What is this spring?

Well, some guesses: If you had work done on the truck, it could have popped off a mechanics tool. It looks too clean and shiny to be an original, under-hood part of your car. It has some corrosion, so it can rust. Hard to tell how large it is from picture, but as I said, some tools use torsional springs like this. What I can also tell you about it, is it designed to to be on a shaft of some sort, and used torsionally, to bring something back to it's original resting position. Sort of like on a throttle-body spring (this doesn't look close enough), a pair of pliers, coolant pressure tester (scissor type), etc... For this spring to come off, the shaft it rested on would be broken, or for the end of the shaft to fall off/be removed.

Did you have the serpentine belt tensioner replaced or does it look new(er)? Could have popped out of the old broken one and was never found until now. Sometimes vehicles like to "eat" tools as well. When I showed my Nephew how to do his first tune up on his 2000 Grand Prix we lost a spark plug socket. I'm sure that it's still in there somewhere, lol! HTH. -Jim

That defenentely looks like an old belt tensioner spring, there was a short duration of time that it seemed that every Explorer in my area had tensioner failures, every parts store was ordering them daily, and that spring was the only thing that was still attached to the back of the tensioner assembly, everything forward could be found scattered around the engine compartment. You can see the lug of the spring in the pic.

Thanks! That makes sense. A long time ago my tension pulley broke. It sounded like a gunshot when it broke. replaced it myself, but I don't remember the particulars. I could easily have overlooked the spring.

I'll have to agree, it's a tensioner spring, if it's large enough to be? As I stated above, difficult to tell it's size from the picture.

Thanks! I should have given dimensions. It is 2 inches in diameter and 1 1/2 inches across the coils. It is very stiff, so much so that I cannot compress it at all with my hands. dm

Tensioner spring.