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Twice in the past month my 92 Civic has exhibited really bad idle behavior in temperatures around 10-20 degrees F. When first starting the engine, it idles at something like 2000-2500 RPMs (by ear; no tach so I can't tell for sure), and after driving for 5-10 minutes, the behavior changes and it cycles between a low idle (maybe 500-800) and the high idle (2000+) in a pattern of about 2 seconds per cycle. The problem goes away entirely once the temperature increases enough.

My assumption is that there's moisture freezing somewhere, likely inside the IAC valve or elsewhere around the throttle body. Tracking down the exact location seems unpleasant (especially considering the weather I'd be working in) so I'd mainly like to know:

  1. Is this cause for any concern aside from obvious things (emissions, mild surging while driving, etc.)?

  2. Is this a common problem with an easy known fix I could try without spending effort trying to track it down myself?

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When you say, "... goes away entirely once the temperature increases enough", do you mean to say when the ambient temperature gets high enough, or the engine temperature gets high enough? –  Paulster2 Jan 27 at 12:00
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2 Answers

I don't know when exactly Honda stopped programming this behavior into their cars, but I do know that what you described is a warm-up sequence that some older Hondas go through by design. First a high idle, then an alternating high and low idle (1-2 seconds/cycle), then normal low idle.

I would say that the behavior you describe is normal, and not indicative of any problem.

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Very interesting. I hadn't even thought of this possibility. If you or anyone else can find a reference to substantiate this I'll consider the matter closed and accept this answer. –  R.. Jan 29 at 4:15
    
@R I did a cursory search online and was surprised that I didn't find a clear reference to the behavior. I have personal experience with an old carburated Accord DX hatchback that did exactly this, but only when the car was in neutral (if i remember correctly). –  mac Jan 29 at 16:10
    
I bet you could verify this theory by unplugging the coolant temp sensor and emulating it's cold temperature value with a resistor. AFAIK the coolant temp sensor is the primary feedback the ECU gets about engine temperature. I think it's unintentional, though. –  mattmachine Feb 7 at 22:45
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Not being familiar with the technical layout of Honda engines this may be a long-shot. However, my Peugeot of the same era had identical symptoms. I replaced the idle air control valve and the problem was instantly solved.

Youtube comes to the rescue: How to Clean the IACV- Idle Air Control Valve .

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I've seen this symptom in various vehicles, but not temperature-dependent, where it was the IAC valve. I seafoamed my IAC valve in the past 6 months mainly as preventative maintenance, and it doesn't have this issue under normal temperatures, so I doubt this is the issue. –  R.. Jan 29 at 4:17
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