Rough when cold, fine when warm: why that happens

“Rough when cold, fine when warm” feels unserious precisely because it cures itself. In fact it is the most informative state an engine has: a cold start exposes what a warm engine compensates for.

Why a cold start exposes everything

Clearances are larger. The metal has not expanded, compression is lower, sealing is worse.

The mixture is richer. The module deliberately adds fuel because some of it condenses on cold walls.

The spark has a harder job. A cold mixture ignites less readily, and a weak plug or coil shows up here.

Oxygen sensors are not working yet — they are not up to temperature, so the module runs open loop from tables. With no feedback, any deviation goes uncompensated.

Once warm, clearances close up, the mixture leans out, feedback engages — and the symptom hides.

The usual causes

Spark plugs. A worn electrode gives a weak spark: enough when warm, not enough when cold. The cheapest hypothesis; electrode conditions are covered in the spark plug reading article.

A coil on the edge. It breaks down with cold condensation and works once warm.

An air leak. A rubber hose contracts in the cold and opens a gap. The classic cause of “only in the morning”.

Low compression in one cylinder. Cold compression is lower still, so that cylinder does nothing for the first few minutes.

An injector with poor atomisation — noticeable on a rich cold mixture, invisible when warm.

The crankcase ventilation valve — freezing in winter and producing either a leak or excess pressure.

What to watch in live data

The key rule: read the parameters during a cold start, not after the engine warms at the workshop.

Per-cylinder misfire counters in the first minutes. One cylinder with a rising counter is the answer; the general mechanics are in the misfire article.

Fuel trims when cold — a strongly positive value means an air leak.

Coolant temperature — so you can see at what temperature the symptom disappears. That indicates whether it is clearances or a sensor.

The temperature sensor reading. If it lies and reports “warm” on a cold engine, the module prepares the wrong mixture — and the symptom looks mechanical; covered in the temperature sensor article.

Why not to postpone

Misfires destroy the catalyst — unburnt fuel burns inside it.

Fuel washes the oil film off the cylinder walls during cold running — precisely when wear is highest.

The symptom grows. What clears in a minute today lasts five a season later.

The procedure

1. Read the codes — even with no lamp there may be pending ones.
2. Log live data during a cold start — this is the main step.
3. Inspect the spark plugs — the cheapest step.
4. Check the hoses for cracks, especially while cold.
5. Compare misfire counters across cylinders.
6. Test compression if everything else is clean.

In short

• A cold start exposes what a warm engine compensates for.
• Take the data cold, not after the workshop warms it up.
• An air leak through a contracted hose is the classic “only in the morning”.
• One cylinder with a rising misfire counter settles the question.
• Cold misfires destroy the catalyst and wash oil off the bores.

Download the app — misfire counters, fuel trims, and temperature are read with an ELM327 adapter over Bluetooth SPP on Android.