At CMD Automotive, we see all kinds of performance issues come through the door—and we pride ourselves on getting to the root of the problem, not just clearing the check engine light and sending you on your way. Here’s a recent example from a 2014 BMW 535i with about 108,000 miles on the odometer. The Complaint The customer came in reporting a rough idle and poor performance on cold startup. The check engine light was on. Classic symptoms, but they can point to a number of possible causes—so we began with a full scan using BMW’s factory ISTA diagnostic platform. What We Found We pulled the following fault codes: P0300 – Random/Multiple Cylinder Misfire P0301 – Cylinder 1 Misfire P0303 – Cylinder 3 Misfire P0306 – Cylinder 6 Misfire From there, ISTA’s test plans revealed excessive cylinder imbalance and overly rich fuel trims at idle. Something wasn’t right in the combustion process, but which part? We tested each cylinder individually. AFR (air-fuel ratio) sensors were all operating correctly and reporting accurate information. We then measured volumetric efficiency—essentially, how well the engine is able to breathe—and

At CMD Automotive, we see all kinds of performance issues come through the door—and we pride ourselves on getting to the root of the problem, not just clearing the check engine light and sending you on your way.

Here’s a recent example from a 2014 BMW 535i with about 108,000 miles on the odometer.

Before and After photo of intake valves after carbonization has been removed using walnut shells

The Complaint

The customer came in reporting a rough idle and poor performance on cold startup. The check engine light was on. Classic symptoms, but they can point to a number of possible causes—so we began with a full scan using BMW’s factory ISTA diagnostic platform.

What We Found

We pulled the following fault codes:

  • P0300 – Random/Multiple Cylinder Misfire
  • P0301 – Cylinder 1 Misfire
  • P0303 – Cylinder 3 Misfire
  • P0306 – Cylinder 6 Misfire

From there, ISTA’s test plans revealed excessive cylinder imbalance and overly rich fuel trims at idle. Something wasn’t right in the combustion process, but which part?

We tested each cylinder individually. AFR (air-fuel ratio) sensors were all operating correctly and reporting accurate information. We then measured volumetric efficiency—essentially, how well the engine is able to breathe—and the results confirmed it: the engine was struggling to take in enough air.

Pinpointing the Problem

To get a closer look, we used a high-resolution inspection camera and scoped inside the intake manifold. That’s where we found it: heavy carbon buildup on the intake valves. This is a common issue with direct injection engines like this BMW’s. Over time, oil vapors and fuel residue can “coke up” the valves, restricting airflow and creating the rich condition we were seeing in the fuel trims.

The Fix

We removed the intake manifold and performed a walnut shell blasting—BMW’s recommended method for cleaning intake valves. Afterward, we installed new spark plugs and reassembled the engine.

With everything back together, we cleared the codes, ran another round of testing, and verified that fuel trims had returned to normal. Idle was smooth. Problem solved.

Have a European Vehicle with Idle Issues?

If your BMW, Audi, Mercedes, or other vehicle is running rough or throwing misfire codes, bring it in. We’ll get to the bottom of it—with the right tools, factory-trained techs, and no guesswork.

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