When I am asked to advise on a car purchase for a client, I rank the mechanical condition of the car behind body and chassis, simply because of the expense of repairing the body of the car. Too often many of us are willing to purchase a car that JUST NEEDS A PAINT JOB, and yet may pass a car over because the engine sounds a little ragged. Take my word for it, unless the car that you want to purchase is a Rolls Royce, chances are the rust work, chassis repairs, prep and paint will far outstrip the cost of mechanical repairs on a lot of cars. This is not to say that mechanical repairs are cheap, they are not, but you can often purchase a car that sounds rough or runs badly which may not need all that much work to render it perfectly useable. It's what you know and can diagnose rather than what the seller may tell you.
Now the warnings. Barn finds.
A car that has sat unused for three years will almost always need brakes including master cylinder rebuild, wheel cylinder rebuilds or replacement, all new lines and usually new shoes or discs if so equipped. The seals in the motor are likely to leak and should be replaced as a matter of course and you will probably have to rebuild the carb. You will usually find that lots of other small and often irritating things occur such as lazy or intermittent power windows, light switches etc. Belts and things will often need a close examination before you can trust them. You should also try to find out why the car was put away. An awful lot of cars are put in barns and garages because they are not fit for the road. Contrary to popular belief, they rarely heal during a stay in a barn.
Cars that have sat for longer periods should be torn down mechanically, checked and rebuilt if needed, before they should be trusted on the road. Of course anything that has sat in a barn, shed or garage for any length of time should be checked for rodent damage and this will mean a careful sniff inside. Mouse urine doesn't need to be described, if the car smells really bad inside, it is probably mouse urine and unless you want to replace the interior, walk away from the car. You can't get rid of it, you can only make it worse by layering equally foul smelling air fresheners over the original stink. It doesn't work and I think that on a hot day it can actually kill you. If it doesn't, you can be sure that the lady in your life will if you make her sit in it.
Barn finds are neat, I enjoy making them myself, but having found at least fifty cars in barns I learned long ago that they are inevitably a lot more work and expense than they may appear.
Cars for sale at the local cruise night or neighborhood driveway. Check out the owner. If he enjoys impressing the Cruise night crowd by doing brake burn outs to show just what lousy traction bias plys actually have, chances are the car may need a little work. Abuse is often a difficult condition to diagnose because the parts are not worn out, they are stressed out, and this often does not become apparent until they break. If you suspect a squirrel may have been driving the car that you want to buy, get in and gently put it into forward and reverse a few times. If you can hear the driveline moving around under the car and clunking a lot before the car moves, you could have transmission, rear end and or universal problems. While the car is moving listen for any unusual mechanical sounds. Drive the car slowly, keeping noise levels low so that you can hear things that you shouldn't be hearing from a healthy car. Ease your speed up slowly, listening and feeling the car at different speeds. Try to get the car to at least sixty miles per hour so that you can feel the harmonics of the car. Wheel balance and alignment problems often manifest themselves at speeds approaching or just past sixty. If it feels uncomfortable at those speeds and makes noise, wanders more than the bias plys can account for, or has other bad habits, it may be a good idea to take a pass. During this examination you will have to bear in mind the age of the car. Forties and early fifties cars can do these speeds but rarely feel comfortable doing it. Older cars from the twenties and thirties reach their optimum speeds much lower, but driveline problems will manifest themselves at lower speeds.
Listen to the engine carefully. Misses and clanking coming from the top of the motor may just indicate that the car needs a tune up or valve adjustment, but if you can hear anything that seems to be coming from the bottom of the motor, you should be ready to do major work if you purchase the car.
Look at the radiator to see if it has wet spots or shows damage or old repairs. Open the cap and see if it is full of sludge, and try to have the car run for long enough to determine if it is heating up and if the engine feels unnaturally hot. This will indicate cooling problems. Don't dismiss cooling problems because even though the radiators on most cars are fairly inexpensive to repair, cooling problems if they have been left for a long time can cause huge amounts of damage to the engine.
If you are satisfied with the major components of the car, check all the small things like windshield wipers, door handles, locks and other minor equipment. Don't gloss over this stuff because as a whole it can get very expensive to repair and a lot of the minor parts such as washer motors and reservoirs, wiper arms and motors, heater fans and interior controls can be very difficult to find and purchase, if they are available at all.
You should also check to see if the heater core is leaking in the interior. Coolant stains under the core are a dead giveaway. If you can't see any stains but the car fills with a sweet smell as it is being driven, the heater core is leaking. The last indicator is that the windows steam up when the car is parked after running or when it is being driven and is warm. As a matter of fact, the defroster will sometimes spray drops of coolant all over the windows as you turn it on high to clear the mist which of course it won't do anyway as it is the cause of the mist.
If you do smell that sweetness, park the car as soon as possible. Do not ignore it as that sickly sweet smell of atomized coolant is also poisonous.
The front end components are sometimes quite difficult to check unless you can jack the car up and even then you have to know what you are doing to properly diagnose front end problems. You should get down and have a careful look at the various components of the front suspension and steering. Check for bent bars and other visible damage. Take a careful look at ball joints and steering arms. See if they look dry or are covered in old dirty and sand filled grease which is often actually abrasive because of the dirt in it. Look for any areas that have wear marks on them that common sense would dictate do not belong. Areas on the frame for instance, showing marks left by wheels rubbing or steering components striking them should be looked for. Inspect attaching points to see if they appear loose or show wear from the parts moving when they should not. Look at the steering box and check it to see if it has leaked out all of its lubricant. A heavy crust of dirt built up on it will usually indicate that it has been leaking and may be empty and worn.
Inspecting a car to determine its mechanical condition is largely common sense. If, after your inspection you are not comfortable with the car but still want it, express your concerns to a professional and see if you can hire a good mechanic to accompany you to a second inspection. If the owner of the car objects to a second inspection or tries to talk you out of it, don't buy the car unless you don't care that it may cost you a small fortune to put it right.
The long and the short of the whole process of buying a car is that you should buy the best car that you can find. Always remember that a perfect car needing no work for fifteen thousand dollars will often cost you a lot less money than a car needing a few things for sixty five hundred. Mechanical bills can add up very quickly, and parts prices have risen astronomically in the last few years. Availability of parts is also becoming worse as they are either used up or lost to the smelters. Be warned. It really is a case of buyer beware, and if you let love overpower common sense, the love affair may be very short lived indeed.