Turbocharged vehicles have become commonplace on South African roads. They’re more efficient than the unblown variety, and their torquey power delivery makes them fun to drive, especially at low engine speeds.
Unfortunately, feedback from the marketplace signals some ominous reliability problems afflicting turbocharged diesel engines in South Africa. It seems that a significant proportion suffer premature failure, often due to overheating. A lot of manufacturers’ money is being spent sorting out the problems, but these are so widespread that the best advice we can give is to be extremely wary when buying a turbodiesel until the matter has been sorted out.
Our sources reveal that the industry is making a concerted effort to pin down the causes, which seem to be a combination of the following:
1. The high mean effective pressure (MEP) increases the piston and bearing loads, especially when labouring at low revolutions, due to bad driving habits. This can occur, for example, when a caravan is towed at low speed in a high gear. In fact, on some vehicles, the cooling system is so marginal that any full throttle running is best avoided except for short periods.
2. High MEP increases blow-by, which is the leakage of combustion gases past the piston rings into the crankcase breather system, resulting in excessive oil vapour causing carbon deposits in the intake manifold, combustion chamber and rings. Beyond a certain point, the process accelerates so that the carbon build-up occurs at a faster rate until the engine fails.
3. Carbon build-up in the intake manifold is also aided by the presence of an exhaust gas recirculation system. If these deposits are not regularly removed, they start to restrict the air intake, causing a drop in delivered volume. Choking the air supply leads to power loss as well as an increase in heat inside the engine, which soon leads to failure.
4. The pressure existing at the intake valve depends on the control exercised by the turbo wastegate, which is essentially a pressure relief valve. On many turbos, the wastegate is set to restore the power that an unblown engine would have lost at Gauteng altitudes. However, it sometimes happens that at a particularly low
atmospheric pressure, the turbo has to critically overspeed to reach its designed pressure, leading to bearing damage. Some manufacturers have countered this problem by adjusting the control unit to deliver less fuel at full load.
5. Turbo blade erosion can also result in overspeeding, with the same end result as described in 4.
6. Delayed oil and oil filter changing accelerate wear, leading to turbo bearing failure. A blocked air filter will reduce intake airflow sufficiently to cause overheating. In fact, it is good practice to use the best quality diesel oil you can find. We know of one manufacturer that insists on a specially imported unique formulation oil for one of its engines.
7. As mentioned in 1, on some vehicles, the cooling systems are so marginal that the addition of an aftermarket bumper or spotlights is enough to cause overheating. Other reasons for overheating include a partially blocked radiator, a malfunctioning automatic fan coupling, or retarded injection timing. Most of the malfunctions so far described also apply to turbocharged petrol engines, but the design margins are normally higher, so that there is less likelihood of engine failure.
Turbocharged diesel
engine theory
It seems paradoxical that diesel engines should heat up when they get less air, ie more fuel, because petrol engine mixtures are often enriched to cool them down. The reason is that diesel combustion takes place around a small area of fuel droplets in the vicinity of the injectors, but the rest of the combustion chamber contains pure air. This excess air plays a major role in keeping the piston and cylinder head relatively cool. However, reducing the amount of air does not reduce the fuel delivery, so that the same amount of heat goes into the system but there is less cooling air, hence a rise in internal temperature. Most diesel engines run at between 20 and 40 per cent excess air, and if this is reduced the engine starts to smoke. It follows that smoking is usually a sign of overfuelling, and should be investigated.
Turbocharged petrol engines
Petrol engines operate within a narrow range around a 14,7:1 air-to-fuel mass ratio, with the richer mixtures being suitable for warm-up, idling and acceleration, and the chemically correct ratio reserved for cruising at a constant speed. On most new models, the lambda sensor will strive to keep the mixture at this value, and the engine will be designed to cope with the range of temperatures that these mixture variations will create. Most turbo faults result in less air reaching the combustion chamber, causing an over-rich mixture. This shows up as a lack of power as less heat is released because the evaporation of the extra fuel cools down the intake system, and also because the rich mixture speeds up the combustion process. In South Africa, some turbo petrol engines seem to have problems at altitude only, mainly due to excessive blowby and overspeeding.
Driving a turbocharged vehicle
1. Do not let the engine idle for extended periods, because the combination of low pressures and low shaft rotation speeds may cause a seepage of oil into the turbine or compressor housing. This may not cause any harm, but the resultant smoking may alarm the driver. 2. Raise the engine speed to above idling only after normal oil pressure has been reached, because this is the only guarantee that the turbo bearing has sufficient oil. This process may take more time if the engine has not been run for a long period, or if it is very cold. 3. Gradually reduce the load on an engine that has been worked hard, so that the turbo temperature is reduced while still maintaining oil pressure and normal cooling. This means that if you have been using close to full throttle you should slow down for a minute or two before stopping, and when you stop from normal cruising speeds, the engine should be allowed to idle for about a minute before switching off. 4. Avoid blipping the throttle with the vehicle stationary, because this causes the turbo to accelerate unnecessarily. 5. Keep in mind that the internal engine stresses are highest at very low engine speed and full load, or very high engine speed and no load. An engine is happiest at 30 to 70 per cent of its load and speed range, so that if you keep it there it will reward you with a long life.
What to do if you have a turbocharged vehicle, or intend buying one:
Drive it as explained above, at least read the article on servicing elsewhere in this issue, and have it serviced regularly. If you intend buying a new vehicle, find out how good the warranty is, and follow the conditions to the letter so that you’re covered if it packs up. If you intend buying a second-hand turbodiesel, be very careful, and if possible, get a diesel expert to check the engine. Bear in mind that most aftermarket secondhand warranties are really insurance covers, and will not pay for a full engine overhaul.
Conclusion
Turbodiesel imports are currently on the rise, not only are they fuel-efficient and fun to drive, but the government would like to encourage the use of diesel fuel because there is a surplus of the stuff. On the other hand, turbodiesel design is currently at the level where petrol engines were 20 years ago in terms of life expectancy. The designs are good, but there is precious little margin for errors in driving, servicing or cooling, and most people in the trade do not understand them. None of this bodes well for trade-in values. Another point to consider is that it can take 100 000, or even 200 000 kilometres of motoring before you recoup the extra price of a turbodiesel versus its equivalent petrol-engined stablemate. Another negative is that besides being more frequent, the services are generaly more expensive. Our advice is – think hard before you buy one. The only good news is that no matter what positive spin the manufacturers and importers might try and put on the situation, they are aware of the problems and are trying to solve them.