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I've noticed that, when I first start my car, there is an intermittent popping or clicking sound coming from up near the engine (sometimes it almost sounds like a very quick burst of air escaping a high pressure valve - sort of a quick PFFFT). It happens maybe every 6 or 7 seconds when the car is accelerating or moving. It will not happen when the car is idle. About 5 or 6 minutes into my drive, the sound will stop.

This issue started this Spring (4 or 5 months ago) and has been getting progressively worse over time.

I've noticed that the issue only occurs when I have the A/C on. If the A/C is off, I never hear the noise. There does not seem to be anything wrong with the A/C, however. It's still cold, the fan runs fine, etc.

Any ideas what this could be? I'm mostly wondering if it's an annoyance or something that I should be worried about. If it's just an annoyance, I can live with some noise - I wouldn't be driving a Kia Rio if I wanted luxurious transportation. However, if it could be an indication of a broader problem with my car, I obviously want to fix it before it gets worse and causes a potentially more costly repair.

(I've asked about this issue before, though I had significantly less information at the time, and I thought it was engine-related.)

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  • Did you have any AC-related work done recently? Refrigerant top-up maybe?
    – Zaid
    Commented Aug 12, 2015 at 12:03
  • Good question, but no...
    – John Chrysostom
    Commented Aug 12, 2015 at 12:08

3 Answers 3

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The source of the pffft sound is the AC system's expansion valve. It is normal to hear it when the AC engages/disengages; it doesn't indicate that the expansion valve has an issue per se.

However,

what is not normal is that the AC is cycling on and off so frequently. 6-7 seconds tells me that something is disengaging the compressor clutch. This could be abnormally high pressure or abnormally low pressure (likely low pressure since the issue goes away after some driving).

Given that no AC-related work has been performed on the vehicle, I'm inclined to believe that there may be insufficient refrigerant or presence of debris in the line. I strongly recommend checking the AC's high-side and low-side pressures against the values which Kia is expecting for your vehicle.

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  • This makes a ton of sense. Thank you. I'll check the high-side pressure tonight and, if that looks good, I'll take it to somebody to have the low-side looked at.
    – John Chrysostom
    Commented Aug 12, 2015 at 12:46
  • @JohnChrysostom : You're welcome. Do keep us posted
    – Zaid
    Commented Aug 12, 2015 at 12:48
  • So, checked low-side last night (had them swapped in yesterday's comment). Pressure was on the low end of the recommended range, so I tried adding a bit of coolant. Interestingly, the system didn't seem to want to accept coolant for some reason - it all came leaking out of the hose. I've never experienced this when recharging A/C before, and it's possible that my charging hose/gauge was faulty (they're really old). I was reluctant to try too hard to get it to accept coolant, as I understand you can kill your compressor if you add too much without monitoring the high side pressure.
    – John Chrysostom
    Commented Aug 13, 2015 at 11:41
  • I think my next step is going to be to go to a mechanic to get high side pressure checked, and to verify that there is no debris in the system. I'll check back.
    – John Chrysostom
    Commented Aug 13, 2015 at 11:42
  • @JohnChrysostom : Was the AC running when you tried to add more refrigerant? Also, how off was the low-pressure reading? What should it be? Are you using the gauge on a recharge can to check the pressures?
    – Zaid
    Commented Aug 13, 2015 at 11:46
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In addition to the suggested insufficient refrigerant or debris, it could also be a pressure safety sensor going bad. That can cause the compressor to cycle on and off even if you have a full charge. Keep that in the back of your mind if it's not the other things.

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  • Thanks for this. There seems to be some weird issues going on with the system (low side pressure was in the low end of the recommended range, but didn't accept additional coolant), so I'm thinking my best bet is to go to a mechanic before I blow out my compressor. I'm usually a pretty good troubleshooter, but I don't have the tools to do this safely without risking a costly compressor replacement.
    – John Chrysostom
    Commented Aug 13, 2015 at 11:44
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Stopped by a mechanic on my way home from work. The popping sound is the compressor clutch. The reason it's making noise is that the friction material is getting worn down, so you're hearing metal on metal when it's slapping together. It sounds little bit like air escaping (the PFFFT sound described earlier) because it's slipping a bit without the friction material.

The reason it happens every couple of seconds is that, on small cars, the computer often will turn the compressor off when the engine is under load in order to save power. So, when I'm accelerating, the compressor is being turned off, then when the RPMs drop back down, SLAP, the clutch comes back together. Once I'm up to highway speeds consistently instead of accelerating, it stops altogether.

So, long story short: it's an annoyance for now but, as things keep wearing down, I may eventually need to replace the compressor clutch or (God forbid) the whole compressor - sometimes the clutch is attached.

Thanks for the help, everybody.

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