Checking diesel before buying


Choosing a used car is a complicated issue, involving both personal preferences, financial possibilities, the technical condition of the vehicle and even legal issues. In the following article we will look at the topic of checking the condition of a diesel vehicle. From issues we can pay attention to without additional resources to methods and tools used by professionals. We will show how to use the EDIA-PRO device, well-known in specialized workshops, to quickly assess the condition of an engine. A used car can be a great choice, but it can turn out to be a “bottomless well.” While this is obvious, let us remind you: often a car is sold when excessive repair costs make it unprofitable to continue maintaining the car. This can happen even with relatively young, seemingly impeccably maintained copies.

What to pay attention to – general advice

Before buying a used car, check its technical condition. When it comes to modern construction, overlooking certain issues can be very costly. Even without additional equipment, it is possible to learn a lot about the condition of the drive unit by carefully observing its operation. Traditionally, the main criterion was the vehicle’s mileage – this led to a situation where almost every used car on offer was below the magic 200,000. km. Suggesting mainly mileage is not very reasonable in modern diesels. The high mileage obtained in a short period of time confirms that the car was driven on long trips, the kind of trips for which diesel engines were created. If we intend to drive the diesel very little, and suggest mainly low fuel consumption, then perhaps a gasoline engine will be a better option. Below is a list of points to check on diesel cars.

  1. Look in the engine compartment for any leaks of oil or other operating fluids. Has the engine been washed? Keep in mind that it is one thing to wipe the dust covers off and another to thoroughly wash the entire engine just before selling it. There should be no visible leaks under the engine, transmission or bridges.
  2. Check the oil. If the oil level is low, it may indicate significant oil consumption. Even if the consumption is correct, a low oil level does not bode well – the owner could regularly drive with this condition. Take a look at the dipstick, also look under the oil filler cap – whether there is no so-called “oil” there. “mayonnaise” (oil mixed with coolant that accumulates when the head or head gasket is damaged).
  3. Check the coolant – its level and appearance. The liquid should be clear, with nothing floating in it. Look under the cap of the expansion tank, it should be relatively clean there. In particular, there must be no oil or other oily residues (“mayonnaise”).
  4. Levels of other operating fluids. Let’s pay attention to the level of brake fluid or power steering fluid (if any). Abnormal levels may indicate that the vehicle has not been properly serviced.
  5. Dashboard lights. Before starting the engine, turn on the ignition and wait a few seconds for the glow plugs to heat up. The lights for the engine, ABS/ESP system or airbag system should come on when you turn the key and go off when you start the engine or after a few seconds.
  6. Cold engine startup. Check that the engine is cold. Make sure the radio or air vents are turned off before starting the engine. The startup should run smoothly and should not be accompanied by suspicious metallic clattering, crackling or rattling. After starting, the engine’s operation should be even, with no unusual noises or smoke.
  7. Test drive – during the test drive, pay particular attention to the smoothness of starting and the absence of suspicious noises coming from the engine as well as the suspension. Don’t get into higher revs before the engine reaches operating temperature.
  8. The car should not smoke, clear smoke from the exhaust pipe regardless of its color can herald problems (in addition to water vapor which normally occurs in the exhaust, and becomes apparent at low temperatures). Vehicles with a diesel particulate filter are basically not allowed to emit soot at all – if they smoke black, the DPF has been removed. Pay attention to the timing of the sudden addition of gas. Lack of power, blue smoke, and a loud whistling noise increasing with engine speed may suggest a malfunctioning turbocharger. This is one of the expensive components that wear out (a turbocharger without variable geometry is indeed cheaper).
  9. Go for a visual inspection with another person. From the outside, it is easier to assess both the smoke and the noises accompanying the engine when starting and driving. You can’t hear much in well-dampened modern cars, and rearward visibility is sometimes limited. A second person is also useful for other reasons; he or she can draw attention to an overlooked detail or prevent overly impulsive purchasing decisions.
  10. Dual-mass wheel. During the test drive at a speed of approx. of several tens of kilometers per hour, take your foot off the gas and then add gas. Repeat the operation several times, paying attention to possible metallic knocking coming from the dual-mass wheel. Similar knocks can be heard when the engine is extinguished. Shifting gears should be easy and seamless. The gearbox can be damaged by driving with a defective dual-mass wheel or by replacing it with a rigid wheel.
  11. After completing the drive, reevaluate engine starting and idling, this time on a warmed-up engine. It happens that the engine is willing to start only when it is cold.


