Hrm.. Well, if your ICP is bad, then it means that your 6.4l seems to have transmogrified into a 7.3l without your knowledge.
Either that, or you might have posted your question in the 6.4l forums instead of the 7.3l or 6.0l forums..
In all seriousness though, on a 7.3l, you will probably end up with a P1211 code, and ragged idle, surging acceleration.
On a 6.0l it will kind of do the same thing with the idle and acceleration, but I don't know what codes it might throw.
More information on your truck would be helpful in diagnosing your problem..