Soooooo let me see if I understand you, by that do you mean that if he let go of the steering wheel while going straight the car will pull to one side?
Check the belt and see if its at proper tension. My daughter noticed poor steering (actually would pull out of her hands on corners). New belt took care of it.
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TC/BS and their "Forced Air GTP Daytona, Typhoon and Cummins 8.3"
I could be wrong, but my brother had what sounds like to be the same problem with his Ford Taurus. Did you check if there were any problems with the steering column? Just a thought.