Hello everyone! So strange problem started up. My mustang would surge between 1200-2000 rpm when coasting above around 30mph in neutral and the check engine light came on for the IAC. I replaced it with a motorcraft one and now the light is off but it still surges between 1200-1500. The car runs and idles flawlessly besides that. Could it be a bad TPS as well?
I once asked EFI expert Probst (SAE) what does the PCM do when car is coasting at speed, throttle closed? Shut off injectors? NO! feeds minute amount of fuel to them. I would think the same thing is true if transmission is in neutral, though the fuel amount likely is less.
Maybe, but doubtful. Try it in yours (any of them). I bet the rpms will probably fall down around idle range and hang there until you put it back in gear.
Not that I advocate cruising in neutral. @Jack118711 don’t be cruising in neutral.
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