Navigating the engine bays of older cars was a fairly straightforward proposition. Now, well...one does not simply walk into Mordor. As car's have gotten more capable, cleaner and better equipped, engine bays have come to resemble dark and foreboding forests of exotic-looking components, hoses and wiring harnesses just waiting to snare the unwary traveler. And that can be a problem if you ever plan to check or change your fluids, or perform the kinds of routine inspections that all manufacturers call for. But with a good map to guide you through, you can do all these things yourself, instead of returning your ride to the "dealershire" from whence it came.
Pop the hood on most cars, and most of the time you'll immediately see four things: an air filter box, the tubing from the box to the engine, a battery and either the top of the engine's "valve cover," or its snake-like bundle of "intake manifold runners." What you see at first glance will vary greatly by the vehicle and engine type, and whether or not there's a plastic cover over the top of the engine, which is often the case these days. Odds are best that you have either an "inline" engine like a four-cylinder or an inline-six, or a "V-configured engine" like a V-6 or a V-8. If you have an inline engine, you'll probably see the top of the valve cover first; this is usually where you'll find the oil fill cap. If you have a V-engine, you might see the intake manifold runners first, or you may have a valve cover staring you in the face if the engine sits sideways in the chassis, as with a front-wheel-drive car. For these purposes, start at the air filter box, which is typically large, square, black, plastic and sitting just behind one of the headlights.
You'll see a large, flexible tube connecting the airbox to the engine's main air valve, called the "throttle body." In the middle of the tube you may see the "mass airflow sensor," an "intake air temperature sensor," and a second, smaller tube running to the "intake manifold" or valve cover. This is the "positive crankcase ventilation" tube, and the "PCV" valve is at the other end. On the throttle body you'll generally see a set of cables connected to the gas pedal inside, and a "throttle position sensor" on the other side of the throttle body. The large, snake-like bundle of metal or plastic tubes the throttle body bolts to is the intake manifold. This caries air to the individual cylinders from the "plenum," or the large, central chamber on top that the throttle body bolts to. On this plenum you'll typically find a "manifold absolute pressure" or MAP sensor, along with the "idle air control valve," an "exhaust gas recirculation" or "EGR" solenoid, and any number of vacuum lines and hoses. At the base of the manifold, where it bolts to the "cylinder head," just below the valve cover, you'll typically find the fuel injectors. The metal tube on top of the injectors is the "fuel rail," at one end of which you may find a "fuel pressure regulator."
On top of the valve cover, you may see either a set of thick "spark plug wires" poking out, or a series of square boxes with smaller wires connected to them. In either case, directly beneath them, in the cylinder head, you'll find the spark plugs. If you see the small, individual boxes, these are the "ignition coils," and your engine has a "coil-on-plug ignition." If you have spark plug wires instead, you can follow them to the ignition coil or coils. You'll typically find one to four individual coils on these kinds of vehicles; these assemblies are known as "coil packs." On older vehicles, the plug wires would terminate at the "distributor," which would be connected either directly or through a separate wire to a single coil. On newer cars, the "ignition control module" is often integrated into the coil pack, but it can also be a separate unit. On the other side of the head, or the lower side, on a V-configured engine, you'll find the "exhaust manifold" -- or "header," if it's a series of tubes instead of one solid "log." In this manifold or header you'll find the "primary oxygen sensor," or "sensor No. 1" on a code scanner. Further down you'll find the catalytic converter, and then "Sensor No. 2" mounting in the exhaust tubing after it.
The "front" of the engine is the side with the accessory drive belt; it's still considered the "front," even if it's facing one of the wheels because the engine sits sideways in the chassis. Your engine may have one or more belts here. A "serpentine belt" is long, wide and thin, and snakes between all of the pulleys on the front of the engine; a "V-belt" is narrow and fat, and wraps around a few of the pulleys. The big, bottom-center pulley is the "crankshaft pulley," the heart of your engine, which drives all of the other pulleys. Often you'll find a thick "harmonic balancer" behind this pulley, and the "crankshaft position sensor" right next to it. Accessory locations can vary greatly by the engine, but the water pump pulley is generally just above the crankshaft pulley, and the "power steering pump" and air conditioner compressor pulleys are often on either side. You can recognize the PS pump by the "fluid reservoir" on top, where you add power steering fluid. The AC compressor should have foam-insulated lines going into the "firewall," the fireproof bulkhead that separates the engine compartment from the passenger compartment. The alternator is typically mounted high up; you can often see the copper "windings" inside.
The "back" of the engine, the side opposite the accessories, bolts to the transmission through a large, flared, metal cap called the "bellhousing." This is an alternate location for the crankshaft position sensor, but it's the only location for the engine's starter. You'll usually find it mounted very low on the back of the engine or the bellhousing, and you can usually recognize it by the small, cylindrical "starter solenoid" attached to it, or the thick, red wire coming out of the back. This wire often leads straight back to the battery. On the transmission you'll likely find the dipstick tube, where you check fluid level and add if necessary. You may also find a number of sensors and electrical connections. One of them near the bellhousing will usually control the "torque converter lockup clutch," or "TCC."
Further back and nearer the bottom, you'll probably find two or three adjacent electrical connectors; these go to the "transmission shift solenoids." Some transmissions will also have a "vehicle speed sensor" near the output, where the "driveshaft," or the "constant velocity" -- aka "CV" -- joints plug in. Somewhere nearby you'll also find the "steering column" coming down and out of the firewall; it connects to the "steering box" or "rack" on the bottom, which you'll note is connected via two hoses to the power steering pump.
At the very front of the engine compartment you'll find the radiator, which might be obscured by a plastic "fan shroud" containing the cooling fan or fans. On the front side of the radiator, hidden from view, is the radiator-like "AC condenser." Often you'll find the "coolant overflow tank" near the radiator, but it may be located elsewhere at a higher point in the engine bay. This is where you check and add your coolant, and it's connected to the radiator with a thin, rubber hose. The radiator itself connects to the engine with two very large hoses; you'll most often find the "thermostat" inside the "thermostat housing" where the top hose connects to the engine. On either side of the engine you'll probably find one or more fuse or relay boxes, which are often labeled as such. If your car doesn't have a battery under the hood, you'll often find the positive" jump terminal" on or near one of these boxes. It may have a red, plastic cover with a "+" sign. This is where you'll connect the red jumper cable if your battery ever goes dead. Connect the black one to a metal bracket on the engine.
Right above the wheels you may find the "strut towers," and bolted to the firewall, usually high up on the driver side, you'll find the "brake master cylinder," where you check and add brake fluid. This typically bolts to the large, round "brake booster" chamber. On the other side of the firewall, you may find the windshield wiper fluid reservoir, the wiper motor, and a large protrusion which contains your "heater core," AC evaporator" and "blower motor."
Like any place on Earth, there are an infinite number of details and variations of such you could include on this map. Some engines have snail-shaped "turbochargers" bolted to the exhaust manifold, others have belt-driven "superchargers" bolted to the top of the intake. "Overhead cam engines" generally have "camshaft sprockets," "camshaft position sensors" and "timing belts or chains" hidden beneath round covers on the front of the valve cover. Under the engine, you'll often find the oil filter hanging down, and an oil drain plug in the very bottom of the engine's "oil pan." You may find the "oil dipstick tube" anywhere around the engine, though it's usually plainly marked with a colored loop handle on top.
We could go on forever listing everything you might or might not find under your hood -- but now armed with a basic map, you can start exploring the nether-regions of your engine bay without fear of getting lost in the deep, dark woods.
