Recently purchased 98 civic ex lsvtec. So my car idles extremely low sometimes to the point where it shuts off on its own, around 500rpm to be exact, also lets say im cruising and i push the clutch in and the rpm drops all the way down to 0 car looks like its about to shut iff but it doesnt and the rpm goes back up to around 500rpm right away. Also im not sure if this has anything to do with it but I hear a hissing sound coming from the passenger side under the dash. Could it be a bad heater core? Maybe air in the cooling system causing the low idle? I want to check it out tomorrow i havent really had time to wrench on it yet. Any other suggestions on what might be causing my really low idle?

 

Sounds like a vacuum leak

 

inspect for vacuum leaks first
and check the intake tubing itself for leaks which could be throwing sensors off, then post back if everything checks out

most likely a vacuum leak/something just got disconnected



side note: metal short-ram intake tubes normally have a kind of whistling sound; and you may just need to adjust an idle screw if there is one, or adjust the timing depending if it's been running like that since the swap (if previous owner just ran it like that)

 

Interesting, I thought vacuum leaks caused your idle to go up?

 

The noise could be the refrigerant flowing in the A/C, which if it is still getting cold, is normal. There are no vacuum lines inside the cabin.

 

the car never made this noise before, ac was stripped out by the previous owner about a year ago

 

Ok, this is obviously an old post, but by chance did you ever figure out what this hissing noise was? I have the same sound coming from under the dash of my 98 ex that the previous owner y7 swapped. Also have had some funny idling issues. The car used to idle high on cold start until it got warm, higher than I'd expect it to. Then the other day it threw a CEL that I suspected to be the TPS because it began having a surging idle and the throttle got real jerky. After driving it for a few miles like that the idling went back to normal and the throttle jerk went away, except now it idles relatively low, about 500 rpm. I'm just really curious to what that hissing sound under the dash is, sounds just like a vacuum leak but I've been told that there are no vacuum lines inside the cab. Any help is appreciated.