Computer diagnostic capabilities

More and more people are choosing to use the services of those offering to check their cars with a diagnostic tester. Checking your car in this way is a very good idea, but you should look realistically at the limitations of this method.

Connecting the car to the so-called. The diagnostic computer is a good method of verifying the condition of the car and a way to detect some tampering. In many situations, it is possible to determine the actual mileage of a vehicle, which is often recorded in several places. If any warning light was on constantly and someone connected it to another circuit, it will be up to the diagnostician to determine. Similarly, it is possible to detect the use of an emulator to replace a particulate filter or an emulator for the SCR selective catalytic reduction system. Keep in mind that a diesel without a DPF does not actually meet the homologation requirements, which could prove painful for the owner in case of possible control solutions. The same may be true of the EGR valve. The exhaust gas recirculation valve is also one of the components to meet the emission standard.

computer diagnostics, car

Reading error codes not only from the engine controller, but also from the ABS/ESP module, the airbag system or the comfort module will help figure out faults and possible attempts to hide them. This will be all the more important the richer the equipment of a particular car.

With this method we will learn about the errors reported by the controllers, such as problems with a selected sensor or component, but we will not be sure which components qualify for replacement. The engine controller is very good at detecting an abnormal signal from the sensor, but it doesn’t know what mechanical condition the pistons or suspension are in – without a professional, you can’t do it. The same is true for other components of the vehicle, for example, there are cases of replacing fired airbags with resistors – from the point of view of the system controller, they appear to be fully operational. There are also cars in which someone has intentionally modified the controller so as to hide fault codes or the condition of the vehicle. In such cases, too, the diagnostic computer may not reveal this.

We check the car with EDIA-PRO

The EDIA-PRO device for rapid initial diagnosis of the injection system, successfully used in diesel service, is also ideal for checking a vehicle equipped with common rail injection before purchase. The system is based on non-invasive measurement during operation, which makes it possible to independently assess the condition of the engine. We can carry out such a measurement in as little as 10 minutes (the measurement time depends largely on gaining access to the wires leading to the injectors. There is no need to punch holes or pierce wires so the procedure is harmless to the vehicle under test. The saved files can be interpreted calmly on the computer later, so we can keep vehicle access time to a minimum in many cases.

When diagnosing a car in a workshop, we are interested in determining the exact cause of the malfunction, as we aim to fix it. Using the EDIA-PRO device, we can speed up our diagnosis by targeting specific areas where the problem may exist. This saves time and money by focusing on the most suspicious elements.

If we are evaluating a car before buying it, the situation is simpler because we are interested in the general condition of the engine and injection system, and the vehicle is not going to be repaired at the moment. Simply identifying a problem with the injectors is far more important than pinpointing which one is most depleted.

How to evaluate a diesel vehicle step by step

1. we connect to the system in the car. We insert the measuring probes between the wires of the injectors and use an adapter to connect to the signal from the pressure sensor (without the sensor signal, we are still able to check the evenness of the engine).

edia pro connection

2 Select “Time analysis”, longer measurement of 10 or 15 seconds and start the engine after the start of the measurement so as to record the moment of starting. The measurement is best performed on a warmed-up engine or in conditions where there are some problems (e.g., suspicious knocking, difficult starting, etc.). During startup, we focus on pressure buildup – the pressure should reach the right value within up to a second of starting the starter.

