INCORRECTLY SET VALVE CLEARANCES

This fault is avoidable but for many people can be difficult to diagnose. Symptoms can vary, but may include the car not driving as it should, and/or MOT test failure on emissions. However, the engine may sound fine when running at idle.

The cause may be incorrect valve clearances; checking/re-setting these (they’re manually adjustable) is part of the service schedule and they need to be checked because the gaps can change over time. Even the tiniest change (slightly loose or tight gaps) can cause the engine management system to compensate, ie delivering more or less fuel according to how the ECU reads the situation from the info it’s receiving.

Running with the correct clearances will make a noticeable difference in terms of performance and fuel consumption.

From a diagnostic point of view the operator can check fuel trims and actual values or read back pressure, but in any case if there is any doubt at all about the correctness of the settings, it is a very good idea to check and, if necessary, re-set the clearances.

While the valve cover is removed, inspect the sealing gasket and ensure that it is in perfect condition (or renew regardless). A quick condition check is to place the valve cover, with gasket still attached to it, on to a flat surface and see if there are any gaps between the gasket and this surface, indicating a worn gasket.