LS Swap Cranks But Won't Start: The Tuning Checklist

An LS swap that cranks strongly and never fires is one of the most demoralizing points in a build, mostly because the possible causes span three completely different domains: mechanical, electrical, and calibration. People burn weekends because they start in the wrong domain.

The short answer: confirm spark, fuel pressure, and compression first — if any of those is missing, no calibration change will start the engine. If all three are present and it still will not fire, the calibration becomes a strong suspect, and the three usual culprits, in order of frequency, are anti-theft still enabled, cam and crank sensor configuration that does not match the physical engine, and injector data so far off that the controller cannot deliver a startable mixture.

Work the list in that order. Guessing costs days.

Step zero: prove the three fundamentals

Before opening a tuning program at all, establish these three. Each takes minutes.

Spark. Pull a plug, ground it against the head, and crank. If there is no spark, the problem is coil power, the ignition control wiring, or a missing crank signal — none of which is fixed by editing tables.

Fuel pressure. Put a gauge on the rail. The pump should prime with the key on and pressure should hold. A pump that never runs is usually a relay, a control wire, or the ground — again, not a calibration issue in a swap where the wiring is new.

Compression. If the engine came from an unknown source, verify it. A no-start on an engine with a jumped timing chain or a swallowed valve will not be solved by anything on your laptop.

These three are non-negotiable because they define what "will not start" even means. An engine with all three present that still refuses to fire is telling you something specific: the controller is either not fueling, not sparking at the right moment, or fueling so far from a combustible mixture that it cannot light. That is a much narrower problem.

Culprit one: anti-theft is still enabled

This is the most common calibration cause of an LS swap no-start, and it has an unmistakable symptom.

The factory anti-theft system means the engine controller expects a confirmation message from the vehicle's body control module that a valid key was used. In the donor car, that module exists and the conversation happens. In your swap, that module is almost certainly gone.

Without the confirmation, the controller allows the engine to fire and then cuts fuel after roughly one to three seconds. The pattern is distinctive: strong crank, immediate catch, brief run, sudden death — as if someone flipped a switch. If that is what your engine does, stop investigating everything else and check that anti-theft is disabled in the calibration.

The related family of factory security features — PassLock and VATS-style systems across GM generations — all express the same way. Disabling anti-theft in a standalone swap calibration is a normal, expected part of a swap and is unrelated to emissions controls; it is a security handshake with a module you no longer have.

Culprit two: cam and crank correlation

The controller determines where the engine is in its four-stroke cycle by combining the crankshaft position signal with the camshaft position signal. If the calibration's idea of the crank reluctor wheel does not match the wheel physically bolted to your engine, it cannot resolve position.

The classic mismatch on LS engines is the two reluctor generations — a 24-tooth wheel on earlier engines and a 58-tooth wheel on later ones. Put a calibration expecting one on an engine fitted with the other and you get either no fire at all or violently erratic firing. The same class of problem occurs when a cam sensor is relocated between generations or when a harness pins the sensor to the wrong input.

Verify what is physically on your engine, then verify the calibration matches it. This is a configuration check, not a tuning judgment, and it takes minutes. It is also easy to get wrong when the engine's history is uncertain — a "5.3 out of a truck" can be several different configurations.

Culprit three: injector and airflow configuration

If the controller is commanding fuel but the engine never catches, look at whether the fuel it is commanding bears any relationship to what the injectors deliver.

Every injector has a flow rate and a set of dead-time or offset values describing how long it actually opens versus how long it was commanded. If the calibration expects one injector and a very different one is installed — a common situation in a swap that mixes engine, intake, and fuel-system parts from different sources — the pulse width the controller calculates produces a mixture far from combustible. Too rich to light and too lean to light both look identical from the driver's seat.

Enter the correct injector data before you spend hours elsewhere. Our injector data page covers what values you need, the injector size calculator sanity-checks whether the injectors suit the engine at all, and the injector slopes reference explains what the high and low slope values actually represent.

The same logic applies to airflow configuration. If the swap uses a mass-airflow sensor, the calibration must expect one and the MAF transfer function must roughly suit it. If the swap deleted the MAF and runs speed density, the calibration must be configured for that, with a volumetric efficiency table that is at least approximately right. A calibration set up for a sensor that is not installed produces immediate, confusing failures.

Using a datalog to split the problem in two

Here is the diagnostic that saves the most time, and almost nobody does it early enough: log the engine while cranking.

What the crank log shows	What it means	Where to look next
No RPM registered at all	Controller never sees the engine turning	Crank sensor, its wiring, or reluctor mismatch
RPM present, injector pulse width zero	Controller sees rotation and refuses to fuel	Calibration — anti-theft, or a disabled fuel function
RPM present, injectors pulsing, no catch	Fueling is happening but the mixture will not light	Injector data, airflow config, or fuel not reaching the rail
Fires then dies after 1-3 seconds	Security cut	Anti-theft still enabled in the calibration
Erratic firing, unstable RPM	Position resolution failing	Cam and crank correlation, reluctor type

That table splits a vague "it will not start" into five distinct problems with five distinct next steps. Cranking logs are short, easy to capture, and dramatically more informative than another round of swapping parts. Our datalog channels article covers reading the channels in general; on a no-start you only need the three above.

