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The 3-4 seconds after ignition is because the ECM is in "START" mode as opposed to "RUN" mode or "WARM UP" mode. Fuel curve is different, ignition timing is different, combustion mix is different, exhaust gas has different makeup. It's all a big mess of look-up tables and "START" is a whole different table than "WARM UP" or "RUN." There are varied indicators that push the engine onto a different table. One of them can be vacuum.