Rough Idle When Cold

If your engine shakes and stumbles for the first few minutes after starting, something in the cold-start enrichment system isn't working right.

Engines need a richer fuel mixture when cold because fuel doesn't vaporize as well at low temperatures. The engine computer relies on the coolant temperature sensor to determine how much extra fuel to inject. If that sensor reads incorrectly, or if the fuel injectors are dirty and can't deliver the right amount, the engine runs rough until it warms up. Worn spark plugs that struggle to ignite a cold, rich mixture are another common cause.

On cold starts, the engine also runs at a slightly higher idle speed to warm up faster. If the idle air control valve is sticking or the electronic throttle body is sluggish, the engine may not achieve that higher cold-idle RPM, resulting in roughness and even stalling. Thick, old engine oil can also add resistance to a cold engine, making the rough idle more noticeable in the first couple of minutes.

Jacksonville winters are mild compared to up north, but morning temperatures still dip enough to expose cold-start issues. Ez Mobile Mechanic will diagnose your rough cold idle at your location — we're a mobile mechanic in Jacksonville who comes to your home or workplace. No tow truck, no shop visit needed. Call (904) 788-7272.