Case Study: Diagnosing an Unstable Idle on a 2019 Mercedes-AMG A35

 

Customer Concern

 

A 2019 Mercedes-AMG A35 was brought to us with two primary concerns:

• A high and unstable idle speed.
• A noticeable smell of fuel, particularly when stationary or in traffic.

As with any vehicle, these symptoms can have numerous causes. Rather than making assumptions, the investigation began with a structured diagnostic assessment.

Please note: This article discusses one individual vehicle. Similar symptoms do not necessarily indicate the same fault on every Mercedes-AMG A35 or vehicle with this engine.

 

Initial Diagnostics

 

The first step was to interrogate the engine management system and assess the live operating data.

A fault code relating to injector quantity adjustment was present, along with an intermittent turbocharger pressure sensor fault.

The live data also highlighted several findings:

• Long-term fuel trim of approximately 14%.
• Oxygen sensor data indicating a rich-running condition.
• Normal coolant temperature, allowing temperature-related fuelling issues to be ruled out.

Although these readings suggested the engine management system was reducing fuel delivery, the overall picture pointed towards incomplete combustion, rather than excessive fuel being injected.

 

Looking Beyond the Fault Code

 

The injector correction values suggested one cylinder was contributing differently from the others.

At this point, manufacturer technical information was reviewed.

Mercedes Technical Service Bulletin 12299 references cylinder head valve seat concerns on certain versions of this engine, with symptoms that can include:

• Reduced cylinder sealing.
• Increased injector correction values.
• Rough or unstable idle.

However, technical bulletins should never be treated as a diagnosis. They simply help guide further testing.

 

Advanced Testing

 

To investigate further, additional diagnostic testing was carried out.

This included:

• Relative compression testing.
• Intake pressure pulse analysis.
• Exhaust pressure pulse analysis.
• Ignition synchronisation testing.

The relative compression test identified cylinder one as performing differently from the remaining cylinders.

The most significant evidence came from the exhaust pressure pulse analysis, where a repeatable anomaly was present every time cylinder one entered its power stroke.

Because the pattern was consistent and repeatable, attention turned towards a possible mechanical sealing issue.

 

Gathering Further Evidence

 

The spark plugs were removed for inspection.

Cylinder one’s spark plug displayed carbon deposits that were not present to the same extent on the remaining cylinders, despite the plugs being relatively new.

An articulating borescope was then used to inspect inside the combustion chamber.

This revealed evidence that the exhaust valves on cylinder one were not sealing correctly. Combustion soot deposits were visible around the valve seat area, supporting the findings from the earlier diagnostic testing.

Importantly, the cylinder bore itself appeared to be in good condition, with no obvious piston damage visible during the inspection.

 

The Diagnosis

 

When all of the evidence was considered together, the findings consistently pointed towards a fault within the cylinder head affecting cylinder one’s exhaust valve sealing.

The conclusion wasn’t based on a single fault code or one test result.

Instead, it was reached by combining:

• Fault code analysis.
• Live data interpretation.
• Fuel trim analysis.
• Relative compression testing.
• Pressure pulse diagnostics.
• Spark plug inspection.
• Borescope examination.

Each stage added another piece of evidence until the diagnosis could be made with confidence.

 

Recommended Repair

 

Based on the evidence gathered, replacement of the cylinder head assembly was recommended.

In accordance with Mercedes’ technical guidance, replacement of the timing chain components at the same time was also advised due to the overlap in labour and updated component design.

As with any major engine repair, the final scope of work would be confirmed once the engine had been dismantled and all components could be inspected.