While the engine runs
Engine runs rough when cold and fine when warm: what it could be
If the engine hesitates, shakes or lacks power for the first few minutes and then runs fine, the control unit is not getting the cold-start mixture right. The usual causes are the coolant temperature sensor, a vacuum leak, the injectors or, on diesels, the glow plugs.
Check without hurryWhile the engine runs
Reviewed by Daniel Morales, founder of Reprorace SL, on
Is it serious? Can I keep driving?
Usually not urgent, but do not ignore it. You can usually drive, gently until the engine warms up. If the check engine light flashes or the car stalls at an intersection, see a shop soon: an engine that stalls when cold can leave you stranded in a bad spot.
A careful, general answer: only someone who inspects the car can say whether it is safe to drive. When in doubt, stop somewhere safe and call for assistance.
Most likely causes
- Coolant temperature sensor: if it reads warmer than reality, the control unit does not enrich the mixture when cold.
- Vacuum leaks: hoses shrink in the cold and let in unmeasured air.
- Injectors: poor spray shows up more when cold.
- Spark plugs or glow plugs: worn plugs on gas engines; glow plugs that do not work on diesels.
- EGR valve not closing properly: lets exhaust gas in when the engine cannot take it.
Listed from more to less common in general; on your car they may be different. A fault-code reading and a check at the garage confirm which one it is.
To understand it better: Why does my car go into limp mode when it is cold? What does the engine thermostat do and what does P0128 mean? What are engine misfires (P0300)?
Car system: Cooling
What you can check yourself
Note at what temperature, or after how many minutes, the problem goes away.
Notice whether it also happens after a frosty night or only on some days.
On a diesel, check that the glow plug light goes out normally.
Read the codes before the engine warms up if you can: some faults only log when cold.
Safe checks only: with the engine off and cold whenever you look under the hood, without taking anything apart and without touching hot or moving parts.
Related fault codes
- P0117Engine coolant temperature sensor 1, circuit: signal low
- P0118Engine coolant temperature sensor 1, circuit: signal high
- P0128Coolant thermostat: coolant temperature below the thermostat regulating temperature
- P0171Mixture too lean (bank 1)
- P0300Random or multiple cylinder misfire detected
- P0380Glow plug/heater, circuit A: malfunction
Generic codes that often come with this symptom. Seeing one does not confirm the cause: it is where the diagnosis starts.
How the ECO-LITE helps diagnose it
Plugged into the OBD socket, the ECO-LITE reads the fault codes of the control units that answer, with the freeze-frame data captured when the fault happened, and shows live data on your phone or computer. With your permission, your garage can see them remotely before you bring the car in.
What to look at here: coolant and intake air temperature at start-up (they should be close after a night parked), fuel trims and misfires when cold.
Frequently asked questions
Is it normal for the engine to sound different when cold?
Yes, the idle is a bit higher and the engine is louder until it warms up. Hesitation, shaking or stalling are not normal.
Why is there no code?
Some cold-start faults do not cross the control unit's thresholds. Live data at start-up helps find them.
Should I warm the car up at idle?
There is no need: it is better to start and drive gently for the first few miles.
Keep reading
- Engine cooling: how it works and what fails
- Car jerks when accelerating: causes and what to do
- Car slow to start: causes and what to do
- Rough idle: why the engine shakes or hunts when stationary
- Car symptoms: what they mean and what to do
- Check engine light on: what to do
- How to read and clear your car's fault codes
- OBD fault codes: what each one means