Ford Fiesta Engine Problems

You have reported it while the warranty is in force so I would believe they are obligated to honour the warranty whether or not they report back before the warranty ends. Keep all records of the reports you have made, and any conversations you have had with the dealer/Ford, and any undertakings you have been given by either party. I they don't come good and fix your car you would then have the ammunition to mount a solid case for compensation.

As you state Ford recommends 91-octane fuel for your car, so you would expect it to run on regular unleaded without a problem. I assume you have tried E10 unleaded, which generally has an octane rating of at least 94. If not it might be worth trying that, but   other than that go back to your dealer and point out to him that   you're not happy with your car and that you want it fixed.