A brand-new engine for today's modern cars can cost thousands of dollars out-of-warranty. With a new engine out of reach for many car owners, it's no wonder used engines are a highly popular and comparatively affordable alternative. Used engines are inexpensive and readily available, in most cases, but they can also be a bit of an unknown quantity. When buying a used engine, it's important to know what you're looking for and understand the pitfalls and caveats that often come with purchasing any used component.

Take Stock of Appearances

With a used engine from an auto salvage yard, what you see is usually what you'll get. Unless the engine was removed the moment it reached the yard and stored for safekeeping, chances are other customers may have cherry-picked any number of parts from the engine or its accessories (such as the alternator, starter, sensors, etc.).

For this reason alone, you'll want to have an original schematic as well as a reference image of the engine in question. This gives you an opportunity to see what's missing, as well as ensuring the engine itself matches up to what you need in your vehicle.

Look Out for Rust

Auto salvage yards aren't kind environments to engines. Most of the time, the hoods are left open or taken off altogether, exposing the engine to the harsh elements. If the engine was partly disassembled for any reason and left that way for an extended period of time, that could allow rust and corrosion to take place deep within the inner workings of the engine. This can cause damage in a wide variety of areas, including the coolant passages, pistons and cylinders.

The amount of rust you may encounter depends on the age of the engine itself, the climate of the area where the auto salvage yard is located and the amount of time it spent exposed to the outdoors. So if you're looking for a used engine for your 1950s-era cruiser, don't be alarmed if you see sizable surface rust. As long as the engine manages to turn freely, you can deal with most rust problems.

Check the Dipstick

Although it is usually standard procedure for an auto salvage yard to drain all of a vehicle's vital fluids, there may be plenty of left-over residue on the dipstick and within the engine oil pan. You can use this residue to help gauge the engine's health prior to arriving at the salvage yard. Here are some things you'll want to look out for as you search for that perfect used engine:.

• Metal flakes and shards – Metal shavings, flakes and shards often indicate a mechanical malfunction somewhere within the engine. It could be any number of things, from a failed oil pump to worn down camshafts, broken spark plugs or a prior repair gone wrong.
• Looks like a milkshake or sludge – When engine coolant mixes into the engine oil, the resultant mess resembles a rather unappetizing chocolate milkshake in appearance. Milk chocolate-like engine oil is usually a good indicator of a damaged head gasket or a warped cylinder head.
• Strong gasoline odor – Blow-by caused by worn or damaged piston rings can cause the engine oil to smell more like gasoline than oil. Excessive blow-by often indicates an engine that's in need of significant rebuilding once it's rescued from its temporary home.

Give It a Spin

Most importantly, the engine should turn freely upon turning the crankshaft pulley. Depending on where or if the engine is located in the engine bay, you can use socket wrench with the appropriate socket to rotate the pulley and move the pistons.

Any interruptions in movement should be carefully examined, as it could mean additional problems preparing the used engine for its next phase of life.

With careful inspection and some TLC, most used auto parts you find can be used for your vehicle.