Another Code 543 question... <SOLVED>

[Cole94]

94 Explorer XLT 4.0

Rough idle, but it does fine going down the road... until it warms up, then it starts missing really bad.
I read codes and got a "543: Secondary Fuel Circut Open."
I'm going to use my scanner and do a wiggle test on the electrical for the fuel system tomorrow.

Ignoring the code, could it be bad/stale fuel or water in the fuel? When the engine warms up it goes to open circuit and leans the mixture, and if there was water in the fuel it would run bad, right? Is there a test for water in the fuel?

 

[Cole94]

Thanks for all of the helpful responses </sarcasm>

In a last-ditch attempt to fix it, me and my dad went and got a fresh tank of fuel and put a bottle of HEET in it, but I got 50 miles in 10 gallons and it was getting hard to start.
Finally got fed up with it and dad sent it to the dealership... they hooked up their expensive computer and couldn't find ANYTHING... not even a code. They tested the fuel... sure enough, the octane level was "too low to even run a lawnmower," and the fuel was gelled (below freezing temps night before). Guess it was water in the fuel after all... They are draining the tank, cleaning it, and putting a new fuel filter on.

I hope this can help someone else down the road.