Long term and short term fuel trims intake leak, low idle, improving fuel mileage. Mpg

[Burban22]

From some YouTube videos, I’ve educated myself on monitoring short term long-term fuel trims to diagnose intake, leaks, or bad oxygen sensors. I recently replaced all four oxygen sensors new spark plugs new wires suburban runs very good but fuel mileage isn’t the best 14-16 on the highway at 70mph.

It starts and idles fine, but on hot days, when shifting between reverse and drive idle will drop very low seems like it will almost die, but has never died. I cleaned the throttle body plate, which helped a little.

What do you think of the fuel trims in the video below? From what I’ve learned 3% plus or minus for short term field trim is considered normal. Long-term fuel trim should be under 10% plus or minus.

Seems like Bank 2 (passenger?) has maybe a small intake leak?

Computer has been Blackbear tuned.
Exhaust manifold gaskets replaced and broken manifold bolts replaced.

No CEL codes just trying to get the best mpg from this pig

 

[Burban22]

2003 suburban z71 5.3 LM7

Here’s some of the info on reading STFT and LTFT

 

[Joseph Garcia]

@swathdiver ; @Fless

 

[Fless]

I would suspect that something is causing over-fueling (rich, not lean) based on the high negative LT fuel trims. I see -16% popping up routinely on both banks, and that's the computer taking away fuel. I'd suspect a faulty IAC, dirty MAF, faulty spark plug or coil or wire, leaky fuel injector, or an intermittent purge valve on the engine.

A few things to verify then:
1. Clean the MAF or replace it with OE
2. Clean the IAC or replace it with OE
3. Monitor with your scan tool for individual cylinder misfires
4. Check and/or replace the purge valve on the engine (can be intermittent; if it sticks open it can let fuel vapor in from the canister when it's not commanded to, enriching the intake air with fuel vapor). Hopefully your evap canister doesn't have liquid fuel in it but that could be an issue when the purge valve opens - any problems fueling the tank?
5. Wouldn't hurt to monitor the fuel rail pressure; it should be well regulated at around 43-45 psi
6. Do an injector balance test to see if any are out of balance with the others
7. One last SWA thought -- do the values between the engine coolant temp sensor and the actual engine temp match? Engine temp can be checked with an infrared temp gun on the block or head. Not a likely cause but it might be good to rule it out

Hope that helps. I'd be curious if you find a direct cause.

 

[Burban22]

I would suspect that something is causing over-fueling (rich, not lean) based on the high negative LT fuel trims. I see -16% popping up routinely on both banks, and that's the computer taking away fuel. I'd suspect a faulty IAC, dirty MAF, faulty spark plug or coil or wire, leaky fuel injector, or an intermittent purge valve on the engine.

A few things to verify then:
1. Clean the MAF or replace it with OE
2. Clean the IAC or replace it with OE
3. Monitor with your scan tool for individual cylinder misfires
4. Check and/or replace the purge valve on the engine (can be intermittent; if it sticks open it can let fuel vapor in from the canister when it's not commanded to, enriching the intake air with fuel vapor). Hopefully your evap canister doesn't have liquid fuel in it but that could be an issue when the purge valve opens - any problems fueling the tank?
5. Wouldn't hurt to monitor the fuel rail pressure; it should be well regulated at around 43-45 psi
6. Do an injector balance test to see if any are out of balance with the others
7. One last SWA thought -- do the values between the engine coolant temp sensor and the actual engine temp match? Engine temp can be checked with an infrared temp gun on the block or head. Not a likely cause but it might be good to rule it out

Hope that helps. I'd be curious if you find a direct cause.

Thanks for the detailed reply!

The burb runs great and I’ve seen 15 - 16 on highway going 70 (hand calculated)

I’ve replaced:
Fuel pump
Fuel filter
Intake tube and air filter box is aFE performance
New NGK (from rock auto) spark plugs
New Taylor racing plug wires
4 new Denso o2 sensors
Deleted stock muffler and put in large magnaflow
New exhaust manifold gaskets and all broken bolts fixed.
Blackbear tune

MAF has been cleaned. It’s a Delphi one. It looks to not be original.

I checked evap canister when I changed fuel pump and the canister is empty. I replaced fuel tank pressure sensor which finally solved evap check engine code.

No problems filling

The purge valve is the one on the intake manifold that clicks when engine is up to operating temp? That seems to be working fine.

The engine temp gauge and scan tool match. It runs 190F

 

[Burban22]

Are you saying I should parts cannon replace the maf and iac to see if fuel trims change?

 

[Fless]

No, but I wouldn't necessarily trust the evap purge valve on the engine. I'd also monitor the fuel pressure to be sure the FPR is working properly.

Also think about having an injector balance test done; if nothing else to rule that out.

I may have missed it, but what year, engine, and is it the Flex Fuel engine? Brand and model of fuel pump used?

 

[Burban22]

No, but I wouldn't necessarily trust the evap purge valve on the engine. I'd also monitor the fuel pressure to be sure the FPR is working properly.

Also think about having an injector balance test done; if nothing else to rule that out.

I may have missed it, but what year, engine, and is it the Flex Fuel engine? Brand and model of fuel pump used?

I’ll order a replacement delco or Delphi purge valve.
173k miles
2003 LM7 5.3 non flex fuel but I put a flex fuel pump in it with “robust fuel level sender”. The original fuel pump still worked but the gauge sender wasn’t reading level properly.

Here’s the Carter

P74905M​

fuel pump I put in

https://a.co/d/2BVXP47

Betting my BlueDriver or innova 5610 scanner doesn’t do injector balancing.

So it’s not an intake gasket leak? I’m guessing intake gaskets were done once or maybe they still are original 20’years old and 173k?

 

[Fless]

Generally speaking the FF pumps tend to have lower output pressure but more volume (FF injectors can put more fuel in the cylinder to compensate for high percentages of ethanol). I'd still recommend a simple fuel pressure test, initial KOEO as well as running psi. Your '03 won't have a PID for that so it would need to be done with a fuel pressure gauge on the schrader valve on the passenger fuel rail. A fuel pressure gauge can usually be "loaned" for free by an auto parts store that has tool rental.

An intake (vacuum) leak would cause high positive fuel trims at idle but those would likely come down when rpms go up.

With all the changes I'd see if your scanner would reset the LTFTs and monitor them (STFTs too) going forward.

 

[Burban22]

Will disconnecting battery for an hour or longer reset the fuel trims?

Probably taking it on a 200 mile road trip soon maybe it will relearn on that trip?

My scanners don’t have an option to clear fuel trims but it says all emission test have run and passed