(Un) safe colours

Naming someone or something colorless will definitely not be taken as a compliment. It is quite different in the case of motor vehicles. Specifically, it is about exhaust fumes coming out. Their colorlessness is most desirable, unlike the whole range of other colors – from white, gray, blue and ending with black. The lack of color of exhaust fumes indicates the correct operation and condition of the engine and other components, while their coloring – a more or less serious defect.


This color of exhaust does not necessarily mean that something bad is happening and the intervention of the mechanic is necessary. White smoke can be ordinary water vapor, a completely natural phenomenon when driving at low ambient temperatures. The task of the driver is then to check the level of coolant in the expansion tank. The problem begins when the loss of refrigerant fluid is significant and the radiator and all pipes show no signs of cracking or defects. In this case, there is leakage of coolant into the combustion chamber. Why is this happening? In most cases, a leaky head gasket is responsible for this. In some cases, the head itself or the power unit block also breaks. Regardless of everything, it is necessary to work and worst of all – costly disassembly of the entire head.


On the other hand, gray exhaust fumes usually indicate a too rich fuel-air mixture (the air-fuel mixture contains less oxygen than is needed to burn a specific portion of fuel). In many cases, gray clouds coming out of the exhaust pipe are also characterized by an intense smell. Too much fuel dosage is often caused by a damaged temperature sensor.


The blue color, unfortunately, heralds serious problems with the power unit. Why? Blue exhaust fumes – regardless of whether we are dealing with a gasoline or diesel engine, they arise from the combustion of not only fuel and air, but also oil. The transition of blue to a more intense blue, indicates a large admixture of the latter in a traditional mixture. It should be remembered that blue exhaust gases do not have to appear throughout the entire engine work cycle. Sometimes they can be observed on the so-called idling or under load. Piston rings are suitable for replacement, and the cylinders must be ground. Blue clouds coming out of the exhaust can also indicate damage to the valve seals: they are then only visible when reducing the revolutions of the power unit. Turbocharger leaks also have the same “effect”.


The darkest smoke coming out of the tailpipe is most commonly found in diesels. In most cases, it occurs when the throttle opens abruptly, as well as when driving at high speed. Why is this happening? This is due to the failure of one or more components of the injection system, i.e. injectors, injection pump or exhaust gas recirculation system. Most often, however, the reason for the intense formation of black exhaust gases is damage to the injectors: instead of properly spraying the fuel under pressure, it begins to leak (in the workshop jargon: the injectors pour). Damaged injectors should be replaced: they can be replaced with new or regenerated ones. The bleak color of exhaust gases – contrary to popular belief – also applies to gasoline units. The color of exhaust gas black usually results from engine control system failure, which results in injecting too much fuel into the combustion chamber. In addition, black exhaust smoke may also appear on vehicles in which unprofessional chiptuning has been carried out.

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