Street Tuned AI

07/31/2026

How to Read Fuel Trims Correctly

If you've ever fired up VCM Scanner and stared at the endless stream of data, you've probably noticed those bouncing percentages under Short Term Fuel Trims (STFT) and Long Term Fuel Trims (LTFT). For a lot of guys just getting into tuning, these numbers look like a foreign language. But once you understand what they are telling you, they become your best friend for dialing in a setup.

Figuring out how to read fuel trims correctly is the foundation of getting your part-throttle drivability right. Whether you are trying to figure out Why Your LS Swap Runs Rich or you just want to smooth out your daily driver, your fuel trims hold the answers. Let's break down exactly what these numbers mean, how to interpret them, and how to use them to fix your tune.

What Are Fuel Trims?
At its core, a fuel trim is just the engine computer (PCM) making adjustments to the amount of fuel being injected into the engine. The PCM has a target air-fuel ratio (usually 14.7:1 for gasoline at part throttle, known as Stoich). It uses the front oxygen sensors (O2 sensors) to measure the actual exhaust gas. If the mixture is too lean or too rich compared to the target, the PCM "trims" the fuel delivery to get it back to 14.7:1.

This closed-loop fueling system relies on two types of trims working together: Short Term and Long Term.

Short Term Fuel Trims (STFT)
Short Term Fuel Trims are the immediate, split-second corrections the PCM makes. As the O2 sensors switch back and forth between rich and lean, the STFTs bounce around constantly. Think of STFT as the PCM's knee-jerk reaction to what's happening right now. If you blip the throttle and a rush of unmetered air hits the cylinders, the STFT will instantly spike positive to add fuel and prevent a lean stumble.

Long Term Fuel Trims (LTFT)
Long Term Fuel Trims are the learned corrections over time. If the PCM notices that the STFTs are constantly having to add 10% more fuel in a specific RPM and load area, it will shift that 10% correction over to the LTFT. Once the LTFT learns that +10% correction, the STFT can go back to bouncing around near zero.

LTFTs are stored in the PCM's memory. This is why when you flash a new tune or disconnect the battery, the LTFTs reset to zero and the car might drive a little rough until it "relearns" the trims.

Positive vs. Negative Trims: What Do They Mean?
This is where a lot of guys get tripped up. You see a positive number and think "too much fuel," but it's actually the exact opposite.

Positive Fuel Trims (+%) mean the PCM is adding fuel. Why is it adding fuel? Because the O2 sensors are reading a lean condition. There is too much air and not enough fuel, so the computer is stepping in to save the day by increasing injector pulse width.

Negative Fuel Trims (-%) mean the PCM is pulling fuel away. It does this because the O2 sensors are reading a rich condition. There is too much fuel in the exhaust, so the computer shortens the injector pulse width to lean things out.

A good rule of thumb: Positive means the engine was lean, Negative means the engine was rich. The trim is the correction, not the condition.

Diagnosing Lean and Rich Conditions
Once you know how to read fuel trims correctly, you can use them as a diagnostic tool before you even touch the VE or MAF tables.

If you see your LTFTs pegged at +20% at idle, but they drop down closer to zero when you give it some throttle and get the RPMs up, you almost certainly have a vacuum leak. At idle, a small vacuum leak lets in a lot of unmetered air relative to the total airflow, causing a massive lean condition. As you open the throttle, that small leak becomes a tiny fraction of the total airflow, so the trims fall back in line.

On the flip side, if your trims are heavily negative across the board, you might have excessively high fuel pressure, leaking injectors, or a MAF sensor that is over-reporting airflow.

Before you start hacking up your tune to fix bad trims, make sure the mechanical side of the engine is sound. A tune can't fix a cracked intake manifold or a dead O2 sensor.

Normal Operation vs. A Real Problem
So, what is an acceptable fuel trim? You are never going to see perfect 0% trims everywhere, all the time. Weather changes, fuel quality varies, and engines wear.

Generally speaking, if your LTFTs are swinging between -5% and +5%, you are in the sweet spot. The tune is solid, and the PCM is just doing its job handling minor environmental changes.

If your trims are consistently in the -10% to +10% range, the car will still run fine, but there is definitely room for improvement in the calibration.

