Hello,

I have been trying to sort out some issues with my Civic
1999 Civic Si (Canadian so EX in US).

My current problem is with the idle. The idle hangs at about 1100 rpm after letting off the throttle. The idle will then rise to 1400 - 1500 rpm and slowly drop down to 1100 rpm.

Pretty much every other time I drive the vehicle, the check engine light comes on and then the idle sits about 1300 rpm and sometimes fluctuates up and down. Once the car throws the check engine light, there is a hesitation just when getting on the gas. So touching the gas pedal 1% - 15% the car feels like it chokes.


I checked the OBDII Code and it always shows 3 codes, all of them P0505.

I know this is "Idle System Malfunction".

If I unplug the IACV electrical connector the engine idle falls down to 7-800 rpm but there is still the stumble when getting on the gas.

If I reset the OBD Codes, upon restart, the car drives fine except for the above mentioned idle fluctuation / high idle issue.

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Here is what I have checked & done so far:

- Distributor Cap & Rotor (done recently due to heave stumbling on accel.)
- TPS (replaced while trying to sort out above stumbling problem
- Removed & cleaned IACV
- Remove Throttle body, cleaned reinstalled
- Fuel Filter (Changed while sorting out stumbling issue above)
- Primary O2 Sensor

Note on the IACV Cleaning:
The IACV on my specific engine looks exactly like the one in the following thread https://honda-tech.com/forums/acura-integra-6/%2A%2A%2Ahow-clean%2A%2A%2A-your-iacv-k-idle-air-control-valve-1575913/

As it only has 2 small holes and can not be taken apart my cleaning of it may not have been good enough.

Note on TPS:
I replaced the TPS and tried to calibrate it, I got it set to 0.5V on closed Throtte but was only hitting about 3.75 on WOT instead of 4.5V?

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I know there may be more than one culprit for what's going on. I still need to check for Vacuum leaks, which are the common tubes to check?

What else could I try?

Thanks for any info.

 

Be sure to check for vacuum leaks. This can cause the P0505 CEL to turn on and as well to create the issues you are having.

Here is the vacuum diagram for your engine.