I'm looking to buy a used car. What do you think I should check (mechanically) before buying it (especially if it's diesel)?
Inside the car
Manual: Check the manual for the service history. Was it services regularly at a authorized dealership?
Ash tray: Smells like cigarettes? The previous owner was a smoker, deal-breaker for me personally.
Interior: Does the amount of wear correspond with the expected amount of wear for a car of that age and mileage?
Trunk: Spare tire present? Jack present? Condition of the carpet?
Under the hood
Engine: Does it look to clean? If the garage cleans the engine it most likely has a defect like an oil leak
Oil cap: White creamy stuff on the inside of the cap? Can indicate a broken head gasket or the car was used for a lot of very short trips. Walk away, it'll cost you to much money for repairs.
Paint condition: Lots of small dents and scratches indicate a sloppy previous owner. Pay attention to door edges and fender corners.
Front fender: Look at the underside, lots of scratches present? The previous owner didn't slow down for speed bumps. Can cost you money for new shocks, ball joints, etc.
Door handles: Lots of scratches around the door handles? A woman owned the car before you, expect to find some small toys and jewelry in the car. ;)
Parking damage on the rims: Lots of damage? Walk away, rim repair is expensive and the steering parts will be more worn than with careful drivers
Steering: Accelerate tot 30 mph (50 km/h) on an empty straight road and get your hands off the wheel. Does the car continue to drive in a straight line?
Brakes: Go to an empty parking lot, accelerate to 30 mph (50 km/h) and press the brake really hard. Does the car brake in a straight line without tugging on the wheel? Do you feel the vibration from the antilock brakes (if present)?
Gearbox/clutch: Does it shift smoothly? Is the clutch worn out? (Engage very late, does not stall when engaging the clutch without applying some gas).
Engine noise: How does it sound? No weird noises?
Shocks: Listen for squeaky noises or thumping sounds.
Engine temperature: Does the car reach it's normal operating temperature after a few miles/kilometers?
Air conditioner/heater: Is the air conditioner really cold? Make sure the fan of the ventilation doesn't make weird noises.
Lights: Everything works?
Timing belt almost due? Try to get a new one including the water pump included in the price
Air conditioner not cold enough? Ask the dealer to refill it with coolant.
Worn tires? No new (good!) tires = no deal
And I think a full tank of gas is part of the deal. :)
These are things I check before buying a car (in addition to Alex's advice):
Make sure it has radiator fluid (at a proper level). If the radiator fluid is empty, it probably has a radiator leak, they probably have not been watching it, and the engine probably has heat damage. The thermometer won't work without radiator fluid so they won't see that the car has been overheating.
Make sure the oil and transmission fluid are at proper levels. Make sure they look clean.
Swipe your finger (or cloth) around the inside of the exhaust (while its cold). If you have a bunch of black soot on your finger (or cloth), it probably burns oil.
With a cold engine, take the radiator fluid cap off and start the engine. If bubbles come up after a few minutes, walk away. It probably has a blown head gasket. It is normal for the radiator fluid level to raise since it will expand due to heat. Make sure the fluid looks clean. If it has stuff in it, they might have used a quick fix product to patch a leak.
With the radiator cap on, drive around and let the engine warm up. Then squeeze the large hose coming from the radiator (with a cloth or gloves to protect from heat). It should be pressurized. If the hose does not pressurize, it may have a radiator leak or something else might be wrong, and the engine may have overheated. You will have to compare against healthy cars to see how much pressure is normal.
Take the oil cap off and take a look inside. If there are a lot of dark dry clumps of oil everywhere... the engine probably overheated at some point and probably burns oil.
Have someone accelerate the car away really fast. If you see clouds of dark blue smoke, it probably burns oil. If you see clouds of white smoke, it could have a blown head gasket. Get a feeling of what it should look like by paying attention to healthy cars.
Check the tires to see if they have evenly worn tread. If the wear is only on one side of the tire, the wheels will probably need an alignment. If it needs alignment, you can try using it to talk them down on the price more. If the tires are near the end of their life, consider that you'll probably have to spend a couple hundred dollars on new tires soon if you get the car.
Make sure that the alternator works. You can check the voltage of the battery with a multi-meter before and after starting the car. The voltage should be higher after the car is started.
If it shows signs of overheating, leaking radiator fluid, having a blown head gasket, or leaking/burning oil... I would not buy the car.
I'm surprised this has not been mentioned.
Have a compression test performed on the engine. This will expose many major engine problems like head gasket leaks, damaged valves/valve lands or rings. It is a simple, inexpensive test that can save you from some of the most expensive repairs a vehicle may need.
I would consider a compression test absolutely mandatory on any engine with forced induction (turbocharged or supercharged).
If the seller shows you the service records/log book, verify the records. Call the dealers whose stamps are in the log book and make sure they have a record of having serviced the car.
Blank log books can be bought cheaply off the internet, and it's easy to find a dodgy mechanic who'll stamp the book while his boss isn't watching.
A friend recently bought a Toyota for around $50,000 with full log book history and 120,000kms on the clock. When he took it to his local dealership for a service, they told him that Toyotas records have the car as having 300,000+ kms. He then called the mechanics whose stamp was in the logbook and they say they have never serviced the car. My friend had the car re-valued and it's only worth about $30,000.
If this happens to you (at least in Australia) you have absolutely no legal recourse and could lose a lot of money.
A few other things which no one has mentioned which are pretty easy to do, or only require a cheap tool.
Check the engine side of the air filter for black oil or sludge. Indicates too much pressure in the crankcase forcing excess oil and vapors back through the air intake. This could be caused by either a clogged PCV system ( cheap to fix ) or because of allot of blowby past the rings due to overly worn rings ( expensive to fix ).
A power balance test is really easy to do on allot of older cars, just pull the plugs one at a time and make sure the rpm's drop by the same amount on each cylinder. If they don't, you've got a problem.
Before signing your name on the contract and completely sealing the deal for a used car, you should also check the dashboard features like the A/C, sound system, light and wiper controls, and horn. You can also check the speedometer for the mileage of the vehicle. After doing that, open the hood and check if the area is dusty or not. You may also want to check the oil in the car. To do that, you may use a dipping stick, which is found under the hood, to check if it's clean or dirty. If the oil is clean, the car is good to go. But if the oil is dirty, oil change is needed to avoid damaging the car’s engine.