Some vehicle issues aren’t obvious at first glance—and diagnosing them requires a methodical, data-driven approach. That was the case with this 2010 Acura MDX, which came into our shop with a check engine light and a rough idle on cold start. The Complaint The customer reported that the engine idled rough on startup but would smooth out once the vehicle warmed up. The check engine light was on. Initial Diagnostics Using the Honda/Acura factory HDS diagnostic system, we pulled the following fault codes: P0300 – Random/Multiple Cylinder Misfire P0301 – Cylinder 1 Misfire P0303 – Cylinder 3 Misfire P0305 – Cylinder 5 Misfire These are all misfire-related codes—concerning, but not definitive on their own. With the engine warm, all live data appeared normal. However, cold-start data showed soft misfires on cylinders 1, 3, and 5 that would clear up shortly after the engine ran for a few moments. Additionally, the data showed a slightly lean condition on startup. Testing the Basics We checked ignition and fuel delivery systems and confirmed everything was in proper working order. Volumetric efficiency testing ruled out any air intake or

Some vehicle issues aren’t obvious at first glance—and diagnosing them requires a methodical, data-driven approach. That was the case with this 2010 Acura MDX, which came into our shop with a check engine light and a rough idle on cold start.

2010 Acura MDX Misfire

The Complaint

The customer reported that the engine idled rough on startup but would smooth out once the vehicle warmed up. The check engine light was on.

Initial Diagnostics

Using the Honda/Acura factory HDS diagnostic system, we pulled the following fault codes:

  • P0300 – Random/Multiple Cylinder Misfire
  • P0301 – Cylinder 1 Misfire
  • P0303 – Cylinder 3 Misfire
  • P0305 – Cylinder 5 Misfire

These are all misfire-related codes—concerning, but not definitive on their own.

With the engine warm, all live data appeared normal. However, cold-start data showed soft misfires on cylinders 1, 3, and 5 that would clear up shortly after the engine ran for a few moments. Additionally, the data showed a slightly lean condition on startup.

Testing the Basics

We checked ignition and fuel delivery systems and confirmed everything was in proper working order. Volumetric efficiency testing ruled out any air intake or breathing issues.

All electronic systems and sensors checked out as well. At this point, we suspected a mechanical fault.

Digging Deeper

We moved on to advanced testing using AST engine analyzer software. By monitoring current draw during cranking with a current probe, we noticed that cylinders 1, 3, and 5 were drawing significantly less current—suggesting low compression in those cylinders.

To confirm this, we performed a cold compression test. Sure enough, cylinders 1, 3, and 5 showed low compression during cranking.

We then used a pressure transducer on the intake manifold to monitor intake pulses during cranking. The results showed weak and erratic intake pulses—again pointing to cylinders 1, 3, and 5.

Root Cause: Valve Clearance

It was determined that the valves had not been adjusted at the recommended service interval (every 60,000 miles). We removed the valve covers and inspected valve clearance.

The physical inspection revealed that the intake valves on cylinders 1, 3, and 5 had extremely tight clearances—so tight, in fact, they were likely not sealing fully on cold start.

We adjusted the clearances back to factory specifications, reassembled the engine, and ran the tests again.

The Result

Compression returned to normal. All misfire codes were cleared. Cold-start behavior was smooth, and no further symptoms were present.

Having Trouble With Rough Idle or Misfire Codes?

At CMD Automotive, we’re equipped to handle complex drivability issues—especially those that don’t show obvious signs. Whether you drive a Honda, Acura, or another make, our technicians have the tools and training to dig deep and get it right the first time.

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