P050D: Cold Start Rough Idle

What it means

The ECU detects rough idle behaviour during the cold-start phase only. Once the engine has warmed up the issue disappears. Typically caused by injector deposits, low compression on one cylinder when cold, or sticky valve guides.

At a glance

• Usually lights the check-engine light
• Affects emissions; the car can fail an emissions test

How it works

During the cold-start phase, the engine control module monitors crankshaft speed changes to detect rough combustion at idle. P050D sets when rough idle exceeds the expected level while the engine is cold, even if the engine smooths out after warming.

Common causes

Most likely first.

1. Injector tip coked - poor cold spray pattern
2. Valve guide sticky when cold

Symptoms

• Check-engine light on
• Rough shaking or misfiring after a cold start
• Engine may stumble or stall while cold

How to diagnose it

1. Read freeze-frame data and note coolant temperature, RPM, and any cylinder-misfire codes recorded during the cold start.
2. After an overnight cold soak, observe whether the shaking or misfire begins immediately and whether it clears as the engine warms.
3. Check live misfire counters by cylinder during the rough period to identify whether one cylinder or several are affected.
4. Inspect the affected cylinder's spark plug, ignition connection, and injector connector for wear, deposits, or poor contact.
5. If ignition and connections are sound, test injector operation and cold compression or leak-down against the manufacturer's specification.

Common fixes

• Clean or replace a fouled spark plug on the affected cylinder.
• Repair a loose or damaged ignition or injector connection.
• Clean or replace an injector with a poor cold spray pattern, if testing confirms the fault.
• Repair a cold-related compression or valve fault identified by testing.

Can I keep driving?

If the engine shakes mildly and then runs smoothly, drive gently once stable, but avoid heavy acceleration while it is misfiring. Stop driving if it stalls, misfires severely, or the check-engine light flashes, since continued misfire can damage the catalyst.