Once you start seeing trims pushing past +/- 15%, you have a problem. The PCM is having to work overtime to keep the engine happy, and if you hit the maximum trim limit (usually around +/- 25%), the PCM can't correct anymore, and the engine will actually run lean or rich.

How Trims Relate to VE Table Accuracy
The main reason we care about fuel trims when tuning is to correct the underlying airflow models—specifically the Volumetric Efficiency (VE) table and the Mass Airflow (MAF) curve.

If your VE table is accurate, the PCM calculates the correct amount of air entering the engine, injects the right amount of fuel, and the O2 sensors confirm that the mixture is dead-on 14.7:1. In this perfect scenario, your fuel trims would be zero.

When your trims are consistently positive or negative in certain areas, it means your VE table is lying to the PCM. If you log your LTFTs and STFTs against your VE table layout in VCM Scanner, you can literally copy the trim percentages and paste them into your tune editor using the "Multiply by %" function. This adjusts the VE table to match reality. If you want a deep dive into this process, check out our guide on VE Table Tuning Explained.

Keep in mind, fuel trims only work in closed loop (part throttle). When you go wide open throttle (WOT), the PCM ignores the O2 sensors and relies entirely on the VE and MAF tables. If your VE table is off by 15% at part throttle, it's probably off at WOT too, which is why getting your trims dialed in is critical before you start doing full-throttle pulls. For WOT tuning, you'll need a wideband—read up on Wideband Setup for HP Tuners Explained if you haven't tackled that yet.

Frequently Asked Questions
Why do my fuel trims go to zero at wide open throttle? When you go wide open throttle, the PCM enters Power Enrichment (PE) mode and switches to open loop. It stops looking at the narrow-band O2 sensors because they can only accurately read around 14.7:1, and you need a richer mixture for WOT. Since it ignores the sensors, the trims lock at zero (or sometimes lock at the last positive LTFT value to protect the engine).

Should I tune using STFT or LTFT? Most tuners prefer to disable LTFTs and tune using only STFTs (or a wideband error ratio). LTFTs carry over learned data, which can skew your logs if you are hitting different cells quickly. Tuning with STFTs gives you immediate, real-time feedback on how far off your VE or MAF tables are.

Can bad O2 sensors cause weird fuel trims? Absolutely. If an O2 sensor gets lazy or fails, it might falsely report a lean condition. The PCM will respond by dumping fuel into that bank, resulting in maxed-out positive fuel trims and a horribly rich-running engine. Always verify your sensors are switching properly before trusting the trim data.

Get Your Tune Dialed In Faster
Staring at datalogs and manually applying fuel trim corrections across hundreds of cells takes hours of tedious work. If you want to skip the headache and get your VE and MAF tables dialed in perfectly, check out the StreetTunedAI LS/LT Assistant. It analyzes your HP Tuners logs, identifies the exact corrections needed, and builds the updated tables for you in seconds. If you'd rather have an expert handle the whole process from start to finish, our Remote Tuning Service will get your rig running flawlessly without you ever having to leave the garage.

Upload your HP Tuners VCM Scanner CSV and get a real LS/LT tune analysis. Fuel trims, knock, AFR, MAF Hz. Trusted by 400+ tuners and enthusiasts.

07/30/2026

One of the most common questions I get is: Why won't StreetTunedAI give me WOT fuel advice?

The answer is simple Safety. If your log is missing wideband data, my system won't guess your fueling. General AI might give you a number, but StreetTunedAI stops the process until you have the right data. Tuning is about precision, not high-stakes guessing.

Check out how our safety logic keeps your build in one piece at

AI-powered HP Tuners datalog analysis for LS, LT, Coyote, Duramax & Cummins builds. Upload your log, get real tuning direction. No guessing. From $74.99.

07/29/2026

I think my biggest mistake with building streettunedai putting AI in the name.

Because StreetTunedAI is not AI tuning and it never has been

This is not some overnight prompt wrapped in a logo and pointed at people’s fuel tables.

StreetTunedAI does not open your tune file.
It does not write calibration changes.
It does not guess VE, MAF, timing, or boost from a chat box.
It does not replace a tuner.
It does not tell you to make WOT fuel changes from a garbage log.

It starts with the datalog.

The analyzer checks the actual channels in the file, what is missing, what is believable, what looks unsafe, and what the log can actually support.

