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Fuel injection computers have a "Clear Flooded Engine" mode
Key on, engine off
Press gas pedal down to the floor and hold it down all the way
With RPMs at 0, and TPS(throttle position sensor) at 4.5volts, computer will turn OFF fuel injector pulses but spark stays on.
Now crank engine, it should not start, no fuel, if it fires or starts then fuel is leaking in.
If RPMs get above 400(engine fires), Computer will exit "Clear Flooded Engine"
If you take your foot of the gas pedal computer will exit "Clear Flooded Engine"
I use "Clear Flooded Engine" every morning when I start my high mile 4.0l, I want to get oil circulating for about 10-15 seconds, then I let off on gas pedal for start up