LS Swap Startup Checklist
Before starting an LS swap engine for the first time, make sure these items are configured correctly. Working through this list before the first crank can prevent damage and save hours of diagnosis.
Engine & ECU Configuration
These items must be correct in the ECU tune before the engine will start cleanly and run safely.
Engine Setup
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Correct injector size configured
Injector flow rate, dead times, and offset data must match the actual injectors installed on the engine.
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MAP sensor type configured
Sensor scaling must match the MAP sensor installed — GM 1-bar, 2-bar, or 3-bar. Incorrect scaling causes inaccurate load calculations.
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MAF sensor connected (if used)
If running MAF, the sensor must be connected and properly configured. If running speed density only, MAF should be disabled in the tune.
ECU Configuration
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VATS removed
Vehicle Anti-Theft System must be disabled. Without VATS removal the engine will crank but not start in the new chassis.
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Rear O2 sensors disabled if unused
Rear O2 DTCs should be disabled when catalytic converters are not present in the swap build to prevent nuisance fault codes.
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Electric fans configured
Fan trigger temperatures, hysteresis, and A/C request logic must be set to match the cooling system in the new chassis.
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Correct engine displacement set
Displacement must match the engine installed. Incorrect displacement affects VE-based fueling calculations and idle quality.
Transmission & Fuel System
Verify the transmission parameters and fuel system are ready before attempting a first start.
Transmission
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Transmission type configured
The ECU must know whether a manual, 4L60E, 4L80E, or other automatic is installed to enable correct shift and TCC logic.
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4L60E or 4L80E parameters set
Shift tables, line pressure, and TCC lockup strategy should be configured before driving to avoid harsh shifting or converter slip.
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Correct gear ratio entered
Rear axle ratio and tire size must be entered accurately for correct speedometer output and shift point calculations.
Fuel System
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Fuel pressure 58 psi for most LS engines
LS engines run a returnless system at 58 psi. Verify static fuel pressure with a gauge before cranking.
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Fuel pump wired correctly
Confirm the pump primes when the key is cycled and that voltage at the pump is within spec under load.
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Return or returnless system functioning
Check that the return line (if used) flows freely and that no fittings or rails are leaking before the first start.
Cooling System & First Startup Steps
Verify the cooling system before startup, then follow these steps on the first crank to catch problems early.
Cooling System
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Electric fans wired
Confirm the fans run when commanded and that relay wiring is correct. Swap builds rely on ECU-controlled electric fans with no mechanical backup.
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Fan activation temperature configured
Set fan-on and fan-off temperatures in the ECU to match the thermostat and cooling capacity of the new chassis radiator.
First Startup Procedure
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Check oil pressure
Confirm oil pressure builds within the first few seconds. Shut down immediately if oil pressure does not appear on the gauge.
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Check for fuel leaks
Inspect all fuel rails, injector connections, and hard lines visually and by smell before and during the first run.
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Verify AFR with wideband
A wideband O2 gauge should be connected before the first start to verify air/fuel ratio. Target 14.2–14.7:1 at idle on gasoline.
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Allow engine to reach operating temperature
Let the engine reach full operating temp on the first start to verify fan activation, check for coolant leaks, and allow closed-loop fueling to engage.
Need help setting up your LS swap ECU?
Midgard Industries offers Gen III LS ECU startup setups and tuning services. Whether you need VATS removal and a baseline startup tune or a full calibration, send your build details and get a quote.