Wideband missing? No WOT fuel advice.
Knock showing up? No timing added.
MAP data suspect? No VE/load guess.
Fuel trims split bank-to-bank? It does not blame the whole MAF table.
Mechanical problem showing in the data? It does not tell you to tune around it.

That is the part people miss.

The safety logic runs before the explanation.

The AI is not the tuner.
The AI is not making the safety call.
The AI is not free styling calibration advice.

The analyzer checks the log.
AI explains the result in plain English.

StreetTunedAI is safety-focused datalog review for real calibration work. The log review below is a real log review from it if you are on the fence about getting it take the time to read this log review and notice how detailed it is, Streettunedai is not a scam its not a chat bot it works just ask any of the 400 plus users thank you
StreetTunedAI
Patient Pending

www.streettunedai.com
Not a magic tune button.
Not chatbot tuning.
Not “trust me bro” fuel tables.

07/23/2026

This isnt saying experienced tuners should be replaced by AI. Its for people using AI already who need something that actually reviews the whole log before suggesting a change.

Ive seen more people talking about using different AI chats to help tune their vehicles.

If its been helping you thats good. Im not here to tell anybody what they can or cant use.

But heres the reality.

General AI chatbots are built to respond to the question you ask and the way you ask it. If you start asking about fuel trims theres a good chance it will start telling you to change fueling whether the rest of the log supports that or not.

It may sound confident but it doesnt mean it actually looked at the whole picture.

StreetTunedAI works different.

It looks at everything available in the log before it recommends anything.

For example, lets say Bank 2 is showing around positive 18.9 percent fuel trim.

A general AI might tell you to start changing the fuel table.

StreetTunedAI looks at the imbalance between the banks and asks why Bank 2 is requiring that much extra fuel. It may tell you to check for an intake leak, unmetered air, or an exhaust leak on the passenger side before touching the tune.

Because changing the tune to cover up a mechanical problem is not fixing it.

The analyzer follows strict rules and only reports what the log actually supports. If the right information isnt there it doesnt guess. It tells you what channels or data you need to record next.

Then the AI explains those findings in plain English.

Thats the difference.

StreetTunedAI.com

07/18/2026

Open loop vs closed loop what you need to know

https://www.streettunedai.com/blogs/news/open-loop-vs-closed-loop-tuning-what-you-actually-need-to-know

Master open loop vs closed loop tuning. Learn how your PCM uses O2 sensors, when it switches modes, and why dialing in open loop is crucial for WOT.

07/18/2026

AI-powered HP Tuners datalog analysis for LS, LT, Coyote, Duramax & Cummins builds. Upload your log, get real tuning direction. No guessing. From $74.99.

07/16/2026

P0101 LS Swap Fix Guide: Curing the MAF Performance Code

P0101 LS Swap Fix: Why Your MAF Code Keeps Coming Back

If you just fired up your fresh LS swap and immediately got hit with a P0101 code, you are not alone.

P0101 means Mass Air Flow Sensor Performance. It is one of the most common codes people run into after dropping a 5.3, 6.0, LS1, LS2, LS3, or other GM V8 into a chassis it did not come in.

Do not panic, and do not immediately throw a new MAF sensor at it. Sometimes the sensor is bad, but a lot of the time the problem is the intake setup, wiring, MAF scaling, vacuum leaks, or the tune not matching the hardware.

Here is what P0101 means, why it shows up on LS swaps, and how to approach the fix in HP Tuners.

What Does P0101 Mean?

P0101 is a Mass Air Flow Sensor Performance code.

The PCM is watching the MAF sensor and comparing the airflow it reports against what the PCM expects based on the rest of the engine data.

Depending on the vehicle and operating system, the PCM may compare MAF airflow against calculated airflow using MAP, RPM, throttle position, VE, and other airflow model data.

When the MAF signal does not make sense compared to the rest of the data for long enough, the PCM sets P0101.

In plain English, the computer is saying:

“The MAF airflow does not match what the engine appears to be doing.”

That does not always mean the MAF sensor is bad. It means the MAF data and calculated airflow do not agree.

Why P0101 Is Common on LS Swaps

Factory GM calibrations were built around a specific MAF sensor, intake tube, airbox, screen, duct shape, and sensor location.

