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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?
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Jan 27, 2014 at 12:00

3 Answers 3

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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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  • @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
    Commented Jan 29, 2014 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
    Commented Feb 7, 2014 at 22:45
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This might be normal behavior. From the book Auto Fundamentals, pg. 146-147 in the section "Cold Start Aids":

During cold startup on some systems, an auxiliary air regulator admits additional air into the intake manifold to increase the idle speed. The auxiliary air regulator is controlled by a thermostatic switch located in the engine water jacket. At a coolant temperature of -13 F ( -25 C ), the thermostatic switch allows the maximum airflow through the air regulator. As the coolant warms up, the air regulator flow is gradually decreased. When the temperature reaches 140 F ( 60 C ), airflow is completely shut off.

That would probably be in fuel injected vehicles.

It also discusses such mechanisms in regards to carburetor based intake systems in which idle is increased while the engine is warming up ( Fast Idle Cam pg. 179 ):

When the engine is warming up, it is necessary to increase the idle speed to prevent stalling... When the choke is on, the fast idle cam swings out in front of the idle speed adjustment screw. This holds the throttle valve partially open to speed up the engine. As the choke opens, the fast idle cam swings down and the throttle returns to its regular hot idle position.

So basically, it sounds like whatever type of intake system you have, it's going to be set up to have a high idle when the engine is very cold, and the idle will slowly return to normal as the engine heats up.

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  • Thanks. My car's intake system does not appear to be that advanced (separate valves/regulators), but it seems plausible the ECU could be controlling the IAC valve to get this behavior intentionally.
    – R.. GitHub STOP HELPING ICE
    Commented Mar 14, 2015 at 21:21
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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.. GitHub STOP HELPING ICE
    Commented Jan 29, 2014 at 4:17

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