I have a 93 Honda Civic SI 1.6L I think it is a B16 Engine. It has 235,240 miles. It has no modifications strictly stock/work car. I have noticed a decrease in the RPM's it happens not when I start the car up in the morning, or while I'm driving but after it has been running awhile or once it heats up. example: Starting it in the morning, is good 1300 rpm's, about 5min. of driving the car and stop at light or stop sign 1000 rpm's that good, but once I've been driving for about 10 min. when I stop at the light it slowly drops to around 500 rpm's. I tried turning off the car and turning it on same thing, but if I wait 45min to an hour it will be fine till the car warms up. But when it is at 500 rpm's sitting there and I turn on the AC or Heater it goes up to 1000 rpms and stays there like nothing is wrong. I need help, I know a little about cars but this I have no clue. Thank you for looking at my Topic. Any help is Greatly appreciated!!

 

unless it was swapped its not a b16 its a D16Y8 or Y7, and try cleaning your IACV(idle air contril valve) and see if that helps

 

Yea, check if it's B16 or D16.

 

It is a D16. I have had the IAC valve off and cleaned it real good but nothing changed. Thanks for responding!!

 

From what i can uderstand, the warmer it gets, the lower the idle gets? You could check the fast idle thermo valve (FITV) it is located under the throttle body. Take it apart and adjust the wax screw in it.