What causes blue smoke during cold start of BMW N20 engine?
2 Answers
BMW N20 engine emitting blue smoke during cold start may be caused by aging valve stem seals with poor sealing or slight leakage from the valve cover gasket. Aging valve stem seals with poor sealing: Due to aging valve stem seals, they cannot achieve complete sealing when the engine is cold, allowing some engine oil to enter the combustion chamber and burn. After running for a while, the seals expand with heat and achieve complete sealing, thus eliminating the blue smoke phenomenon. Slight leakage from valve cover gasket: With slight leakage from the valve cover gasket, engine oil flows along the cylinder head to the spark plugs. After prolonged parking, some oil accumulates around the spark plugs. During startup, this accumulated oil gets ignited, causing blue smoke. After the engine runs for a while, oil no longer accumulates around the spark plugs, making the phenomenon unnoticeable.
I've driven several N20-powered BMWs, and blue smoke during cold starts is quite common. It's most likely due to aged valve stem seals, especially noticeable on winter mornings. Rubber components harden and shrink over time, compromising the seal and allowing oil to seep down the valve stems into the cylinders, resulting in blue smoke. Additionally, turbocharger oil seals may also leak, particularly in vehicles with over 100,000 kilometers. A simple diagnostic method: if the smoke persists for more than a minute during cold starts, it's a red flag. I recommend first checking oil consumption – exceeding 0.5 liters per 1,000 kilometers is a warning sign. When I last addressed this issue, replacing them with upgraded fluoroelastomer seals worked well. If the piston rings are worn, engine disassembly will be necessary, so early intervention is crucial.