When you swap the engine into a C10, Foxbody, S10, Jeep, OBS truck, or another chassis, that factory intake layout is usually gone.

Now the PCM is using a factory MAF calibration on a custom intake tube that may have a different diameter, different bends, different sensor placement, and different airflow behavior.

That is where the problems start.

Common Causes of P0101 on LS Swaps

Wrong MAF Housing or Intake Tube Size

The MAF calibration is tied to the housing and intake tube it was designed around.

If you take a MAF calibrated for one tube size and install it in a different size tube, the airflow reading will be wrong.

A larger tube usually slows airspeed through the MAF. A smaller tube usually increases airspeed. Either way, the PCM is no longer seeing what the stock calibration expects.

Any major MAF housing or tube size change needs MAF calibration work.

Bad MAF Placement

MAF sensors do not like turbulence.

Sharp bends, couplers, reducers, air filters, blow-through setups, or throttle bodies placed too close to the MAF can disturb the airflow across the sensor.

A MAF installed right after a 90-degree bend or right against the filter can give unstable readings, even if the sensor itself is fine.

For best results, the MAF should be in a clean, straight section of pipe with stable airflow across the element.

Wrong MAF Sensor or Wiring

Many LS swaps use mixed parts.

Truck MAF, LS1 MAF, LS3 card-style MAF, aftermarket harness, repinned connectors, standalone IAT wiring, and speed density conversions all get mixed together.

If the tune is set up for one MAF but the car has a different MAF installed, the airflow curve will be wrong.

If the wiring is wrong, the signal may be unstable or incorrect.

Before tuning around P0101, confirm the MAF type, wiring, connector, signal, power, ground, and IAT setup.

Vacuum Leaks or Unmetered Air

Any air entering the engine after the MAF is unmetered air.

That can come from:

Intake manifold leaks
Throttle body leaks
PCV problems
Brake booster leaks
Loose clamps
Cracked vacuum hoses
Bad intake gaskets
Leaking MAP sensor seal

The MAF reports one amount of air, but the MAP sensor and fuel trims show the engine is getting more air than the MAF measured.

That mismatch can trigger P0101.

MAF Curve Does Not Match the Setup

Even if the tube size is close and the sensor is wired correctly, the MAF curve may still need correction.

Custom intake tubing, different filters, different airbox design, and aftermarket MAF housings can all change how the sensor reads.

The fix is not guessing. The fix is logging and correcting the MAF curve.

How to Fix P0101 on an LS Swap

Step 1: Check the Hardware First

Before opening HP Tuners, check the basics.

Confirm the MAF is installed in the correct direction.

Confirm the MAF sensor matches the tune.

Confirm the wiring is correct.

Confirm the intake tube is sealed.

Confirm there are no vacuum leaks.

Confirm the MAF is not right after a hard bend or right against the air filter.

Confirm the IAT sensor is reading correctly.

Confirm the MAP sensor data looks believable.

A smoke test is a good idea before chasing the tune.

Step 2: Log the Right Channels

In VCM Scanner, log the data needed to see what is happening.

Useful channels include:

Engine RPM
MAF Frequency
MAF Airflow
MAP
IAT
ECT
TPS
Commanded AFR or EQ Ratio
STFT
LTFT
Wideband AFR or Lambda
Fuel system status
Injector pulse width
Current gear if needed

You want to know whether the MAF signal is smooth and believable, or if it is noisy, flat, dropping out, or way out of range.

Step 3: Correct the MAF Curve

If the hardware is good, the MAF curve needs to match the actual intake setup.

Use fuel trims for closed-loop areas and a properly configured wideband for open-loop, PE, and WOT areas.

The basic idea is simple:

If the engine is lean in a certain MAF frequency range, the MAF curve is usually too low there.

If the engine is rich in a certain MAF frequency range, the MAF curve is usually too high there.

Make controlled corrections to the MAF Airflow vs Frequency table and re-log.

Do not make huge changes from one short pull. Work through the curve cleanly and verify the correction.

Step 4: Check the VE Model Too

P0101 can also show up when the MAF and speed density calculations disagree.

That means the MAF curve might not be the only issue. If the VE table is way off, the calculated airflow side can also be wrong.

For best results, the MAF and VE models should both be close.

