How to Diagnose a Problem with the First Cylinder

This video shows the approach a good mechanic should take when diagnosing a fault in the absence of warning lights and no faults found in memory. The symptom presented by the vehicle is a very irregular idle, so the defect is likely related to the fuel supply, particularly identifying which cylinder. Start by assessing what happens to the engine's rotational speed by disconnecting the injectors one by one: disconnecting the first injector keeps the defect unchanged, indicating that the problem is likely in the first cylinder. In fact, the symptom worsens when disconnecting the second injector and so on with the others. It is clear that the fault concerns the first cylinder, and you should proceed to measure the compression of each cylinder as follows: measure the maximum compression, that at one turn of the starter motor, and then at five turns (the starter can be activated from the relay box by making a simple bridge and removing the fuel pump relay to prevent fuel injection into the cylinders). This test also shows that there is a compression issue in the first cylinder. Complete the check by performing a cylinder leak test by introducing compressed air: it is immediately noticeable that the cylinder cannot maintain pressure and that air is escaping from the throttle body. This leads to the conclusion that the problem lies in the intake valve seat of the first cylinder, which is also confirmed by the height difference between the intake and exhaust valves after removing the rocker cover.