Hi, i am running a ford focus st with around 290-300bhp, it has been ran on shell v power, i stupidly filled up with normal unleaded, i ran the car lightly and drained the tank and is now back running on shell, however even though it has improved it is still misfiring at high revs ( pre detonating i think ) my question is does the ecu take a while to re learn the higher octane fuel?
Your computer will attempt to compensate for the lower octane fuel. The fuel would not cause misfires but may cause pre detonation on high load situations. The problem may be okay after refilling the fuel tank a second time. If misfire continues then have engine computer scanned for misfires and possible problems then. The lower octane fuel unless contaminated should not cause a check engine light or misfire.
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