Is the EFI speed density or MAF? How long does it stay at the elevated RPMs? Start it, let it run for a few seconds (10 - 15), shut off, restart. Both systems have a "cold start" mode and need a moment to settle in. My speed density system would bring the engine up to around 2500 - 2800 after the battery was disconnected and took a couple of restarts to get it to settle in to a proper idle. After the computer "learned" all of it's parameters it still fired up to 2500 then immediatly settled into a normal idle.