Supreme_One

Hello, I been having problems with my 98 civic ex automatic, recently I just changed the Idle air control valve cause it was giving me problems and the check engine was on, the code it was giving was the p0505 which is the IACV so I changed(bought it new) but now that I changed and cleaned the throttle body the check engine went back on(it was fine for two days) and it's giving me the same code! What could be wrong! Very confused need your help guys I will really appreciate it! Thanks!

ezone

P0505 is not defined as "replace the IAC valve".

The definition is "Idle control system malfunction".

The code is only a clue.
You now have to figure out the rest of the equation: "WHY does it idle wrong?"

Too high? Too low? Base idle set? Vacuum leaks? ECT wrong? TPS wrong? Idle learn?

A proper service manual should have handy troubleshooting info you can use to find the correct answers.

Supreme_One

Thanks for the answer back, first the car accelerated all the way to 2000 RPM and that's when the check engine went back on, but now that I drive it it drops to 300-500 RPM when I come to a full stop, then I put parking and it goes up to 1100 RPM

ezone

Wow, that's kinda all over the place. I'm not sure what to think here.
Is P0505 the only code?

Here's what the computer had to detect in order to set that code:
Malfunction Threshold
The actual idle speed is at least 200 rpm greater than or at least 100 rpm less than the target idle speed for at least 20 seconds.

The computer can't tell WHY the idle is wrong.
All it knows is that it couldn't control the speed for whatever reason.
Figuring out why is your job now.

You got some investigating to do.
I'm seriously suggesting a proper service manual because there are several possibilities.

Do you have a scanner available that can read live engine data for TPS and ECT?

What happens when you unplug the IAC valve?
(You said this has an automatic trans, right?)

What happens when you plug off the IAC passage hole in the throttle body opening with your finger?

Have you checked the base idle speed setting?



These are kind of rhetorical questions here. Each answer leads to a different next question. You really need a manual.

How does it run otherwise?
Power good and proper, or does it lack a bunch of power?

Travie_Vtec

I'm working on the same thing with my car.

Check for vac leaks.

Burp your coolant.

Clean/replace IACV (i have one for sale.. $30)

Change the TPS and/or ECT sensor

Change Throttle body gasket, IACV gasket, or intake manifold gasket.

Adjust Idle screw

Supreme_One

I have checked all that, but not the tps, does that have to anything to do with it? Btw thanks the answer back appreciate it

Travie_Vtec

no problem yeah, some times the tps will do it... so ive heard.

And have you removed your cruise control by any chance

johnattan_1984

Honda dealer tech here, I wanted to share something I came up with at the shop.
2003 Honda Civic Ex with high idle and P0505 behind the CEL. Coolant was good along with the readings. Idle was high and not surging. The code was set because the pcm could not control the idle. I found in the data list the power steering sensor in the high pressure line toggling on/off without me touching the steering wheel.
When I unplugged the sensor, the idle dropped to 720 and no DTC.
this is a confirmed fix
PCM looks for other sensor parameters to set idle.