53.18. Indicate the method for determining the cause of unstable idle speed.
UAВкажіть метод визначення причини нестабільних обертів холостого ходу.
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This exam question is focused not on the direct requirements of road safety, but on the technical condition of the vehicle and the driver's ability to recognize typical signs of engine malfunctions. Unstable idle speed can lead to sudden engine stoppage at a traffic light or in a traffic jam, complicate the start of movement, and create dangerous situations. Therefore, this topic is useful for preparing for the theoretical exam even when it goes beyond the literal norms of the traffic rules.
From the perspective of the Traffic Rules of Ukraine, this question does not test a specific section of the traffic rules, since engine diagnostic methods are not regulated in the rules. In fact, it belongs to the block "technical operation/maintenance of the vehicle," which often appears in training test formats alongside topics about the technical condition of the car. The logic of the correct action is based on the fact that at idle speed, the "failure" of just one cylinder is enough to make the engine speed uneven, and the fastest way to check this is by observing the engine's reaction when the fuel supply to individual cylinders is sequentially interrupted.
That is why the best solution is the method of "stopping" a cylinder by sequentially disconnecting the fuel injectors: if after disconnecting a particular injector the engine's operation noticeably worsens, this cylinder was likely working properly before; if almost nothing changes, the injector or the cylinder itself (ignition, compression) is suspect. The option of "listening" can only provide indirect signs (knocks, misfires), but does not localize the cause as reliably, and the "comparative method" without a clear criterion (what exactly and with what to compare) is too general and does not provide a practical algorithm, which is expected in a similar exam question.
Traffic Rules clauses directly checked by this question
The Traffic Rules of Ukraine do not contain any clause that establishes or describes engine diagnostic methods (in particular, the method of determining the cause of unstable idle speed by sequentially disconnecting fuel injectors). Accordingly, knowledge of specific clauses of the Traffic Rules is not tested by this exam question, since it relates to the topic of technical maintenance/repair and engine diagnostics, and not to the rules of the road.
Therefore, the correct answer is "The shutdown method by sequentially disconnecting fuel injectors," given that, according to the definition of the Traffic Rules of Ukraine, methods for identifying the causes of unstable engine operation (including injector checks) are not regulated by the Traffic Rules and belong to the technical diagnostics of the vehicle.
Unstable idle speed is most often associated with one of the cylinders operating unevenly or even "dropping out" of operation entirely. At idle, the engine has minimal power reserve, so even a minor issue with fuel supply to one cylinder immediately manifests as fluctuating RPM, vibrations, and uneven running.
To quickly determine which specific cylinder or injector is the cause, the “cylinder shut-off” method is used: fuel injectors are disconnected one by one while observing how the engine's operation changes. The logic is simple: if a cylinder was working properly, then after cutting off its fuel supply, it will stop contributing to the engine’s operation, and the RPM will noticeably worsen. If the cylinder was already barely working due to a faulty injector or another issue, then disconnecting it will “change nothing,” since it was not providing normal operation anyway.
In practice, this check is performed on a warmed-up engine by sequentially disabling the injectors one at a time (for example, by disconnecting the electrical connector or using diagnostic equipment if such a function is available). After each disconnection, the reaction is assessed: if the engine starts running noticeably worse, it indicates that the disconnected injector and the corresponding cylinder were functional; if there is no significant difference, that particular injection channel or cylinder is suspect and requires further inspection.
This method is called the “shut-off method” because we artificially stop the operation of an individual cylinder by cutting off its fuel supply, and based on the engine’s reaction, we conclude where the malfunction is. This allows for quick localization of the cause of unstable idle speed without complicated measurements at the initial stage.
Therefore, the correct answer is the “shut-off method by sequentially disconnecting the fuel injectors,” since step-by-step disconnection of each injector allows you to determine the faulty injector or cylinder causing unstable idle speed by observing changes (or lack thereof) in engine operation.