Do not blindly blame the MAF if the VE table has never been corrected for a cam, intake, boost, displacement change, or MAP sensor change.

Step 5: Adjust Diagnostic Thresholds Only After the Tune Is Close

HP Tuners gives access to airflow diagnostic settings on many GM platforms.

You can widen the P0101 diagnostic thresholds in some cases, but this should not be the first move.

If the MAF curve is wrong, the intake has leaks, or the wiring is bad, widening the diagnostic is just hiding the problem.

Only adjust the diagnostic after the hardware is fixed and the airflow model is close.

When Speed Density Makes More Sense

Sometimes the MAF is not worth fighting.

Speed density may be the better route if:

The intake layout cannot support clean MAF placement
The cam creates unstable low-speed MAF airflow
The setup is boosted and the MAF location is problematic
The MAF frequency range is maxed out
The vehicle is a tight swap with no good straight intake section

Speed density uses MAP, RPM, IAT, displacement, and the VE model to calculate airflow.

It can work very well, but it still has to be tuned correctly. You also need the correct MAP sensor, correct sensor scaling, correct DTC setup, and a dialed-in VE table.

Do not switch to speed density just to avoid fixing basic mechanical or wiring problems.

FAQ

Can I just turn off P0101 in HP Tuners?

You can disable the code, but that does not fix the airflow problem.

If the tune is still relying on the MAF, turning off P0101 is usually a bad idea. The PCM may still be using bad airflow data, which can cause poor fueling, hesitation, rich or lean conditions, and drivability issues.

Only disable or change MAF-related diagnostics when the airflow strategy has been intentionally set up for it.

Will a bigger MAF fix P0101?

No.

A bigger MAF housing without proper calibration usually makes the problem worse.

Any change in MAF housing size or sensor style needs the correct MAF calibration. The PCM has to know how much airflow each frequency actually represents.

Why does my LS swap run rich after P0101 sets?

When P0101 sets, the PCM may reduce trust in the MAF and rely more heavily on backup airflow calculations, depending on the operating system and failure mode.

If the VE table, MAP sensor scaling, injector data, or airflow model is wrong, fueling can go rich or lean after the code sets.

The code is not the root problem. The airflow model mismatch is.

Should I tune MAF or speed density first?

For a MAF-based setup, it is common to isolate and correct the VE model first, then correct the MAF curve, then verify them together.

The exact workflow depends on the operating system and setup.

The important part is not mixing corrections blindly. Know which airflow model the PCM is using before applying changes.

Final Thoughts

P0101 on an LS swap is not random.

It usually means the PCM does not believe the MAF airflow compared to the rest of the engine data.

Start with the basics:

Check the MAF type.
Check the wiring.
Check the intake layout.
Check for leaks.
Log the right channels.
Correct the MAF curve.
Verify the VE model.

Do not just throw parts at it and do not just turn the code off.

If you want help reviewing your HP Tuners CSV, the StreetTunedAI LS/LT Assistant can look through the log and point out likely MAF, VE, fueling, sensor, and airflow model issues.
www.streettunedai.com

AI-powered HP Tuners datalog analysis for LS, LT, Coyote, Duramax & Cummins builds. Upload your log, get real tuning direction. No guessing. From $74.99.

07/14/2026

Why Your Truck Stalls Coming to a Stop

Why Your Truck Stalls at a Stop: HP Tuners Idle and Decel Fixes

There is nothing more annoying than pulling up to a red light in your freshly tuned truck and having the RPM dip, stumble, and die.

You restart it, give it a little throttle to keep it alive, and hope nobody behind you noticed.

If your truck stalls when coming to a stop, you are not alone. It is one of the most common drivability problems on cammed LS trucks, LS swaps, boosted setups, and anything with major airflow or converter changes.

The fix starts with the log.

Do not guess. Catch the stall in VCM Scanner and look at what the engine lost right before it died: air, fuel, spark, or converter release.

Here are the most common causes.

1. Base Running Airflow Is Too Low

Base Running Airflow, or BRAF, is the base amount of air the engine needs to idle at the commanded RPM.

When you lift off the throttle and the truck slows down, the engine has to return to idle cleanly. If the base airflow is too low, the RPM can fall below target and the engine may not recover.

