In previous weeks, we were discussing the topic of buying a second-hand. The outdoor, adventure-loving part of me has always been partial to getting a big turbodiesel 4x4. Perhaps a used Chevrolet Trailblazer or Mitsubishi Pajero Sport or Nissan Pathfinder. I can picture myself in any of these vehicles, caravan in tow, navigating the forgotten dirt roads of the Kruger. 

The one nagging thing which always prevents me from signing on the dotted line (and I have come that close a few times), is remembering how my mechanic told, quite emphatically, never to buy a used turbodiesel. Period. 

Indeed, diesel engines have enough pricy components on their own, long before the turbo charger gets bolted on. And, as wear and tear components go, go they will. It’s just a matter of time and mileage. The prices of new injectors, a new turbocharger or diesel pump could strike fear into the hearts of grown men, especially if you bought the vehicle used, and its market value is not much more than the aforementioned repairs. 

There is not a lot you can do prolong the life or your injectors. Their lifespan is around 150 000 kms, but reaching that requires good quality diesel, and performing oil and filter changes at the prescribed intervals. 

High Pressure Fuel Pump 

There are a variety of factors that can cause problems with your fuel pump, and at least some of them are avoidable. If you wish to reduce the risk of having to fork out an eye-watering sum for a new replacement diesel high-pressure pump, you may wish to read on. 

When talking of the diesel pump, we referring to the high-pressure pump specifically, because it tends to be the most expensive part to replace. Its job is to pressurise diesel fuel to around 13 800 KPa (for context, a typical car tyre is inflated to 200Kpa) that permits the fuel injectors to pulse fuel into the engine with the utmost accuracy. 

Unsurprisingly, they can fail and cause the engine to either run badly, or stop running completely, but there are things that you can do to prevent a hefty repair bill. The only caveat, however, is that 

the fault could lie with cheaper parts of the system, such as electrical sensors, so be wary if you are told that the pump is faulty, without other causes being investigated first. 

Main causes of pump failure and how to avoid them: 

• Running out of fuel: Certain diesel pumps (such as the Bosch CP4) can be damaged severely by incorrect bleeding, which is required after the fuel tank runs dry and is refilled. Some vehicles require diagnostic equipment to be bled correctly. To be safe, avoid running out of diesel! 

• Modifications: Certain engine tuning raises the pump pressures and causes thermal overheating, which can shorten its life. Also, certain cetane-boosting diesel fuel additives, if not dosed correctly, affect the lubricating quality of diesel fuel, which increases internal pump wear. 

• Misfuelling: Should you fill-up with petrol accidentally, the pump can be damaged before the engine is even started. This is because petrol possesses zero lubricating qualities for diesel fuel pumps. Do not even turn-on the ignition; seek advice and organise transportation, so that the fuel system can be drained professionally. 

• Poor maintenance: Diesel fuel filter housings require annual drainage of corrosion-causing water. Regardless of manufacturers saying that they can be left for longer, I advise replacing the fuel filter cartridge annually. Use good quality filters, as poor-quality substitutes tend to fall apart, and the fragments damage the pump subsequently. 

• Fuel contamination is the top cause for failure. Therefore, avoid filling-up your car with unknown diesel from a drum. If you use a fuel can, ensure that the diesel has been filtered and that you know its original source. Apart from being illegal, unless duty has been paid on it, never use other fuels in your modern diesel, including pure biodiesel, unless you are buying it from a credible supplier that warrants that the fuel will not damage the delicate pump. 

Turbocharger 

Many people make the mistake of regarding the turbocharger as an add-on component to an engine, comparable to, for example, the alternator. Therefore, the running assumption is, when it fails, it is the turbo’s fault, and a new unit is needed. 

In truth, as the turbo is such an integral part of an engine, external problems often cause failure and will do so again to the new unit if the root cause isn’t addressed. Many people make this mistake, and spend thousands of rands installing a new turbo, without fixing the root cause, for example oil contamination. The result is that the new turbo fails soon after. Let’s have a look at the top turbo killers according to Car Magazine . 

Main causes of turbocharger failure and how to avoid them: 

• Lack of lubrication - The number one killer is a lack of lubrication. Owing to the fact a turbo can spin quicker than 200 000 r/min means the boundary-layer lubrication at the bearings is critical. If for some reason the correct-specification oil does not reach these areas at the correct pressure, turbo failure is imminent. Lubrication problems may include incorrect oil, faulty oil pumps and a blocked oil feed (or drain) pipe. When a turbo is replaced, a dry start-up – if the correct lubrication procedures were not followed – can kill a turbo in a matter of seconds. Even at idle, the turbo spins at about 10 000 r/min. 

