The advantages of diesel engines are as well known as their disadvantages. More economical thanks to higher efficiency, they are more complex and expensive because of the specifics of their design. Of course, paying less at the pumps is pleasant. However, this barrel of honey also has its spoonful of tar: the moment the diesel starts to misfire even a little, questions immediately arise – is it the injection pump or the injectors…?
Indeed, repairing a diesel is no gift. Parts are expensive, so when it comes to diesel maintenance one must be especially careful. However, in many cases the alarm raised by seemingly obvious signs of needing extensive repair can turn out to be completely unnecessary. After all, it can happen that a daunting and inevitable problem suddenly finds a simple and affordable solution.
For example, a typical case. The car's diesel engine suddenly loses responsiveness and sensitivity to the accelerator pedal, the RPMs wander, and when revving, thick black smoke pours from the exhaust pipe. “Injection pump or injectors,” the neighbors at the parking lot speculate. “Yes, unlucky, but what can you do – it’s a diesel.” Yet with such signs of an apparently inevitable, costly, and complex repair, there is no need to despair. In any case, until the true cause of the engine’s obvious ailment is accurately diagnosed and the repair methods are defined, there is no point in lamenting.
In many cases, the cause of diesel engine problems can be simply dirt. Dirt that has accumulated where it has no business to be – in the intake tract of the engine’s manifold. And if such an unpleasantness occurs, the diesel engine begins to behave as if it has suddenly and clearly worn out prematurely to the limit.
First of all, it is worth understanding the causes of intake tract contamination in diesels. There are many such causes, but it all starts, as is known, with fuel quality. If the cetane number of diesel fuel is reduced, below 50, which is by no means rare at our stations, such fuel has an increased tendency to form carbon deposits. This issue is amplified by the design features of modern diesel engines. In the intake manifold of most of them an EGR valve is obligatorily installed, an important component of the exhaust gas recirculation system.
The widespread adoption of this system is due to its ability to reduce the NOx percentage in diesel exhaust gases. During low‑load engine operation, this valve enables the air supplied to the engine cylinders to be diluted with exhaust gases – up to 15% of the total intake mass. Deprived of oxygen, this portion of exhaust gases acts in the cylinders similarly to an inert gas, slowing the reaction rate, lowering the fuel combustion temperature to a level where NOx does not form. At the same time, besides the direct impact on the engine’s environmental characteristics, the recirculation system solves another extremely important technical problem. Adding an inert component to the air charge supplied to the cylinders eliminates the risk of knock in regimes where the engine runs on a lean mixture. Under high and continuous loads the diesel engine operates without supplying such a component, i.e., exhaust gases, to the cylinders. In these regimes the EGR valve is closed, and therefore there is nothing in the intake manifold that could promote soot formation on its walls. However, this mode of operation, when cruising on a highway, is far from the most common in the overall picture of city‑car usage. Thus, soot formation in the intake manifold is only a matter of time, sooner or later, and any city‑car engine will eventually develop this problem.
In addition to the characteristics of the exhaust gas recirculation system, the vacuum pump that vents excess crankcase gases into the intake manifold also contributes to dirt formation in the intake manifold. Obviously, the oily mist that enters the intake manifold from the engine crankcase creates all the conditions for increased contamination of all surfaces, including the EGR valve itself. In a contaminated state this valve can completely fail to perform its normal functions, resulting in exhaust gases constantly being present in the intake manifold.
But it is not only exhaust gases and oil mist that can cause intake‑manifold contamination in a diesel. Another contributing factor can be the operation of the engine with faulty glow plugs. Starting a cold engine under such conditions is difficult for obvious reasons, and fuel also enters the manifold, worsening the overall contamination picture. Over time, considering all these causes, it is no surprise that the intake manifold becomes thoroughly coated with a thick layer of fuel‑oil‑like grime, whose presence in this location gives rise to a whole range of diesel‑operation problems.
