I have a 3.0 Flex fuel ranger. 1999 with 186k miles. I have only owned the truck for about 2 years and have never put in E85. I also don’t know if the previous owner did as well, the truck was a farm truck before I bought it. Is it bad to use E85 after not using it for a while? I have never had a flex fuel vehicle so I am not sure how it works.

 

Flex fuel has to do with the air fuel ratio for Gasoline and Ethanol
Gasoline uses 14.7 to 1 air to fuel
Ethanol(E85) uses 9.7 to 1
And these are WEIGHT Ratios

14.7 pounds of air to 1 pound of gasoline
9.7 pounds of air to 1 pound of E85

The 3 liter engine pulls in the same amount(weight) of air based on RPM, that doesn't change
You can see that more E85 fuel needs to be added for each pound of air the engine pulls in, so MPG will be less with E85
So you need to weigh that against the price difference in the two fuels

In a 1999 Ranger there is a Fuel Type sensor near the fuel filter in the frame rail, it was a very expensive sensor to replace, $650 if you can find one
This sensor was removed in 2001 and up when new computer software was used

Some people that had this sensor fail replaced it with a much less expensive "Fuel Sensor Emulator", but that means vehicle can no longer run on E85
So I would look under the truck to make sure the 3 wire sensor is still plugged in and not a Fuel Sensor Emulator, google it to see what it looks like
The sensor is on the fuel line, emulator is not

Once that's been determined and sensor is there, you can run E85 with any mix with gasoline, i.e. even if you have 1/2 or 3/4 tank of gasoline you can add E85, the sensor determines the ratio of ethanol to gasoline to set the correct air/fuel ratio to use