I posted this question concerning my flex fuel 2011 Town Car in the GM/CV forum and got no response.
The service manual specifies two spark plugs: AGSF-22FM1 with a gap of .041-.047" for "flex fuel only"; and, AGSF-32N with a gap of .052-.056" for "gasoline fuel only".
I plan to use only gasoline fuel. My thinking is that the AGSF-32N (or a comparable plug, such as the Autolite AP104) with its higher heat range and wider gap will run better and maybe give slightly better fuel economy on gasoline because it was designed for gasoline. The flex fuel AGSF-22FM1 is designed for E85 which requires a cooler plug to prevent pre-ignition and may run less efficiently on gasoline.
However, the car runs great on the flex fuel AGSF-22FM1plugs; and, when I removed and inspected them I found them to be burning clean with no evidence of fouling.
I would appreciate any informed comments, particularly from anyone who has switched a 4.6L flex fuel engine to the AGSF-32N or comparable plug.
Maybe no one switched plugs with a flex fuel vehicle. I for one haven't. So I can't help there. In our shop we use either the Motorcraft or Autolites that are made by the same company.
Thanks for the reply Bert.
Seems like there should be a few DIYers out there with older flex fuel Fords that when the time came for new plugs put in the gasoline plug instead of the flex fuel plug. I am just wondering whether it is worth doing now at 2,000 miles on the odometer.
Based on the advice I received from a highly regarded idependent repair shop owner/technician and radio program host, I removed the AGSF-22FM1 flex fuel plugs and installed the AGSF-32N gasoline plugs. He confirmed my thinking that the gasoline plugs will perform better in gasoline than the flex fuel plugs. He also said that spark plugs are a critically important part of the ignition system and that OEM plugs are usually the best choice, but aftermarket plugs such as Autolite are OK.
The car runs smoother and seems to have more "pep". I will report back later on MPG.
I have driven about 3,000 miles, including a 2,000 mile road trip, since replacing the AGSF-22FM1 with the AGSF-32N spark plugs. It appears that I am getting 1 to 2 more MPG. It also starts quicker and has a more responsive throttle response.