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My 2010 1.8L engine on Civic idles at about 625RPM but and has a mild rough idle feel at this speed. If I gently increase the RPM to 630ish the rough idle goes away. Is this some sort of resonance issue with this 1.8L engine or???

 
Rep Power: 367

too low. did you disconnect battery? if so, it might still going through an idle relearn

 
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I did replace the battery and did the following idle relearn: Run engine at 3000 RPM for two fan cycles the let idle at rest position on acc pedal for 10 mins. Have had this low idle RPM issue for some time and still persists after several relearns. I'll try an EGR clean and throttle bottle clean next but just curious as to the 625 RPM idle speed being the norm for this 1.8L engine and how increasing it by 5 or so RPM eliminates the rough idle/vibration sensation

 
Rep Power: 224

Are you reading the engine speed with a scanner? There's no way the cluster is accurate enough to tell that small difference.

Here is the idle inspection procedure. Skip the part about connecting HDS. Instead, connect a scanner or something that lets you read engine speed from the ECU.
https://civic.hondafitjazz.com/A00/H...000MAAT00.html

 
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I don't have an accurate way to get a true RPM reading but neverless when I very slightly depress the accelerator and in turn very slightly increase RPM's the rough idle/vibration symptom disappears and I'm curious if this is related to some sort of resonance effect

 
Rep Power: 367

lots of OBD readers that connect to your smartphone using Bluetooth in Amazon, very cheap
then download app Torque to display info