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It only happens when I refuel and then drive and everytime I come to a complete stop it drops RPM from a normal 1k to 500 and then goes back up to 1k I am not sure what’s wrong I use 93 premium because I got 2017 mustang ecoboost twin turbo. Haven’t stalled yet after driving for maybe 20 min it never does it again. Please help?

  • Welcome to Motor Vehicle Maintenance & Repair! I'd personally be wondering why your engine is running at 1k rpms after it's warmed up. I know my truck idles at ~450-475rpms after it's fully warmed up. It's a 2018 Silverado with a direct injection LTx engine. I don't know for sure, but I believe manufacturers started tuning them with a lower idle for better fuel consumption. – Pᴀᴜʟsᴛᴇʀ2 Oct 7 at 21:03
  • Well the mustang manufacturers usually set it 700-1000 rpm idle, I just don’t understand why it drops after refueling then after 20 min it does not do that until next refueling. – Dominic Musick Oct 8 at 3:44
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I'm not very experienced on this subject, but these are my thoughts:

The problem may have to do with the air pressure inside the fuel tank. When you remove the fuel cap, the pressure is equalized to the surrounding air pressure.

When you're driving, fuel is sucked out of the tank which could cause the air pressure inside the tank to drop. (it causes a small 'vacuum').

Maybe this is somehow related? Maybe something in the evap system is broken? I'll leave the answering to someone more experienced, but hopefully this puts you in the right direction.

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