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When I start the engine cold in my '93 Honda Accord, it will rev up to 2000rpm and slowly go down to 800 rpm as the engine warms up (normal behavior), unless I do one of the following during this process:

  • Touch my gas pedal
  • Move my steering wheel (power steering)
  • Sometimes without a stimulus
  • When jumping a friend's car, as he connected the jumper cables to his car, my engine immediately starting doing this:

The revs will go up to 2000rpm then drop down to 1000rpm and cycle every second! My engine will still warm up and the revs will drop as normal, but they will still cycle to 1000rpm (up and down, up and down...) until the engine is warm (idles at 800rpm). I often get weird looks from people as I'm driving around, since it must sound like I'm the one reving my engine.

I took the car to a mechanic but they said it could be anything going wrong, since the computer is not throwing an error. I'm not sure if this is related, but in the dead of winter when it's <10 F, my temperature gauge will barely go over the cold mark, even after hours on the highway (my interior blowers will pump out some heat). This behavior lessens as the ambient temperature goes above 40 F or so.

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I had something that is somewhat the same. My cold air intake sensor went bad on my car and (this would happen no matter the temp) my car would rev from 800 to 2000. A good mechanic should be able to hook your car up to a 'real' computer, a computer that actually shows all the parameters of the car (cfm air intake, engine timing, fuel usage, etc...), not just if a code is being thrown. –  Patrick Mar 24 '11 at 15:21
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4 Answers

up vote 2 down vote accepted

I had a high mileage '94 Accord that would do the same thing (irratic idle while warming up) in cold weather. I would just use the gas pedal to keep the revs up until warm when I could.

I would check the Idle Air Control Valve (IACV), having it cleaned may resolve your issue. Sounds like there is also a fast idle valve that you can adjust. Take a look: http://www.honda-tech.com/showthread.php?t=1704468

Good luck! -Ryan

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I finally got around to it and cleaned out my FITV, on the right, circled in yellow below: !FITV and IACV This video was helpful. I basically took the FITV all the way apart as he does and cleaned it with carb cleaner. The "plunger" was seized up, thus preventing the throttling function from working. Works great now! –  uranther Dec 30 '11 at 18:49
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Perhaps you have a bad thermostat and/or a bad coolant temperature sensor. A bad thermostat would prevent the engine from warming up, causing the engine management computer to use a too-high idle speed. Incorrect temperature sensor information could cause the computer to pick the wrong idle speed as well.

Perhaps the temperature issue is unrelated, in which case you could have a bad idle air control valve (assuming this car has one), which would provide an uneven flow of air and therefore erratic engine speed when stopped.

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2nd issue first.

The cold engine & poor cabin heating can only be caused by one thing: a bad thermostat. In this particular car, it comes as part of large plastic housing that also contains the thermostat. This should fix the minor issue of the car not heating properly, but will do nothing to solve the idle issue, which is a more complex problelm to diagnose.

Thermostat & housing

enter image description here

The idle issue could be caused by many things:

Coolant temp sensors reading cold then hot:

Coolant Temp Sensors (NOTE: they are different) below

The coolant temp is located under the distributor. It has 2. One with a single red wire and the second with 2 wires green/yellow, blue/yellow

enter image description here enter image description here

Air Charge Temp Sensor (ACT) below

enter image description here

Possible vacuum leak

Bad ECU

Also, make sure your cooland level is topped up.

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Can I ask a mechanic to test these sensors? And then I could replace one if one (both?) is bad. –  uranther Mar 27 '11 at 0:57
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i had the same loping idle and after cleaning the idle air control and fast idle thermo valve, located on the front of the throttle body plenum above intake manifold, like all the forums said to do, i still had the annoying idle. so next i managed to source the problem which was a leaking intake manifold gasket!! with the engine running, spray wd-40 along the gasket surface between backside of head and intake manifold. if you have a leak the wd-40 will get sucked thru where the gasket is supposed to be sealing. i did the repair myself and it only cost under 80$ for the gaskets/ o-rings ,etc. took a few hrs and at the same time cleaned all the carbon out the egr ports and plenum, put it all back together and it purrs like a kitten now, and this is a 91 accord with 116k on it. my 93 accord has 206k and still running strong!!!! hope this helped anyone looking. honda #1

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