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I have a Maruti Suzuki Alto K10 (a petrol/gasoline 1,000CC, 3-cylinder engine). Its idle RPM (without AC) is in the 840-900 range, and the engine load is 24-28% range (monitor using ODB2 scanner).

However, if I turn on the AC, the engine RPM does not increase; it is always below 1000 level and never passes 1,000 RPM level, but the engine load increases to 40-50% range. Keep it in 40-50% range until compressor clutch disabled (stop rotating). As far as I know, RPM should increase to 1,000 after turning on the AC.

This is OBD2 scanner screen record (initially ac off, after 1st second ac turn on). https://streamable.com/qec7sw

At Engine load is in the 40-50% range, instantly, RPM goes to the 900-1,000 range and quickly drops to the 840-900 range within one or two seconds (even if the AC compressor is on). In my opinion, there seems to be a fluctuation in engine RPM (ranging from 800 to 1000) whenever the compressor is running.

After the AC is on, there is a considerable vibration in the cabin and engine. There's no vibration if I press the accelerator/gas paddle to increase RPM to the 1,200 to 1,500 range.

A few months ago, a professional mechanic cleaned the engine's fuel injectors, spark plugs, and coils.

What could be the reason?

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  • Welcome to Motor Vehicle Maintenance & Repair! Is the behavior the reason you took it to the mechanic? Or has it always behaved like this? Aug 30, 2023 at 10:05
  • Does ac work, compressor running, cooling the interior?
    – F Dryer
    Aug 30, 2023 at 12:55
  • @Pᴀᴜʟsᴛᴇʀ2 I took the mechanic service as per the routing maintenance.
    – Chathu
    Aug 30, 2023 at 13:20
  • @f-dryer Yes, AC work and compressor running.
    – Chathu
    Aug 30, 2023 at 13:22
  • Some compressors cycle on and off along with the cooling fan, creating engine loads felt and seen at idle rpm. Small engines will exhibit this more than larger engines. The engine loads with ac includes the alternator cycling to higher output. Other compressors remain running without cycling on and off, eliminating the annoyance felt while driving.
    – F Dryer
    Aug 30, 2023 at 13:39

3 Answers 3

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As per your question, AC is also among the utilities in the vehicle and besides consumable one. Once you ON an AC the rpm will decreases and when the compressor is disengaged the engine increases in rpm, as it engaged again the rpm decreases and vice versa. I hope it will help, this is from experience.

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Your ac system most likely suffers what most vehicle ac systems suffer, the eventual leak that correlates to lower cooling and rough engine idling. A loss of refrigerant while still providing some cooling also means less oil circulates to return to the compressor for lubrication. Refrigerant moves oil throughout a system as a mist. Lose refrigerant and the compressor has less lube oil returning to the compressor, resulting in a noisy compressor and accelerated wear. When enough refrigerant is lost, a sensor detects this and sends a disable signal to halt power to the compressor. Find the leak, make proper repairs, evacuate the system of air and moisture, ensure a vacuum holds after the vacuum pump stops to ensure atmospheric air pressure doesn't seep back in, then refill the system to restore ac back to factory condition. Repairs can be done as a diy project but if gauges and vacuum pump is are unfamiliar, then leave this work to a repair shop. What isn't mentioned; which refrigerant is used and does your system have dye installed from the factory?

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  • But Whenever I turn on AC I can see compressor engages and hear compressor clutch engaging small click sound. When the compressor on, engine load is between 40-50%.
    – Chathu
    Aug 31, 2023 at 15:46
  • I have added OBD2 scanner screen record. streamable.com/qec7sw
    – Chathu
    Aug 31, 2023 at 16:01
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The increase in load correlating to cabin vibration is relatively normal, and isn't necessarily a sign of anything wrong with your vehicle, unless it's especially prominent. As another has said, there could be a leak in your A/C system, but I wouldn't bet on it. You could also have a loose alternator (or other) belt, bad injector, bad tensioner, or more and it's good to check those things both now and somewhat regularly but again some vibration is simply an unpreventable but unfortunate side effect of aging for engines.

Additionally, the variations in RPM is also normal, and is due to the high load you mentioned and the engine responding accordingly. Engines use what's called an idle speed controller to maintain a specific RPM at idle, and because the engine is an oscillator and the loads and air inputs are imperfect, trying to maintain the speed perfectly with a large load is a tough job for the engine and revs will bounce somewhat. Imperfect and older A/C systems will be more likely to cause more load, therefore more noise/vibration/harshness, but again not a definite sign of an acute issue unless it seems especially significant. If the RPM bounces up and down when not at idle, or if it bounces a lot, you're more likely to have a real problem on your hands.

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