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What Causes Unstable Idle Speed Fluctuations During Cold Start?

1 Answers

OGemma

07/30/25 7:38am

Unstable idle speed fluctuations during cold start are primarily caused by excessive carbon buildup in the throttle body and intake passages. Below are the relevant explanations: 1. Intake manifold or valve leaks: Incorrect engine air intake can result in overly rich or lean air-fuel mixture, leading to abnormal combustion. This causes engine idle fluctuations. When leaks affect engine vacuum, severe vibrations occur, particularly impacting cold idle. Common causes include aged intake manifold gaskets causing leaks or damaged intake manifolds. 2. Excessive carbon deposits in throttle body and intake passages: Carbon buildup and dirt accumulation in the throttle body and surrounding intake passages alter the air channel cross-sectional area, preventing the control unit from precisely regulating idle air intake. This results in improper combustion due to overly rich or lean mixtures. Common causes include throttle body carbon deposits and oil contamination/carbon buildup in surrounding intake passages.

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