At night, or after I've gone really fast on the freeway (100mph), my car fluctuates RPM's (like a vacuum leak) from 500 to 1500 rpm. but after the car has cooled down, or it's been a day, the car idles perfect. anyone have any idea why?

 

Yeah, I would like to know also. I have the same problem

 

I had that problem on my 93 SOHC VTEC but it did it all the time. Turned out to be a valve under the throttle body had threaded loose and it wouldn't allow the coolant to flow right confusing the ECU as to whether it was cold idle or hot so it just went with random idle. I honestly don't know what the valve is called but it's under the tb there is a coolant line that runs into it from the head & another that should come from the IAC valve. There are (2) 8mm phillips head screws that hold the cover plate onto it on the firewall side, the plastic valve inside is threaded and mine had spun loose.

 

fast idle valve?

 

No I think its the Idle Air control Valve

 

uhh, no. the iacv is on the back of the intake manifold.

 

could be iacv or fast idle under the tb...mine did that a lot before. then it stopped... then it just went bonkers and i decided to take it to honda. it was a bent threttle plate (either from a nitrous backfire i had or just tightening the throttle cable too tight:hammer

 

i don't think honda/acura would appreciate my car a whole lot

 

it's weird but my car did the same thing... and it would ONLY be after fast freeway runs (coming home from school on the turnpike). Then when i was rolling into a stop it would stick at 1500 then when i stopped completely... it would drop to normal. then sometimes dropping to like almost stalling... and this is in my 00 civic. weird... since then it has evolved into an iacv CEL and i took it to honda... they sad my throttle plate is bent... too lazy to change it.