5.3 negative LTFT, feels sluggish

[Hans Myers]

Have a 2007 chevy truck with the 5.3, has about 100k on the engine. Truck feels sluggish until the engine hits 3.5k rpm, then will sorta suck you into the seat. Best way I can put it is that it always feels like I am towing something. 

 

My long term fuel trims are at -11 and -13 on bank 1 and 2.

 

Have replaced-

-spark plugs

-MAF

-changed CAI back to stock air cleaner

-evap vent solenoid (the one by the gas tank)

-new fuel pump (delphi)

 

(all these parts are less than 2 months old)

 

nothing is dragging, alignment is perfect, new shocks/struts so the truck has the proper rake angle. 

 

fuel pressure is steady at 62 PSI, drove the truck with the gauge hooked up and it did not drop under load. Injectors do not leak, with the truck off for around 5 mins the pressure did not go ANY lower than 55 psi. 

 

at my wits end, not sure where else to look. fuel economy is pretty bad too, the best I usually get on the freeway is about 15 on flat ground at 70. Used to get about 17-18. Getting expensive since I have a 275 mile drive I make once or twice a month. 

 

thanks for any feedback. 

 

[swathdiver]

2 hours ago, Hans Myers said:

Have a 2007 chevy truck with the 5.3, has about 100k on the engine. Truck feels sluggish until the engine hits 3.5k rpm, then will sorta suck you into the seat. Best way I can put it is that it always feels like I am towing something. 

 

My long term fuel trims are at -11 and -13 on bank 1 and 2.

 

Have replaced-

-spark plugs

-MAF

-changed CAI back to stock air cleaner

-evap vent solenoid (the one by the gas tank)

-new fuel pump (delphi)

 

(all these parts are less than 2 months old)

 

nothing is dragging, alignment is perfect, new shocks/struts so the truck has the proper rake angle. 

 

fuel pressure is steady at 62 PSI, drove the truck with the gauge hooked up and it did not drop under load. Injectors do not leak, with the truck off for around 5 mins the pressure did not go ANY lower than 55 psi. 

 

at my wits end, not sure where else to look. fuel economy is pretty bad too, the best I usually get on the freeway is about 15 on flat ground at 70. Used to get about 17-18. Getting expensive since I have a 275 mile drive I make once or twice a month. 

 

thanks for any feedback. 

 

Probably time for new intake gaskets.  Try re-torqueing them first.  You can watch the O2 sensors and spray water, propane, MAP gas, whatever, along the intake and heads and see if the numbers spike, along with the idle.

[Hans Myers]

1 hour ago, swathdiver said:

Probably time for new intake gaskets.  Try re-torqueing them first.  You can watch the O2 sensors and spray water, propane, MAP gas, whatever, along the intake and heads and see if the numbers spike, along with the idle.

 

Yeah I could give that a try. Correct me if I am wrong, but wouldn't an air intake leak cause positive fuel trims, as the ECU has to compensate for the extra air? 

[CamGTP]

If you have a fancy scan tool you can look at the ethanol or alcohol percentage/content that the truck thinks is in the fuel. If that number is far greater than the standard 8-10% content that you find in mostly all pump stations running E10 fuels, then you have found a possible issue.

 

The nothing below 3,000rpm is kinda par for the course when it comes to 5.3's and 4L60e's with the newer computers. It's not in it's main power band until then and you also have the computer limiting it a little because of torque management and delays built into the calibration. After market tuning and those pedal commander things do help a lot with overall throttle response and performance. Not exactly giving you more power but just giving you what was there.

[ic3man5]

See my post here, its not your exact engine/year but I was having fuel trim issues and had a vacuum leak causing negative LTFT.

 

 

[Hans Myers]

On 9/26/2021 at 8:32 PM, CamGTP said:

If you have a fancy scan tool you can look at the ethanol or alcohol percentage/content that the truck thinks is in the fuel. If that number is far greater than the standard 8-10% content that you find in mostly all pump stations running E10 fuels, then you have found a possible issue.

 

The nothing below 3,000rpm is kinda par for the course when it comes to 5.3's and 4L60e's with the newer computers. It's not in it's main power band until then and you also have the computer limiting it a little because of torque management and delays built into the calibration. After market tuning and those pedal commander things do help a lot with overall throttle response and performance. Not exactly giving you more power but just giving you what was there.

I have a scan tool with live data. Ethanol reading is normal, at 10%, which is what I filled it with.

[Hans Myers]

On 9/27/2021 at 12:36 AM, ic3man5 said:

See my post here, its not your exact engine/year but I was having fuel trim issues and had a vacuum leak causing negative LTFT.

 

 

I'll have to take a look at that, thanks.

[Hans Myers]

Well, still no progress. Replaced my evap purge valve, seems to be a little better after that but not by much. Also replaced my TPS as well, which made no difference. 

 

Couple things I have noticed, which seem to be a little weird. My fuel trims go farther down into the negatives when I run the A/C. When it is not running, the trims go up by a number or two. 

 

My MAP sensor reads about 10 in-hg at idle, and about 11.8 with the A/C on. MAF is flowing at 0.8lb/min w/out A/C, and 0.9 with it on. 

 

Also wanted to mention that the fuel trims have went down to -17 and -14. 

Edited October 4, 2021 by Hans Myers

[CamGTP]

What are the chances that you have a minor leaking of the a few injectors. Like they are dribbling fuel out.

 

I'm sure what else to say at the moment, anything else is kinda just shot gunning parts at it.

[Hans Myers]

12 hours ago, CamGTP said:

What are the chances that you have a minor leaking of the a few injectors. Like they are dribbling fuel out.

 

I'm sure what else to say at the moment, anything else is kinda just shot gunning parts at it.

I kinda doubt that since the fuel trims are fairly similar on both banks. Did a leak test and it held at 55 psi for a good 20 mins. 

 

Possibly a bad pcv system? Not sure where else to look. I would have thought the problem would have been solved with the parts I have replaced.

 

 

[Hans Myers]

well, I was going over the pictures I took of the fuel trims over a few weeks. Bank 2 is consistently 3 points lower than bank 1 at all times. 

 

Just for the hell of it, I am going to switch injectors from bank 1 to bank 2, and vice versa. If I see that my trims change on each bank I will know if that is the issue.

[swathdiver]

If this is a FlexFuel engine, reset the alcohol content and LTFTs after running the tank down as much as possible and refueling with E10 or E0.  The Tech-2 or scan tool will reset the alcohol content to 3% and then the computer will re compute from there.  Don't reset until after the computer finishes calculating the new alcohol content after refueling.

 

Mine has been doing this lately, rarely gets close to 0% LTFTs and goes mid teens at idle.  Took a fuel sample from the rail and it is 80% and the computer says 67%.  Time to reset for me too.

[Ianf]

My 2010 4.8 did the same thing and had 2 issues, alcohol percentage was 35%, fixed that and still an issue. Turned out to be the vacuum port on the passenger side valve cover had been knocked off when changing the plugs. Put a rubber cap on it and it clear my negative LTFTs on both banks.

 

[Dave Casillas]

Lean conditions are usually due to a vacuum leak. That's based on my experience tuning 04 gto

 

I kept getting a code and finally replaced the intake tub that joins to the throttle body. Had a hairline crack that overtime lead the the code.