Why does the Regal's idle speed increase when the air conditioning is turned on?
2 Answers
The reasons for the Regal's high idle speed when the air conditioning is turned on include the idle switch not closing, a faulty idle control valve, and intake manifold leaks. Idle switch not closing: The speed signal determines the fuel injection volume and timing. However, the engine is operating under idle conditions with reduced air intake, leading to an overly rich mixture and an increase in speed. Faulty idle control valve: When the idle speed exceeds the set speed, the ECU commands the idle control valve to reduce the intake bypass, decreasing air intake and lowering the engine speed. Issues such as sticking due to oil contamination or carbon buildup, or incomplete throttle closing, can prevent the ECU from correctly adjusting the engine idle, resulting in unstable idle speed. Intake manifold leaks: The opening of the idle control valve and the air intake strictly follow a specific functional relationship—where an increase in the idle control valve's opening corresponds to an increase in air intake. Intake manifold leaks disrupt this relationship, causing the air flow sensor to fail in measuring the actual air intake accurately. This leads to the ECU's inaccurate control of air intake, resulting in unstable engine idle speed.
I've been driving my Buick Regal for quite some years now, and this issue of increased idle speed with AC on is quite common. When the AC is turned on, the compressor requires additional power from the engine to operate, so the vehicle's control system automatically raises the idle speed to prevent engine shaking or stalling. This is a normal compensation mechanism. However, if the idle speed surges too high or fluctuates erratically, then there might be an issue. I once experienced a situation where the idle speed was stuck at 1500 RPM and wouldn't drop. After inspection, it turned out to be excessive carbon buildup on the throttle body, which blocked airflow and caused the computer to misjudge the need for more power. Another common cause is a vacuum leak, where air sneaks into the engine manifold, leading to unstable idle speeds. If the compressor clutch is not engaging properly or is worn out, it can also cause unstable load conditions. I recommend regularly cleaning the throttle body and checking these hose connections during routine maintenance. If the symptoms are severe, like increased engine noise when the AC is on, it's best to visit a professional shop to read the fault codes and address the issue promptly for safety.
