'94 Mustang check engine light

I have a '94 Mustang with the 3.8 engine that was rebuilt 25 000 miles ago.
Idles smooth, starts great and has no hesitation. Everything is as it should
be. A month ago I filled it at a gas station I usually don't go to and after
about 1/8 of a tank being burned the check engine light came on. I checked
all the hoses, wires, basically everything I could put my hands on and it
all seems ok. I disconnected the battery and reset the computer and it was
fine. A few days ago I again filled the tank at the same station because it
was the only one open locally and the next day the check engine light again
came on. I thought maybe it's some bad gas or something similar. Then the
next day when I started the car a huge cloud of white smoke came out the
back for about 5 seconds or so. I know white smoke usually means water and
coolant is being burned but the coolant level is up and hasn't dropped one
bit. I'm not losing oil, coolant or anything. I even took out the MAF sensor
and sprayed it a couple of time with an electrical contact cleaner I bought
specifically for electronics and contacts. It looked like new before and
after I sprayed it, not covered in any gunk at all. My guess is water in the
tank but I'm not too good at
troubleshooting this kind of stuff.

I'm taking it into a garage to get the diagnostics read but I was wondering
if anyone has any ideas what it could be.

Thanks for any and all input or ideas!