An engine failure is always bad news because it involves making some major decisions:
Should you spend your money to replace your old engine? Or would it be smarter to get rid of your old car or truck and buy another?
Is your old engine repairable? Or will it have to be replaced with a new, remanufactured or used engine?
Which of these options is the best way to go, and which will give you the best return for your investment?
If your car or truck is less than 10 years old, and you really like it, or you can't afford to buy another vehicle at this time, your best choice is probably to repair or replace your engine. Cars and trucks depreciate rapidly as time goes by, even if they are not driven much. By the time your vehicle needs an engine, its "book value" or "trade-in" value may be so low that sinking more money into the vehicle does not make economic sense. Consequently, if your car or truck is worth less than about $2000, think long and hard before you put any more money into major repairs. Your money wold be better spent on a newer vehicle.
On the other hand, if your car or truck is more than 10 years old, or you don't like it that much, or you are looking for an excuse to buy another vehicle, don't spend another dime on your old car or truck. Forget repairing or replacing your old engine. Call the junk man to tow it away, or donate your vehicle to charity and take a tax write-off, or sell it cheap as a "mechanic's special" to somebody who thinks they can fix it up and get it running again.
If your old engine has a lot of miles on it (over 150,000 or more), and it is burning oil, running poorly, making noise or has locked up, overhauling it will be expensive:
Rebuilding a high mileage engine often requires boring out the cylinders to accept new oversized pistons. This adds a lot of expense because of the parts and machine shop labor. The engine block may also have to be line bored to restore the alignment and roundness of the crankshaft main bores. The deck surfaces on the engine block may have to be milled to restore flatness and the proper surface finish.
The cylinder heads will also have to be resurfaced, the exhaust valves replaced, maybe the valve seats if it is an aluminum head, and the overhead cam bores may need to be line bores also to restore the bearing surfaces and cam bore alignment.
In addition to the machine work, the engine will have to be completely disassembled, thoroughly cleaned and checked for cracks or other damage that might render the block or heads unrebuildable. If the block and heads are good, the crankshaft will probably have to be reground to undersize to restore the journal surfaces. A new camshaft and lifters or followers may be needed if the old parts show too much wear. Same for the pistons.
Other new parts that will be required include new main, rod and cam bearings, a new timing chain and gear set (or timing belt if it is an OHC engine with a belt), a new oil pump, and any other parts that are damaged or worn too much. It all adds up to a very expensive repair.
Because of all the parts and labor that are required to rebuild a high mileage engine, many repair shops and dealers recommend replacing your old engine with a new "crate engine" or a "remanufactured" (reman) engine. Both come more or less complete, and can usually be installed in a day. There are no delays waiting for machine work or parts for your old engine, and most crate engines and reman engines come with a warranty.
As for used engines, they can be risky. An engine from a low mileage wreck at a salvage yard is probably okay. By low mileage, we mean less than about 60,000 miles. And if the salvage yard will guarantee the engine is good (some will, some won't), it would probably be much less expensive to buy the used engine and have it installed. But if the engine has a lot of miles on it, or it came out of a junked car (not a wrecked car), or the salvage yard won't guarantee it, don't buy it. Keep looking or opt for a new crate engine or reman engine from a reputable supplier.
One very important point to keep in mind if you are shopping around for a good used engine is that it has to be compatible with your engine management system, sensors and wiring harness. Because engine designs and calibrations often change from one year to the next, you may have problems finding an engine from the same year, make and model as your current vehicle, or one that is "close enough" to be compatible.
A new crate engine literally comes in a crate or shipping container and typically has all or mostly new parts in it. Many crate engines come with various "upgrades" that provide larger displacements or more horsepower if that's what you want. Others are essentially duplicates of the factory engine.
A reman engine is a used engine that has been disassembled, cleaned, inspected and rebuilt to like-new specifications. Common wear items such as bearings, rings, timing chains, valve springs, gaskets, seals, oil pump and other parts are replaced, and major parts like crankshafts, camshafts and pistons are replaced with new parts on an "as needed" basis. The finished product is built to meet or exceed OEM engine specifications.
Another advantage of choosing a remanufactured or rebuilt engine over a new engine is that it recycles valuable parts and extends their useful service life. It's an environmentall-friendly approach that reduces scrappage while keeping people employed.
A new crate engine, by comparison, usually uses all new parts (engine block, cylinder heads, crankshaft , connecting rods, pistons, camshaft, valves, etc.). Because of this, a new crate engine costs considerably more than a reman engine, and possibly much more if you opt for performance upgrades.
Crate engines and reman engines usually come with a warranty (longer is always better and worth the extra money).