This is very common on cammed trucks.

A larger cam usually has lower vacuum and needs more idle airflow than the stock tune commands. If the tune still has stock idle airflow values, the PCM has to use idle trims and spark correction to save the engine every time RPM drops.

Symptoms of low base airflow can include:

RPM dips below target idle
Stalling in gear
Stalling when coming to a stop
Idle surge
High idle airflow correction
Engine only stays running if you give it throttle

Fix the base airflow before chasing every other table.

2. Torque Converter Clutch Not Releasing

If the truck feels like a manual car where someone forgot to push the clutch in, look at the torque converter clutch.

The TCC should release before the truck comes to a stop.

If the converter stays locked too long, it can drag the engine down and stall it. This can be a tune issue, a TCC apply/release schedule issue, or a mechanical/hydraulic transmission problem.

Watch TCC slip in the log.

If TCC slip is near zero while the truck is slowing down and RPM is being pulled down hard, the converter may still be locked.

Common causes include:

TCC release speeds too low
TCC apply/release hysteresis wrong
Wrong converter settings
Sticky TCC solenoid
Valve body issue
Converter problem
Wiring or brake switch issue

Do not assume every stop-stall is idle airflow. A locked converter can kill a perfectly good idle tune.

3. IAC or Electronic Throttle Problems

Depending on the truck, you may have drive-by-cable with an IAC valve or drive-by-wire with electronic throttle control.

On a cable throttle setup, the IAC has to flow enough air to catch the idle. If the IAC is dirty, slow, maxed out, or the throttle blade is set wrong, the engine may not get enough air during return-to-idle.

On a DBW setup, the PCM moves the throttle blade directly. A dirty throttle body, airflow model problem, throttle area issue, or bad idle learn can cause poor idle control.

Things to check:

Dirty throttle body
Carbon around throttle blade
Vacuum leaks
IAC counts or position high/maxed out
Throttle position behavior
Idle airflow correction
Idle relearn needed
Throttle body wiring or connector issues

Be careful with throttle stop screw changes. Small mechanical changes can affect TPS, airflow, and idle control. Clean and verify the basics first.

4. Vacuum Leaks or Unmetered Air

A vacuum leak can cause a truck to stall at a stop, especially when the airflow model is already borderline.

Unmetered air can make fueling unstable and idle control messy.

Common leak points:

Intake manifold gaskets
Brake booster hose
PCV system
EVAP lines
Throttle body gasket
MAP sensor seal
Vacuum caps
Catch can plumbing

Look at fuel trims, MAP at idle, idle quality, and wideband behavior if available.

If fuel trims are very positive at idle and light throttle, do not just add airflow. Find the leak.

5. Cam Overlap and Low Vacuum

Big cams sound good, but they make idle control harder.

Overlap lowers vacuum and can make MAP readings unstable at idle and during decel. The engine may need more airflow, more idle speed, better idle spark control, and cleaner VE/MAF data in the low RPM areas.

A cammed truck may stall when coming to a stop if:

Base airflow is too low
Target idle is too low
Idle spark correction is weak
Throttle cracker/follower decay too fast
VE or MAF is wrong in idle/decel areas
Fueling is unstable at low pulse width
Injector data is wrong

A big cam does not mean the truck has to drive terrible, but it does mean the stock idle tables are usually not enough.

6. Idle Spark Is Not Helping Recovery

Spark timing is one of the PCM’s fastest idle control tools.

Airflow changes are slower. Spark can move almost instantly.

When RPM drops below target, the PCM can add timing to help the engine recover. If idle spark is too low, unstable, or the correction tables are not doing enough, the engine can dip and stall.

There is no one perfect idle timing number.

Many cammed LS setups like idle timing somewhere in the mid-teens to low twenties, but the right number depends on the combo.

What matters is that idle timing is stable and the PCM has room to catch RPM when it falls.

Watch for:

Spark flatlining while RPM drops
Timing jumping around aggressively
Main spark and idle spark areas not matching well
No useful underspeed correction
Too much timing pulled during idle recovery

Do not use spark to hide a bad airflow problem, but do not ignore it either.

7. Throttle Cracker and Throttle Follower Are Wrong

Throttle cracker and throttle follower help control airflow when you lift off the throttle and the vehicle is still moving.