What is almost never the cause

Two things absorb enormous amounts of swap troubleshooting time and are rarely the actual problem on a crank-no-start.

Spark timing tables. People open the timing tables early because timing feels like the thing that makes an engine run. But a base timing table that is merely suboptimal will still start an engine. If the engine will not fire at all, timing tables are not the issue — the controller's ability to determine position might be, which is the cam and crank item above and lives in configuration, not in the timing map.

Idle tables. An idle calibration that is wrong makes an engine that starts and then stalls under no load. It does not make an engine that never catches. If you are in the idle tables on a car that has never fired, you are in the wrong place.

Both are worth attention once the engine runs. Neither belongs in the no-start phase.

The order that works

Put together, this is the sequence.

One: confirm spark, fuel pressure, and compression. Do not proceed until all three are present.

Two: capture a cranking datalog and read RPM and injector pulse width. Use the table above to decide which domain you are in.

Three: if the engine fires and dies in a couple of seconds, disable anti-theft in the calibration. This one symptom is that specific.

Four: verify reluctor type and cam sensor configuration match the physical engine.

Five: verify injector data matches the installed injectors, and that the airflow configuration matches the installed sensors.

Six: once it starts and idles, stop treating it as a no-start and switch into the normal calibration loop — baseline datalog, verify supporting data, one change at a time, log after each. Our LS swap tuning page covers what comes after the first successful start, and the complete guide to tuning your car walks the loop in full.

After it starts: what to do in the first hour

The first successful start is not the finish line, and the first hour of runtime is when a fresh swap is most vulnerable.

Watch coolant temperature closely and do not let it climb while you celebrate. Confirm oil pressure immediately. Check fuel pressure again with the engine running, since a pump that primes adequately can still fall short under actual demand. And let it idle and warm up fully before revving it — a swap with an unverified calibration is not a car you want to load until the fundamentals have proven stable.

Then capture a proper baseline datalog and start on supporting data: airflow first, injector characterization confirmed, fuel trims driven toward zero. From that clean baseline, real tuning begins — and the discipline is exactly the same as on any other engine, as the ECU tuning fundamentals article sets out.

The bottom line

An LS swap that cranks but will not start is a diagnosis problem, not a tuning problem, and the fastest route through it is a fixed order: fundamentals, cranking log, anti-theft, position sensors, injector and airflow configuration. The single most useful diagnostic — a datalog captured while cranking — takes two minutes and splits five possible causes into one.

Resist the pull toward timing and idle tables. They matter later. Get the engine to fire, get it to idle, and only then start calibrating. If you want a second set of eyes on a swap calibration before or after that first start, start here or read the HP Tuners help overview.

Frequently asked questions

My LS swap cranks but will not start. Is it a tuning issue?

Sometimes, but check the fundamentals first because they are faster to rule out. Confirm you have spark, fuel pressure, and compression. If any of those three is missing, no calibration change will start the engine. If all three are present and it still will not fire, the calibration is a strong suspect — most often anti-theft still enabled, an incorrect cam and crank sensor configuration, or injector data so wrong the engine cannot get a startable mixture.

What is VATS and why does it stop an LS swap from starting?

VATS is the factory anti-theft system. In the donor vehicle the engine controller expects a message from the body control module confirming a valid key, and without it the controller cuts fuel after a couple of seconds of running. In a swap, that body module is usually gone, so the controller never gets its confirmation. The calibration must have the anti-theft function disabled, and the classic symptom is an engine that fires, runs for one to three seconds, then dies as if a switch was thrown.

How do I tell a wiring problem from a calibration problem?

Datalog it while cranking. If RPM never registers, the crank sensor signal is not reaching the controller and that is wiring or sensor, not calibration. If RPM registers but injector pulse width is zero, the controller sees the engine turning and is deciding not to fuel it — that is calibration. If RPM registers and injectors pulse but the engine never catches, look at the mixture: injector data, airflow configuration, or fuel that is not actually reaching the rail.

Do I need to change the injector data before it will start?

If your injectors differ meaningfully from the ones the calibration expects, yes. A controller commanding a pulse width sized for a much smaller or much larger injector produces a mixture too far off to light. Enter the correct flow rate and offset data before you spend hours chasing other things — it is a five-minute edit that fixes a whole class of no-start.

What about cam and crank correlation on a swap?

The controller uses both signals together to know where the engine is in its cycle. If the reluctor wheel type in the calibration does not match the one physically on the crank — the 24-tooth and 58-tooth generations are the common mismatch — the controller cannot resolve position and either will not fire at all or fires erratically. Verify the reluctor setting matches the actual engine before anything else in the calibration.

My swap starts and then dies after a couple of seconds. What is that?

That specific pattern is the classic anti-theft signature. The engine has spark, fuel, and compression, which is why it fires — and then the controller cuts fuel because it never received the security confirmation it expects. Check that anti-theft is disabled in the calibration before you investigate anything else.

Should I get the swap running before I tune it properly?

Yes. Getting the engine to start and idle reliably is a configuration problem, not a tuning problem, and it comes first. Once it runs, warms up, and holds fuel pressure, you switch into the normal calibration loop: verify airflow and injector data, capture a baseline datalog, then change one thing at a time and log after each change.