My 2003 Ford Explorer XLT V8 5.4l had a dry oil stick, added 5 qts of oil, drove it for the evening and all of a sudden a really loud knocking started. Did it blow a piston?

It likely has a ruined crankshaft or rod bearing...or several. Pistons and or cylinder walls could be scuffed also. If it is knocking and was run very low on oil the end result will not be good.

Are you sure that your Explorer has a 5.4 engine? Or this this an Expedition? No matter the above still applies.

It is a V8 . . 4.5L I think . . regardless, would the cost of repair cost more than $1,000?

If it is a V8, then it is a 4.6 L. You might consider a used engine from a salvage yard (that can be somewhat of a gamble unless you get a warranty) or a rebuilt engine. I think just in labor alone you are looking at over a grand.

Thank you. I assumed as much . . given that we just spent $700 in repairs and gave it to our 17 year old, I am not willing to do much more.

I don't blame you. You could maybe consider a used or rebuilt engine, but unless you can install it yourself you are still looking a a rather large repair bill of $2,000 plus. Best of luck with it.