Ford z x 2 serpintine belt keeps coming off

Are you sure the belt tensioner spring is strong enough to keep the belt tight ?
You should have to move the tensioner to put the belt on, then it should keep the belt tight when you finish.

Generally speaking, the belt is either too loose or one of the pulleys is not straight with the others.

Typically a belt will come off if all the pulleys are not perfectly aligned.
If you recently replaced a power steering pump and the pulley was not properly seated all the way down where the pulley front is flush with the metal shaft from the pump then you will shred the belts and they will come off. If you ONLY swapped the tensioner and idler pulley you may have installed the tensioner so it isn't parallel to the other pulleys but that would be very rare because it would have been difficult to get the belt on if it was not properly installed. There also could be a damaged plastic pulley that could be "cutting" into the belt which would also cause the belt to slip off.

here it is

You shouldn't have to remove those items, I believe you would lift up on it. You canuse a 3/8 or 1/2 inch drive racket, that will fit in the square hole in the tensioner. They also make a tool that you can use, but a bit pricey, around $40.

Yes a new belt the old one has gotten briitle and stiff from all the heating and cooling cycles.
Good luck dont forget to rate.

There are 2 possible engines for your Windstar, the 3.0L and the 3.8L. Below are both diagrams. Best of luck on your repair!
Greg

3.0L






3.8L

it sould be located under the hood on the radiator shroud area

bring belt tensioner to where all the way moved
make sure you have belt set once you get the tensioner moves over the belt slides the rest way on and tensioner goes against belt
make sure belt is lined up properly

It sounds to me like the belt tensioner pulley is bad.Its spring loaded and this pulley is what keeps the belt in place.