edia pro, launch chart

3 We take a second measurement, this time at idle. Select Menu -> Automatic analysis, and then indicate the type of injectors. We observe the differences in the control times of the injectors and compare them with the bottom row of the table. “Cycle time deviation” shows us whether a given cylinder is accelerating or decelerating the engine (a positive deviation means deceleration, a negative deviation means acceleration). Deviations of less than 100us (at idle) are considered to be within the norm. If both injector control time and deviation are increased, this suggests problems with that particular cylinder. Both pressure and engine speed should be stable. Slow rippling of pressure values often indicates problems with the pressure control valve.

edia pro - leanings

4 Begin another measurement, starting with slow speed. Then push the accelerator pedal all the way down and release as soon as the engine accelerates. In this way, we create a short-term power requirement and evaluate the system’s ability to achieve higher pressures. The pressure during such a test should rise faster than the engine speed and reach a value of more than 1/2 the scale of the sensor. Pressure build-up that is too slow may indicate problems with the pump or excessive overflow of injectors. If the pressure rises only slightly above the baseline, we probably have a problem with the pump.

edia pro, pressure build-up

5. before turning off the engine, we still take a measurement in the “Shape Analysis” mode, especially if we suspect a problem with the injectors. This measurement will allow you to superimpose the runs of the injectors – if they differ significantly in shape (they should differ at most slightly in length), this could indicate problems with the injection, circuit or even the engine controller.

(6) If we have the opportunity to delete the error codes later, we make one more measurement – disconnect the injector wires to prevent start-up and make a measurement in the “Compression pressure analysis” mode. Connect the wire with terminals to the battery, click “START” and then spin the starter about. 10 seconds. We obtain an estimate of the compression distribution in the engine. We do not know the absolute values (in units of pressure), but we can compare the uniformity of compression in each cylinder. If the compression comes out evenly, and the injectors work unevenly – we have a problem with the injectors. If the compression is uneven then replacing or regenerating the injectors is of little use without a proper engine overhaul.

edia pro, compression

The advantage of such a measurement is that it is basically impossible to manipulate. While you can manipulate the controller, you can’t cheat clogged injectors or worn seals in the pistons. If the engine runs properly and we are satisfied with the test drive, we have a good chance of enjoying the purchase of our car.

The shrewd professional gains

What if the measurements showed some kind of defect? It turns out that buying a car with a defect can be very profitable. For example, if the uneven operation of the drive unit and the sound of a rattling injector have raised our concern, we are probably not the only ones who have paid attention. If the EDIA-PRO measurement shows that the problem is most likely to be with a single injection, and the engine otherwise seems to have nothing wrong, such a car may be a good choice.

The undoubted advantage is knowledge – if we know that specific injectors can be easily remanufactured (they are usually cheaper), the cost of restoring them to working order will be less. If we know what components cause problems in specific engines then we know where to focus our attention. Sometimes a conspicuous flaw (which the seller has not been able to mask) will cause many customers to abandon their purchase. If we can properly estimate the cost and scope of the necessary repairs, we may find that we can gain a lot by using our knowledge and technical advantage.

If we don’t have the right knowledge or a professional on hand, let’s at least try to analyze the reviews available on the Internet. On the web we can also find approximate costs for replacing components such as dual mass wheels, turbochargers or injectors for specific models. Using the vehicle longer in most cases these topics will not pass us by. Let’s try to take a fairly objective look at the whole car keeping in mind that modern vehicles consist of many expensive components and not all of them will be in perfect condition.

Author: Product Engineer Piotr Libuszowski


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Over 25 years of experience in the automotive industry gives us the position of an expert who sets trends in innovative car diagnostics. Our greatest pride are hundreds of satisfied customers and numerous industry awards.
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DeltaTech Electronics to polski producent i ekspert w branży automotive, który na bazie ponad 25 lat doświadczenia wyznacza trendy w innowacyjnej diagnostyce samochodowej.
To, co nadaje rytm naszej pracy, to wsłuchiwanie się w potrzeby klientów oraz śledzenie aktualnych problemów, z którymi mierzą się warsztaty samochodowe. Owocem tego jest oferta skrojona idealnie „na miarę” ich oczekiwań.

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