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I'm looking to buy a used car. What do you think I should check (mechanically) before buying it (especially if it's diesel)?

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If you are not entirely confident in your ability to check the car over completely, consider scheduling a pre-sale inspection at your local mechanic. They cost < $100 and can inform you of problems that you never even saw. In the case of my recent used car purchase, I spent $80 on an inspection that found a leaking shock and estimated $600 to replace the pair of shocks, and confirmed on paper that the back tires were worn and should be replaced. The seller dropped the price to account for the shock replacement and replaced the tires for me as part of the deal. It was well worth the $80. –  Ricket Jul 22 '11 at 13:23
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Check if they are a <put your future car here> enthusiast website and check if anyone have posted common things to check. I have a Land Cruiser and prior to buying it I check the IH8MUD website and they have a nice FAQ for the weak spots of that particular model. Good Luck! –  Gabriel Mongeon Jul 22 '11 at 19:17
    
And while I generally avoid dealers for maintenance, they're a great place to get an inspection done, as they will usually be able to check for any outstanding warranty or TSB work. –  chris Jul 25 '11 at 14:09
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7 Answers

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, dealbreaker 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.

Exterior

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 speedbumps. 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 jewelery 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

Testdrive

Steering: Accelerate tot 30 mph/50 kmh on an empty straight road and get your hands of the wheel. Does the car continue to drive in a straigt line?

Brakes: Go to an empty parking lot, accelerate to 30 mph/50 kmh 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?

Airco/heater: Is the airco really cold? Make sure the fan of the ventilation doesn't make weird noises.

Lights: Everything works?

Price negotiation

Timing belt almost due? Try to get a new one including the water pump included in the price

Airco 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. :)

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I must add something here. I recently bought a used car for my wife. I glanced at the tires and thought they looked good. Shortly thereafter, my wife starting hearing bumping sounds coming from the tires. I took the car to a mechanic who discovered that the tires were worn unevenly, so that on the outside, but on the side facing the undercarriage, they were worn down to metal in places. I ended up paying for four new tires and an alignment to get the car fixed. –  Rice Flour Cookies Dec 12 '11 at 16:21
    
What does the oil cap point mean? "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."? Some picture? Like this? This question moved here. –  hhh Jun 6 '13 at 21:45
    
I just want to add that if you have found a good deal, but there is a smoke smell, that you can use an ozone generator to get rid of the smell. That's what I did when I bought my 2nd hand car and it really did get rid of the smell completely. So if you have an otherwise great car, it's well worth spending $50 to rent an ozone generator. –  Ben Nov 15 '13 at 23:52
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Several printable checklists to consider:

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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.

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These are things I check before buying a car (in addition to Alex's advice):

  1. 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.

  2. Make sure the oil and transmission fluid are at proper levels. Make sure they look clean.

  3. 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.

  4. 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.

  5. 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.

  6. 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.

  7. 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.

  8. 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.

  9. 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.

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My thermometer definitely works without coolant. (92 Civic) It stays all the way at the top. Unless it's pouring down cold rain, then it runs perfectly cool even with an empty cooling system. :-) –  R.. Mar 25 '12 at 3:37
    
+1 good point about alternator -- always have a multimeter with you. If the alternator is broken, what is the estimated cost to replace it? It looks to be a mechanical part, short googlings gives between 40-500USD depending whether you do it yourself and the part price ofc. –  hhh Jun 6 '13 at 23:28
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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.

Tyra Shortino

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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).

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This can become a bit expensive to do on all second-hand cars to be tested, short googling shows the price to be around 100-400USD. Thanks for sharing +1 –  hhh Jun 6 '13 at 23:36
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You don't have to do it on every car you look at, but I'd say it would be foolish to buy a used forced induction car without doing it. It's certainly much, much cheaper than the cost of repairing any problems that doing the test before buying would've exposed. And a compression tester can be purchased for under $50; performing the test is no more difficult than changing the spark plugs. If a shop quotes you several hundred dollars for it they are overcharging. –  qes Jun 7 '13 at 17:36
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Go here - carbuyingtips.com

The used car section is the soup to nuts information I never shop without!

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