When the key is off, the ECU stops sending signal to injectors. So...if injectors are not leaking, no residual pressure will cause fuel to enter the engine. In fact, fuel pressure should remain in the rail after engine is off.
Carbon on the piston tops will glow hot enough to light fuel vapors after key is off. Run a couple of bottles of TECHRON thru your fuel tank, as directed on the bottle. (Walmart has the best price at $5.95 ea) This is good to do every 2-3 months anyway.
If 1 or 2 injectors were sticking open a little bit, you would likely notice some running problems, and hard start, as the fuel pressure would have dropped to zero, and those cylinders would be flooded. But I doubt even 2 sticking injectors would cause "after run".
Perhaps the cold start injector, which would be the only one common to 'all' the cylinders, is stuck open a tiny bit. It would allow the fuel pressure to drop and possilbly cause your problem. The 02 would see the extra richness, and adjust the regular injectors down a bit to get the good mixture during running condition.
Keep us posted.
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