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LS swap first-start checklist
Everything to check before you crank a swapped LS for the first time. Print it, tick it off in the garage, and keep it with the build.
Engine: ____________ Computer: ____________ Date: ________
Engine and fluids
- Oil filled to the full mark, with a new filter.
- Oil pressure first: crank it with the fuel pump fuse or relay pulled until the gauge shows pressure, then put it back.
- Coolant filled, and the steam tubes routed to a port (a radiator steam port, a tapped water pump or a heater-hose fitting).
- Transmission fluid filled. Shifter connected, in Park or Neutral, with the range switch plugged in.
- Belt on and every accessory turns. Fan, hoses and wiring clear of the headers and the steering.
Power and grounds
- Battery charged: at least 12.0 V before you crank.
- Every computer ground on a clean engine ground, plus engine-to-frame and engine-to-battery grounds. Coil harness grounds bolted down.
- Constant battery power and key-on power to the computer. The key-on feed has to stay at 12 V while cranking; one that drops out in crank is a classic no-start.
- Fuel pump relay wired: the pump runs about 2 seconds at key-on, then stops until the computer sees the engine turning. Listen for it.
- Fuel pressure checked at the rail with a gauge: about 58 psi on a factory-style system. Relay wiring.
Sensors and connectors
- Every injector and coil connector seated and locked.
- Crank, cam, MAP, MAF, intake air temp, coolant temp, throttle and knock sensors plugged in. O2 sensors in or turned off in the tune.
- Crank sensor and computer match: 24x with a 24x computer, 58x with a 58x one. Not sure which computer you have?
- Key on, engine off: throttle position reads 0 and sweeps to nearly 100% at full pedal; MAP reads about 95–102 kPa at sea level (lower at altitude).
- Drive by wire: pedal and throttle body both plugged in. Cable: the throttle opens fully and returns.
The tune
- Know what is programmed in the computer before you crank. Firing an unconfigured swap risks damage.
- VATS turned off in the tune.
- Starter-relay diagnostics turned off if the donor's starter interlock isn't there (they can cause a no-start).
- Every sensor you removed (rear O2, EVAP, EGR, air pump) turned off in the calibration.
- Bigger cam? It needs more startup air to catch on first fire.
First fire
- Fire extinguisher in reach, someone watching for leaks, laptop or scan tool connected and logging.
- Watch oil pressure right away. Shut it off if there is none.
- Let it warm up, watch coolant temperature, and top off the coolant as air works out.
- Cranks but won't start? Check key-on power during crank, fuel pressure, the crank signal and VATS, in that order.
lsswapbible.com/tools/first-start-checklist/
Let us do the tune part. A Start-up File ($79) gets it started and driving: injector data, idle, fans, conservative fuel and spark, and VATS off if you ask. Built on a read of your computer (reading needs no credits).