I am having a idle issue with a 98 civic dx, when I first start the car cold it is normal. Once it gets to a normal operating
temp it begins to rev up and then down. When I turn on the AC it stops and is normal again. I purchased a code reader and
found PO113 and PO505 as a trouble code. I ahve replaced the IAT sensor and is still having the same problem. The PO505 code
sugess the IAC valve system fault. Does anyone know where that valve is located on a 98 civic 1.6 ltr or have any other suggestions
that may solve the problem. This is only a idle issue once you accelerate it stops until you get to a stop

 

i think it may be your throttle position censor TPS, my car does that if its in park or neuatral, i hit the gas rev up to like 6 k, then when the needle drops down it bounces between like 500-1200 rpms. So check that TPS.