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How to check the engine when buying a used car

Updated: January 14, 2019

When buying a used car, the condition of the engine is very important because engine problems are expensive to repair. It's difficult to evaluate the mechanical condition of the engine during a quick test drive, that is why we recommend having a used car properly inspected by a qualified mechanic before signing the contract. Here are a few tips on how to spot signs of engine problems or lack of maintenance when checking a used car.

Checking an engine in a used car
Checking an engine in a used car

It's difficult to evaluate the mechanical condition of the engine during a quick test drive, that is why we recommend having a used car properly inspected by a qualified mechanic before signing the contract. Here are a few tips on how to spot signs of engine problems or lack of maintenance when checking a used car.

Check service records

Service records are not always available, but it helps if the dealer or a person selling the car can produce some proof that the vehicle has been maintained regularly. If you can get access to the service records, look for oil changes and mileage records. It's a plus if you can verify that oil changes were done regularly. Depending on a manufacturer, recommended oil change intervals vary from 3,750 to 10,000 miles. If the vehicle has been driven between oil changes for much longer, the engine might be worn inside. It's also good to know whether a timing belt (if the car has one) has been changed, or what other maintenance has been done.

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Have a look under the hood

Before checking under the hood, make sure, the engine is OFF, the transmission is in "Park" and the parking brake is applied. What you are looking for are the leaks, smell of burnt oil or antifreeze, signs poor quality repairs or lack of maintenance, as well as 'racing' modifications. Dealers often shampoo the engine bay before showing a used car to potential buyers. This means if everything looks clean and shiny, it doesn't mean that the engine is in good shape. Let's see some examples:

Burnt oil smell under the hood

Shiny engine compartment

This Audi looks like new under the hood but when we looked under the hood after a test drive, we noticed a strong smell of burnt oil, which is a sign of oil leaks. Repairing oil leaks is not always cheap. At higher mileage, the piston rings and cylinders wear out, which cause more blow-by gases to enter the crankcase. This increases pressure inside the crankcase. As a result the oil is pushed out trough various seals and gaskets, as well as through the PCV (crankcase ventilation) system. This issue is more common in turbo engines. An engine in good condition is unlikely to have any leaks.

Visible oil leaks

Engine from underneath

Oil leaks might not be visible from under the hood, but here is the trick: look from underneath. Take a photo or video with your phone. Check the lower part of the engine and transmission. Everything has to be dry. This car in the photo has a pretty bad oil leak around the engine undershield.

Engine from underneath

This car, for example, has no leaks. Everything looks dry. Click on the photo to see a larger view.

Coolant and other leaks

Leaking coolant from the radiator

This car runs fine, but there is this coolant leak from the radiator. As a minimum, this car requires a new radiator, but sometimes, a cracked radiator may be a sign of more serious problems. It's best to avoid used cars with this type of issues.

Low oil level, dirty oil

Oil condition on the dipstick

If you can check the oil condition on the dipstick, it can tell a lot. To check the oil, the engine needs to be OFF. Set the parking brake, careful, some engine parts might be hot. The owner's manual in the car has the directions how to check the engine oil. If the oil level is low, it means that either the engine consumes oil or it has been long since the last oil change. When the engine runs low on oil, it wears faster. Normally the oil level should be close to the "Full" mark, like in the lower dipstick in the image. If there is no oil or the level is very low, or if the oil is mixed with coolant (see the image), avoid a car.

With the engine off, check under the oil cap

Oil cap

If you aren't comfortable doing this test, leave it to your mechanic. With the parking brake applied and the engine OFF, remove the oil filler cap. Careful, it might be hot, use a towel or a rag. Look under it, use your flashlight. In some engines you can actually see the internal parts.

Checking under oil cap

For example, in this Mercedes-Benz engine in the photo, the part looks clean.

Checking under oil cap

In this car, you can see there is a lot of black carbon deposits or sludge under the cap. It is a sign of lack of maintenance.

Well maintained vs sludged up engine

This is how an engine looks with the valve cover removed.

Watch out for performance mods

Engine performance mods

Be careful if a car has some performance mods. If done right, modifications can improve the vehicle performance. However, poorly-done engine mods can lead to many problems, especially if parts that were originally on the vehicle are no longer available. If the car has been modified, it's also likely that it has been raced or otherwise abused.

Does the engine have a timing belt?

Timing belt

Not all cars have a timing belt, some cars have a timing chain instead. In most cars, a timing belt needs to be replaced between 60K and 105K miles. The price to replace a timing belt ranges between $200-$450 in a 4-cylinder engine and $550-$760 in a V6. If the car you want to buy has a timing belt, it's good to know if it has been changed. Some mechanics place a sticker on the engine when a timing belt is replaced. You cannot see a timing belt under the hood, it is hidden under covers. To check its condition, your mechanic will need to remove one or two covers, and it's not always easy. A more realistic option is to check the service records to see if a timing belt has been replaced. Read more about a timing belt.

Cold start can reveal many hidden problems

Blue smoke from the car exhaust

The best way to catch hidden engine problems is to start it cold. To do this, it might be a good idea to come to the dealer a little earlier than your appointment time. You will also know if the battery is good, because if the battery is old, it might need to be boosted to start the car. Watch out for engine noises and smoke when the engine is first started. If the engine rattles or makes other loud noises, or there is a blue smoke from the exhaust, look for another vehicle. This car in the photo, for example, showed a blue-gray smoke from the exhaust. It smelled like a burning oil too. A blue smoke means that the engine burns oil.

Test drive

After the car is started, all warning lights on the dash should come off. If the engine symbol (Check Engine) light or Service Engine Soon stays on, the engine computer detected some fault.

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It could be a minor issue, but it also could be a very expensive problem. There is no way of knowing how serious is the problem until the car is properly diagnosed. Read more what "Check Engine" light means.

During the test drive, watch out for engine noises, vibration, lack of power, or any other driveability issues. When started, the engine should run smooth, without shaking or hesitation. If you feel that the engine is hesitating or stumbling when accelerated, there is a problem. The idle speed should be stable too. Test drive the vehicle for as long as possible; sometimes problems may not be obvious during a short drive around the block. It helps if you can test drive in all modes: acceleration, deceleration, stop-and-go traffic, highway cruising. Watch out for the engine temperature on the dash. Once the engine is warmed up, the temperature gauge should stay somewhere around the middle of the scale.
Even if everything seems OK, we strongly recommend having the used car properly inspected by an independent mechanic before buying.



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