• Oil contamination - It is not just sufficient for the turbo to receive oil; the liquid also needs to be clean. As the turboshaft spins at such high rotational speeds in the bearings, any particles or debris in the oil act as grinding paste and wear away the shaft and brass bearings in the case of the journal type. Forensic analysis of this failure mode reveals scoring on the shaft and bearing surfaces. The wear can result in bearing failure and seizure but will initially lead to other problems. 

• Exceptional operating conditions - If a turbo runs outside its design envelope, failure can be catastrophic. This includes over-speeding of the turbo leading to burst compressor and turbine wheels if, for example, there is a problem with the wastegate control (or variable-nozzle turbo actuator). The turbo materials are designed for a specific temperature range; exceeding this is terminal. Hot shutdown is a known killer because oil starts coking to bearing surfaces. Although modern water-cooled turbos with electric water pumps are designed to cope with engine stop/start systems, many experts feel there’s merit in letting a turbo cool down by allowing the engine to idle after sustained hard driving. 

• Foreign object damage - Imagine throwing a rock into the blades of a room fan. The result is similar to a foreign object entering a turbocharger, but the damage is amplified because of the elevated speeds. Run the vehicle without an air filter and the dust particles will sand-blast the compressor blades. On the turbine side, any debris the engine coughs out can be lethal. Even the sealant wrongly applied by some turbo installers on the turbine flange can harden and break off in tiny pieces, ripping the blades to shards. 

• Ruptured pipes and intercoolers - A ruptured oil feed pipe, breather hose, boost pipe or poor-functioning intercooler can lead to total turbo failure or a drastic loss in engine performance. What’s more, a blocked or damaged intercooler can have a big impact on the turbocharger’s performance and longevity. Make sure these components are inspected when you have your vehicle serviced. 

How to take care of your turbo: 

• Always service your engine according to the service schedule, paying special attention to the specification of oil. 

• Fix engine problems immediately, including intake and boost leaks. 

• As good practice (although not strictly needed with modern water-cooled turbos supplied by electric water pumps), let the engine idle for a minute after hard driving. 

• When a turbo fails, find the root cause before replacing the unit. 

• Request the turbo oil-feed pipes to be replaced when a turbo change is required (or after 100 000 km), as a coked supply line has the same effect as cholesterol to arteries. 

• Never dry start a new turbo. Ensure the proper lubrication procedures are followed when a new turbo is installed. 

In Summary 

Buying a diesel used cars is a bit different than buying a regular used car. When you purchase a turbodiesel car, you should be aware of the possible expensive components which need replacing, and, with the help of this article, you should know what to look out for and how to avoid these parts failing prematurely. 

In addition to all the due diligence you should do when purchasing any used car, here are some key factors to look at when looking into buying a used diesel vehicle. 

• Look at Exhaust

A diesel car will burn the diesel a little different than a gas engine. Because the diesel fuel is a lot thicker than gas, there is a small amount of black soot that is emitted from the exhaust. Look at the exhaust to see if there is an excessive build-up of soot on the vehicle. If so, then there is a problem with the engine combustion or in the exhaust itself. 

• Look into the Oil

A big factor that will tell if something is wrong is the diesel used cars oil. Look at the dipstick and see what colour the oil is. If you notice a little bit of white, or a milky substance on the stick, then you will want to keep walking to another vehicle. This means that there are some internal problems with the engine, and it will need to be replaced. You can also look under the vehicle to see if any of the oil is leaking. A quick look around the engine compartment for tell-tale signs of oil around the gaskets is another sign of a problem. 

• Try to Start the Car

A common problem with diesel used cars is that they are notoriously bad starters in the cold. Since diesel fuel is thicker than regular gas, it will get much thicker and harder to move through the lines when it is very cold. Try starting the vehicle from a cold start to see how it cranks over. If it takes a long time, then there are problems with some of the glow plugs in the head. These can be replaced but are expensive to do so. 

• Exhaust Smoke

When the car is running you will see what kind of exhaust is coming from it. A light black colour is good, but if it is too thick, or has a white colour, then there is a problem within the engine. It could be that the fuel/air ratio is off, but it could be that the cylinders need to be bored or replaced.