the 60? External gauge or with a tuner/diag tool from the ECU? If it's the ECU output, I would THINK the computer could adjust for extra 2 psi with less injector duty cycle. These other folks are much smarter than me and could say I'm totally full of it though, LOL.

 

[Marky Dissod]

I’ve read that regardless of regular 5.3L or Flex Fuel, anything over ~58 psi is too high.

Humbug. No reason to fear anything under 66psi / 4.5bar. More fuel pressure = better atomization.
What you need to be concerned with is a misreading of fuel pressure.

 

[JamisMG]

@JamisMG, my fuel pressure ran a rock steady at 58 psi before, during, and after my nightmare. I think the biggest variation window i saw from dataset to dataset was 0.2 psi. It was little enough to call it zero.

Where are you reading the 60? External gauge or with a tuner/diag tool from the ECU? If it's the ECU output, I would THINK the computer could adjust for extra 2 psi with less injector duty cycle. These other folks are much smarter than me and could say I'm totally full of it though, LOL.

On the mechanical gage at the rail I’m reading a steady 60 psi, but on the scanner I’m reading EXACTLY 57.9 psi and when I say “EXACTLY”, it does not budge or deviate even .1 of a difference at idle or any other throttle position. Just 57.9 all day long, which is weird because when I hooked up to an 07 Sierra it jumped around a little.

 

[Kwing]

On the mechanical gage at the rail I’m reading a steady 60 psi, but on the scanner I’m reading EXACTLY 57.9 psi and when I say “EXACTLY”, it does not budge or deviate even .1 of a difference at idle or any other throttle position. Just 57.9 all day long, which is weird because when I hooked up to an 07 Sierra it jumped around a little.

Oh these are gonna be dangerous words to say on the internet... I don't trust analog gauges for super precise readings. Just too much potential for human error, and I'm a pretty dumb human.

Yours could have a really large scale and small range on it that you can have confidence in a 2 psi delta, I obviously don't know, I'm just throwing that out there.

What you're describing on the ECU reading is consistent with what mine was showing. The bouncing you describe on your Sierra is the kind of behavior I would expect from a failing sensor or some kind of fuel system leak.

But take that with a pile of salt, because look how long it took me to figure mine out, LOL.

 

[Kwing]

I just had another thought. You said you're reading about -9.5% trim now vs the -25% you were seeing. Is that only at idle conditions?

I ask because that's about where my trim landed at idle at the end of this journey. At speed, the trims zeroed out, but the ~-9% at idle didn't seem bothersome to me as it was consistent and steady, and I didn't see big swings on the O2 outputs like I was before with the -25% trims.

 

[donjetman]

Fuel in the oil, from it running rich previously, can cause it to -9% from the fuel vapors being sucked thru the PCV system into the intake manifold. Drive it a lot, or change the oil/filter, to get it back to normal.

 

[Kwing]

Fuel in the oil, from it running rich previously, can cause it to -9% from the fuel vapors being sucked thru the PCV system into the intake manifold. Drive it a lot, or change the oil/filter, to get it back to normal.

Now there's something I hadn't thought of! I can absolutely see that, and I could see that thinning out the oil and contributing to my rocker noise I've been dealing with since I finished this.

I see an oil change in my near future

 

[donjetman]

There is a guy named Eric O. that has a YT channel called "South Main Auto" that has done videos covering this situation, and a lot more.

His videos are entertaining and educational IMO.

 

[Fless]

There is a guy named Eric O. that has a YT channel called "South Mail Auto" that has done videos covering this situation, and a lot more.

His videos are entertaining and educational IMO.

South Main Auto LLC

Hey there viewers! My name is Eric O. and I own and operate a small town 3 bay shop in upstate NY. I grew up in the automotive industry (in my parents shop) and have over 25 years (working full time as a mechanic) experience repairing domestic and Asian cars and light trucks. This is all I've...

www.youtube.com

 

[JamisMG]

It’s been a minute, but I found out what’s going on with my jalopy. So, the previous owners put in new injectors (think I mentioned this in my earlier post), and apparently, one of the injectors is different.

All 8 injectors have the same primary part number, and all 8 seem to have additional part markings above the primary PN (see pictures). However, 1 of the injectors seems to have a much lower flow rate compared to the other 7, but also has different part markings above the primary PN - the one with the “784 06” marking.

I ended up taking a non flex fuel intake assembly off an 07 Sierra and swapped it onto my Tahoe. She runs mint now.

So, I’m looking to buy a new set of injectors, but have some concerns about the difference in the ones I currently have. Do you guys know what the numbers circled on the injectors in the pictures mean?