The single most important part of a used car car is the drivetrain: the engine, transmission (or transaxle if a front-wheel-drive model), differential and underhood accessories. The body and interior may be in fantastic shape, but if the drive components are ready to fall out, the car won't be of much use. More to the point, bodywork and interior repairs can generally be done more cheaply and with considerably less urgency than drivetrain work. A split upholstery seam has never yet stranded anyone on the freeway during rush hour.

Have an expert look at the car.
Unfortunately, many potentially big problems in the drivetrain are not detectable just by looking and could even be missed in a road test. That's why we cannot emphasize too strongly that it is a good idea to have a competent, independent mechanic (not affiliated with the seller) examine the car before any final offer to buy is put forward. This doesn't have to be terribly complicated. A local service station will do. Before you get that far along with a potential purchase, however, you can do much of the preliminary detective work yourself, even if you don't have mechanical experience or aptitude.

Examine the engine compartment.
The first thing to do is open the hood and look at the engine compartment. Does it look spanking clean? If so, it has probably been detailed--the engine has been steam cleaned, the hoses treated with a cleaner and some pieces may even have been repainted. Detailing may make it look good, and is not a sign of trouble in itself, but it removes evidence of leaks or other major problems that can help you check the car. If you find yourself looking at a detailed engine, take even more care and thoroughly examine everything.

Look underneath the car.
Then, take a good look underneath the car--before starting the engine. It's best if the car has been sitting a while--preferably overnight. Look for any drips and puddles. These indicate something is escaping that shouldn't be. Many of the engine fluids that might leak can be identified by their color because the manufacturers have thoughtfully colored them with dyes to make it easier for mechanics to diagnose fluid leaks. Greenish-yellow is coolant (anti-freeze), brownish-black is engine oil, reddish or reddish-brown is automatic transmission, brake or power steering fluid, and golden-yellow is manual transmission or differential gear oil.

Check the car's fluids.
After looking underneath, take a look at the fluids in their reservoirs. Pull out the oil dipstick and look at the oil; if it's thick and black, it's old and needs changing. This can be a tip-off that the whole car has been poorly maintained. If the car has an automatic transmission, check that dipstick as well. If the fluid has a red color and smells like oil, it's good; if the fluid has a brownish tint and smells burned, the fluid has "varnished" and could mean the transmission has problems. Look at the coolant in the recovery bottle and open the radiator cap (make sure the engine is cool before doing this!) to look in there, as well. The coolant should be a rich, green-yellow. If it's rusty, there's corrosion in the system. If you see shiny spots floating on top of the coolant, that's oil and that means a serious leak, such as a blown head gasket or a cracked block.

Check belts and hoses.
Take a hard look at the hoses; they should be supple to the touch, not hard. They shouldn't be swollen or spongy either. Check the belts for signs of cracking or fraying. Today's modern belts don't show wear as readily but some clues might still be present. Try rocking each of the pulleys back and forth. They should be absolutely tight. If they wobble, something is definitely not right. (The underhood check can also reveal secrets about the car's body. See the chapter on inspecting the exterior for things to look for under the hood.)

Look under the rear of the cars.
After looking under the front of the car and under the hood, take a look under the rear of the car. If the car has rear-wheel drive (or 4-wheel drive), look for leaks from the differential. Also, look just inside of both wheels for evidence of brake fluid or shock absorber oil leaks. These are not as expensive to fix as drivetrain problems, but they are important and should be repaired before you buy. If the seller doesn't want to fix them, you can use these to leverage price negotiations to your favor.

Now, start the engine.
Now that you have done the "cold" inspection, you are ready to start the engine. It should fire within 5-10 seconds. Failure to start promptly can be due to many things, such as poor tune, timing problems, a weak starting system or excessive drag in the engine. Don't accept at face value the statement, "it just needs a tune-up." If all it needed was a tune-up, the seller probably would have taken care of it already.

As the engine catches, listen for any clanging or grinding; that's evidence of the starter motor hanging up. While not expensive compared to other engine repairs, it isn't cheap. Starters on some imported cars can cost $400 or more--plus labor. Also, listen for any telltale "clacks," "clicks," or rattles. These sounds should be checked by your own mechanic.

As the engine idles up to operating temperature, get out and listen under the hood. Things should be quietly busy: the whir of the accessories being driven by the belts, the whoosh of the fan (if manual and not electric), and the rush of air into the air cleaner should be all that you hear. If you hear any squeaks, pops, growls or whines, that is not normal. Make a note to have your expert check them out.

Look throughout the engine compartment and underneath for any fluids being spurted or oozed. Try to spot where they originate and have your expert check these as well.

Get back in the car and press the accelerator a couple of times--not all the way to the floor, but enough to bring the engine to midrange (about 3,000 rpm if the car has a tachometer). Listen for any squeals or moans. A high-pitched, chattering squeal that disappears is most likely a belt slipping--a fairly easy repair. A longer squeal that changes sound as the engine speed changes could be a bad water pump--a more serious (and expensive) fix.

A rattling sound at the mid-rpm range could be bad wrist pins or a crank rattle. All you need to know about these is that it spells big bucks engine repairs, perhaps $1,300 or more. You'll need your expert to certify the problem, but it might be better to just rule the car out.

Check the accessories.
If the car has power steering, turn the steering wheel right and left about a half-turn. A low-pitched growl indicates something wrong with the steering pump. It could just need fluid, or it could need a rebuild. Make another note for your expert.

Check the transmission.
Now try the transmission. If it's an automatic, put your foot firmly on the brake and shift the transmission into reverse. You should feel the transmission "take" quickly and solidly. There should be no long "whir," nor any sort of jolting clunk as it engages. Gently, but reasonably quickly, shift to neutral and to drive. Again, the engagement should be firm, with no delay or clunk. A "clunk" as the drive train takes up the slack indicates something is worn, possibly the universal joints. While pressing on the brake, rev the engine gradually and lightly. The car should strain a little, wanting to move forward. Listen for any unusual whooshing or whirring sounds, indicating bad valves in the transmission passages (called vanes). Ease off the throttle and set the emergency brake. Again, lightly rev the engine. The car should not creep forward under light throttle. If it does, the emergency brake could just need adjustment or the brakes could be seriously worn. Make another note on your checklist.

After you have gone through this eye and ear procedure, you should have a fair opinion of whether to continue to examine the rest of the car. Remember, however, not to make any final decision until you have had an expert do a separate inspection and give you an opinion on any items you may have noted.