The engine is the heart of the vehicle, and when buying a used car needs to be carefully looked at. Start with a look at the overall condition of the engine area. Is it clean or oil filled, dirty, and clearly neglected? Is it in consistent condition with the rest of the car? Some owners get their dirty engine compartments power washed with degreasing fluid or hot water under high pressure. You should look at engine components to find out what secrets you cannot immediately see.
Check the colour of the oil first. If it's clean (tan or brown), but not black, ask when the last oil change was completed. Check the windshield or door for the sticker from the local quickie-lube's oil sticker and if found, compare against the date and the mileage on the car. If the oil was changed very recently, remember to check the oil again after your test drive, for any change in condition. If the oil is already black after the drive, there is serious wear, which should be reflected in the price.
Make sure the motor oil is at full level, and look under the front of the car for oil leaks, typically coming from valve covers, oil pan, and front and rear engine seals. If oil is visible on the sides of the engine, there is a good indication of these problems. Remove the oil fill cap and inspect it, for signs for engine sludge build up. Also look inside the valve cover for sludge and oil build-up. This inspection will indicate how often the oil was changed and if the engine was neglected, since a well-maintained engine, regardless of mileage, will be fairly clean inside.
Start the motor cold to assess for difficulty starting, or for signs (or smells) of blue smoke, which indicates oil being burned or worn valves. Idle the motor for awhile, then race the motor slightly and check tailpipe for smoke, thus indicating a serious oil-burning problem.
A final check of the engine involves letting it run to reach its operating temperature. Then race the engine slightly and listen for any noises. Knocking (from a rod bearing) or ticking (from a hydraulic lifter) sounds that increase with engine speed usually represents significant problems and costly repairs. If you are not able to assess these sounds yourself, ask to take the car to a vehicle inspection service if you are still interested in the vehicle.