They work like a dashpot.

If airflow decays too fast, RPM can crash and the engine may stall.

If airflow hangs too long, the truck may push through the brakes or hang RPM.

Throttle cracker is mainly vehicle speed and RPM based.

Throttle follower is related to throttle movement and airflow decay after throttle input.

Common problems:

Throttle cracker too low
Throttle follower decay too fast
Follower airflow too low
Cracker airflow removed too aggressively near low speed
Base airflow too low and cracker being used as a band-aid

Fix base idle airflow first, then use cracker and follower to shape the return to idle.

8. Fueling or Injector Data Is Off

A truck can also stall at a stop because fueling goes unstable during the transition back to idle.

This is common with big injectors, wrong injector data, bad short pulse data, incorrect offset data, or poor MAF/VE calibration in the idle and decel areas.

Watch:

STFT
LTFT
Wideband AFR or lambda
Commanded AFR or EQ ratio
Injector pulse width
MAP
MAF frequency
Fuel pressure if available

If the engine goes rich and loads up, that is different from starving for air.

If it goes lean while still fueling, that is different from normal DFCO.

Do not assume every stall is airflow.

How to Diagnose the Stall in HP Tuners

You need to log the stall event.

Useful channels include:

Engine RPM
Desired idle RPM
Vehicle speed
TPS
MAP
MAF frequency
MAF airflow
IAT
ECT
Commanded AFR, lambda, or EQ ratio
Wideband AFR or lambda if installed
STFT
LTFT
Injector pulse width
Spark advance
Idle spark correction
Base running airflow
Idle desired airflow
Dynamic airflow
STIT
LTIT
IAC position if drive-by-cable
Throttle position if DBW
Throttle cracker airflow
Throttle follower airflow
Gear or PRNDL
Brake switch
TCC slip
TCC apply status if available

The data right before the engine dies will usually tell you what happened.

If RPM drops and airflow is too low, look at BRAF and decay airflow.

If RPM drops while TCC slip stays near zero, look at converter release.

If the wideband goes rich and pulse width is high, look at fueling and injector data.

If trims are very positive at idle, check for vacuum leaks.

If spark is not reacting while RPM falls, look at idle spark control.

FAQ

Why does my truck only stall when the AC is on?

The AC compressor adds load to the engine.

If the AC airflow or torque compensation is not right, the PCM may not add enough airflow when the compressor kicks on. That can pull RPM down and stall the engine.

Also check fan behavior, alternator load, idle airflow, and idle spark correction.

Can a bad oxygen sensor cause my truck to stall at a stop?

Yes, but do not blame the O2 sensor first.

A lazy or failing O2 sensor can cause fuel trim problems in closed loop. That can make the engine rich or lean during idle and return-to-idle.

But similar symptoms can also come from exhaust leaks, vacuum leaks, wrong injector data, bad MAF data, or poor airflow tuning.

Do I need to retune idle after changing the torque converter?

Usually, yes.

A different converter changes how much load the engine sees in gear and during decel.

A looser converter usually loads the engine less at idle than a tight stock converter, but the actual behavior depends on the converter, cam, idle speed, and transmission setup.

Check in-gear idle airflow, idle spark, TCC behavior, and return-to-idle.

Should I raise idle speed to stop stalling?

Sometimes.

A cammed or boosted truck may need a higher target idle than stock. Raising idle speed by 50 to 150 RPM can help some combos.

But do not use idle speed to cover up a vacuum leak, wrong injector data, bad airflow settings, or a locked converter.

Why does it only stall when coming to a stop, not sitting still?

That usually points to return-to-idle control.

Look at throttle cracker, throttle follower, DFCO, airflow decay, TCC release, and idle recovery.

If it idles fine in the driveway but dies while rolling to a stop, the problem is often in the transition, not the steady idle.

Final Thoughts

A truck that stalls at a stop is not fixed by guessing.

You need to catch the event in the log and see what disappears first.

Airflow, fuel, spark, or converter release.

Start with the basics:

Check for vacuum leaks.
Confirm injector data.
Confirm fuel pressure if possible.
Get Base Running Airflow close.
Verify idle spark correction.
Check throttle cracker and follower.
Check TCC release.
Review fueling during the return to idle.

Fix one thing at a time.

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