When buying a used engine, the best way to make sure you’re getting what you need is to hear the engine run before it gets pulled from the donor vehicle. Listen for ticks and knocks that could indicate that the engine needs a rebuild, a new timing chain, or is just junk. Small ticks are probably not a big deal unless you plan on running the engine as is. Large knocking noises can be bad. If the engine turns over but won’t run, that might not be a deal breaker, but it could be hiding low compression or internal knocks that you wouldn’t know about without hearing the engine run. Look at the oil to check if it’s nice and honey- colored or black and worn—also give it a smell. If you can smell fuel in the oil or it looks foamy and whitish, that could indicate coolant coming from a bad head gasket. Any dubious evidence should bring the price down or help you decide to keep looking.