A Logical & Intuitive Diagnosis

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Steve Oborne is the owner, with his wife Jennifer Gilbert, of Atelier Oborne Service in Val-Joli. He has been involved in mechanics since childhood. [email protected]. Photo Bianca Diorio

For this column, I’d like to cover a repair done on a pickup truck recently.

The truck in question, a 2017 GMC Sierra, had 104,000 miles on it. The customer complains of a considerable loss of power in addition to the check engine light being on. At the appointment, we will do a road test that confirms 100% of the problem.

Back in the service bay, we start with a reading of the anomaly codes, in order to have a track to follow: P0324 Performance du capteur de cliquetis. The conditions necessary for the code to be registered are very simple; the ECM (Engine Control Module) has detected that the signal from one of the knock sensors is out of range for a cylinder.And to avoid mechanical damage to the motor, as long as the code is present, the motor power will be limited.

Follow the charter

Let’s jump into the charter:Step 1, we are given a list of codes that could be present at the same time, in order to direct us in another charter that would have priority over this one.

This is not our case here. Step 2, we are asked to do visual and auditory checks on the van. The list of parts to check is considerable, but a quick read of this step brings us to the 7th check, “an engine mechanical condition”. This leads to another link that opens to a text that has the title, Strategy Based Diagnostic.

We are informed that any diagnosis should follow a logical process. Quite vague as a guideline, and again, another extensive checklist of things to do follows.

Curiosity and intuition

I’ll stop here, because we never got this far in the charter. There is a very important point to come back to: “any diagnosis should follow a logical process”.In several articles, I often mention that we should always take fifteen minutes of personal research to satisfy our curiosity and our intuitions.

It also involves understanding how such code can be registered as active. The description mentions that the ECM detected clicks (which may be mechanical noises). This Sierra has a very small mechanical noise coming from the right head, where we discovered after investigation that a push rod on a cylinder is slightly crooked, causing a noise that is interpreted by the ECM as a rattle.

For the connoisseurs, yes, it is a variable displacement engine, and yes, it is once again an active hydraulic lifter that has retired (prematurely many would say!). After replacing the hydraulic lifters, the van was able to return to the road with full power.

The faulty rod and lifters. Photo Steve Oborne



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