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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?

R.. GitHub STOP HELPING ICE
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3 Answers3

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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.

Robert S. Barnes
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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.

mac
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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 .

jensgram
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