Can you tell if its coming from the head or below?  Often an engine will
spin a crank bearing if low on oil and cause knocking.  First you need
to find out where the oil went.  Isuzu V6's short piston skirt is well
known to cause it to burn oil up to 1qt per 1,500 miles or so.

While not cheap I doubt you need a new engine.  Probably rebuild/replace
whatever was damaged.

Welcome to the world of the Isuzu Trooper.  Here's the deal as I've
come to understand it:

"Troopers will always burn a lot of oil."

I've been across other Trooper owners at red lights and asked them if
their Trooper burns oil.  The ALL laugh and say Yes!  Always.

My 2001 burns a quart a week but otherwise runs great and is very
dependable.  Best advice is that if the vehicle is running well and
you want to keep it is to just keep it full by checking regularly.
Slick-50 never hurts either.
~GJ~

Mrs. Cullifer,
I'm feeling your pain. Unfortunately, this is, as the other posters have
mentioned, a common problem with this engine model. My 98 3.5 Trooper has
gotton as bad as 300 miles to the quart, while pulling a small trailer at
highway speeds. One quart every 1000 miles for these engines is good.

Now, for your problem. You do have options. You could have a shop pull the
oil pan and see if they can see a visible play in one of the rods on the
crank, which is most likely the cause of your noise, as Miles has said. You
probably spun a rod bearing. Sometimes, you can spin a rod bearing and not
damage the crank journal. What I mean by that is that as the engine runs,
oil is forced through the crankshaft through holes that are drilled in it,
and the holes provide oil to bearings which are really "shells" of soft
metal that keep the pieces of steel (crank journal and connecting rod end)
from making direct contact. Oil is supplied to these bearings, which
actually causes the clearance between the journal and the inner diameter of
the bearing/rod end to be filled with oil, keeping the parts from actually
touching. If they actually touch, you have a "knock". When they touch, due
to lack of oil, heat is generated nearly instantly and causes the bearing
material to get hot and melt, and stick to the crank journal or actually
"spin" in the rod end, which has little tabs to keep the bearings in place,
assuming there's oil to lubricate it. No oil, no keeping the bearing from
spinning. As I said, sometimes you can spin a bearing and not damage the
crankshaft. It is possible to remove the pan, take off the affected
cylinder's head, and replace one rod/bearing/piston set, even if the crank
is somewhat scored - it can be dressed down and an oversize (or undersize)
bearing installed with a NEW rod and piston. If the crankshaft is ruined,
then you're better off pulling the engine and overhauling it, or finding a
used on on Ebay or from a reputable wrecker that deals in Isuzus and you can
trust. I'm sure there are short blocks (built up block, crank, pistons)
available, but I think they may be quite expensive. And you need to have
your heads rebuilt, unless you want to have more oil consumption problems,
depending on the mileage on your 99.

Do you have a local, independent garage that you trust? If so, I'd recommend
you have your Troop towed to them and ask them to listen to it, and try to
do whatever they can to figure out what's wrong. It may cost you several
hours of their time, but it would be worth it. If you don't know a local
shop that is competent, ask around for who your friends use to fix their
cars and go off that.
Hope this helps. Let us know if you have more questions.
r
Steve