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Erratic idle after replacing the IAC valve

Tiny
98HONDCIVIC
  • MEMBER
  • 1998 HONDA CIVIC
  • 1.6L
  • 4 CYL
  • 2WD
  • MANUAL
  • 217,000 MILES
After replacing IAC valve, vehicle now idle up and down on its own. Vacuum leak?
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Friday, March 19th, 2021 AT 10:22 AM

3 Replies

Tiny
KASEKENNY
  • MECHANIC
  • 18,907 POSTS
More then likely this is a vacuum leak. What is happening there is air coming in which the PCM is trying to match the fuel to it and the RPM raises, then it drops and raises again.

Use this guide to figure out where it is:

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

On most vehicles there is a learn procedure that needs to be done after the replacement of this but I don't see one on your vehicle. Which means you should not need to perform this but if you find no leak you can try and perform an ECM reset which will force the ECM to relearn the throttle an all other monitors. It is no different then shutting off your phone and rebooting it when it locks up.

See below for this detail. Let me know what you find. Thanks
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Saturday, March 20th, 2021 AT 8:40 AM
Tiny
98HONDCIVIC
  • MEMBER
  • 2 POSTS
Thank you for the input. However I later found that when putting the throttle body back on (I had to take it off to access IAC valve) that I used the wrong gasket. I replaced the gasket with the correct one and the idle surging stopped. But now the car still idles very low. Around 400 RPMs and sometimes very high around 2000 RPMs. But it no longer bounces around like it did. I replaced the MAP sensor but it didn’t do much. What should I do to get a solid steady idle? Could it still be a vacuum leak? Or some other sensor?
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Saturday, March 20th, 2021 AT 8:55 PM
Tiny
KASEKENNY
  • MECHANIC
  • 18,907 POSTS
Yes. I would treat this about the same. Run through the vacuum leak guide below to see if you can find the issue.

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge
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Sunday, March 21st, 2021 AT 4:03 PM

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