Hi all! Hope someone can help me through this confusing time

Car is a 98 Civic EX automatic & NYS inspection time is up, and I've had a P0505 for quite some time. Ultimately this was my fault as I broke a perfectly functioning IACV way back when, and then I replaced the entire TB due to another issue, and the replacement TB came with a bad IACV and triggered a P0505. I have just now ordered a brand new IACV, and after install, I had a huge idle bounce, so I tightened down the idle adjustment screw all the way and the idle was normal. No CEL.

I then realized later on I had unplugged the hose going from the charcoal EVAP cannister to the solenoid on top of the IM, and forgotten to plug it back in afterwards. So, I plugged it in. Started the car, 30 seconds later, my rapid idle surge and P0505 is back. At this point I'm completely confused, because introducing a vacuum leak to my car is correcting my problem.

Vac Leak = No CEL, 1000RPM idle, no jerking.
No Vac Leak = P0505, 1000-1300 rapid bouncing, jerking on light throttle.

Before I ordered the new IACV, I have tried "deleting" the IACV by using a cardboard blockoff plate and leaving the IACV plugged in, which yes brought a very low idle, but it was still triggering a P0505 after a few seconds of idle. So it seems that perhaps the IACV was never bad, but it was my lack of a vacuum leak was preventing the car from idling normal? How can I even begin to diagnose this issue? Should I loosen the idle screw and plug the hose back in?

Thanks for anyones help in advance. I'd greatly appreciate it!

 

Are all vacuum hoses connected correctly? The only vacuum hose not shown in the diagram is the PCV.

 

Yep looks about right to me.

In reference to this pic, I have the hose disconnected between the purge solenoid and the cannister.

 

Which end of the hose was disconnected - canister end or purge valve end?

 

Purge side

If I plug it in and eliminate the leak, this is where it triggers the P0505.

The electrical plug for the purge solenoid is still connected.

 

When the hose is disconnected from the purge valve and the idle is NOT surging, does the idle surging return when you use your finger to cover the purge valve nipple? Conversely, how is the idle speed affected when you cover the end of the hose with your finger?

 

Have not tried doing so. I am going to hope the CEL stays off tomorrow morning so I can get the actual inspection, then I’ll play around with it afterwards.

I will do this with something completely sealed so I can let it sit for a few minutes. Think I have some block off nipples somewhere.

 

If covering the purge valve nipple with your finger induces the surging, then the next step would be to unplug the hose from the other side of the purge valve to see whether the surging is again eliminated and, if so, whether covering the end of the disconnected hose with your finger causes the surging to return. In this way, you should begin to pinpoint the problem.

 

Soooo I’m warming up my car before work this morning and ablut 5-10 minutes in I hear the idle rapid bouncing... go outside aaaand P0505.

This is while that hose is still unplugged.

So, maybe the purge solenoid closed. I’m going to disconnect from the I/M and see what happens.

 

Hey,

So I took off the purge from the IM and then it idled wayyy too high, around 4k. Then I took a further inspection and it looks like I missed the hose for the cruise control vac... couldve sworn I saw it but it was infact, disconnected from the IM. I reconnected all the hoses, drove it for a bit, 30 or so miles no CEL and rolled in and passed inspection with flying colors. Still no CEL throughout the day.

I'm really in shock a missing vac line caused an idle system code, but it is what it is!

Thanks for your help regardless!!