I know, I know, I'm sure you guys have gotten stuff like this before, but I simply cannot find any other threads with this same issue and it's driving me absolutely nuts.

When the engine is cold (a minute or so after startup in the morning) and while coasting in D4 at around 20-30 mph on a slight downhill slope, the idle will bounce up and down between about 800-1000rpm rapidly for about 5 seconds and then stop. Doesn't happen when it's all warmed up. This isn't a typical idle issue involving the IACV or anything of that nature, as the IACV works fine and the car idles great in park and runs beautifully. All vacuum hose are plugged in correctly and are in good condition, upper/lower manifold gaskets and head gasket are brand new and properly tightened down, cooling system purged of air, new ECT sensor and sender, valve lash properly adjusted, timing all lined up. I could list other things that I can't think of off the top of my head, but you get the gist of it. The only existing/recurring code is the P0420.

Any pointers are appreciated! I REALLY want to get rid of this irritating problem!

 

If you haven't cleaned the throttle body that would be my suggestion. Watch Scotty Kilmer's Youtube video "How to fix a car that idles poorly" He has a lot of other good DIY videos for car repair. The code is for the catalytic converter so you may want to check the back pressure on the cat. Scotty has a video for that too. Overall it doesn't sound like much of a problem so I wouldn't worry about it too much unless it keeps getting worse.

 

I'll have to check them out. I cleaned the TB and the intake manifold REALLY GOOD the first time I replaced the head gasket nearly six months ago. You don't suppose any of the sensors could have been messed up by cleaning the TB, could they have? I used Easy-Off oven cleaner to remove the years of carbon and gunk. Could it also be that the EGR valve/passages need to be cleaned?

I read the live vehicle data on a scan tool while trying to figure out the P0420 code, and the short and long term fuel trims were both consistently below the factory spec of +1. Short term was around -4 or -5 and the long term was about the same. So I'm led to believe that the code is false, especially as the code never came back until I redid the IM gasket to seal off a bad vacuum leak (wrong gasket caused a P0505). So maybe the vacuum leak sort of "fixed" the overly rich mixture until I redid it, at which time the P0420 came back again. I've read from some sources that O2 sensors on their way out could cause the issue, especially considering that the engine is running so darn rich.

Oh, and I tried putting the transmission in neutral while it was happening, but it didn't change anything. Thoughts?

 

The EGR passages get clogged and cause air richness problems. Not sure why it would only do it right after you start but it can't hurt to clean them. Also a throttle position sensor that is starting to go might cause your problem IMHO. The potentials unfortunately are many and since it is such a small problem it is even harder. I've got a minor vibration problem in mine that I can't seem to solve and it drives me crazy so I understand your pain. Unfortunately I am out of ideas for you and for me.

 

When you are "coasting" I'll assume your foot is off the gas, that would trigger the IAC valve since the throttle plate is closed (or supposed to be closed) That being said, I would clean the throttle body and remove and inspect the IAC valve AND the connector.

EGR issues won't cause your symptoms, and "air richness", that's a new one for me.

 

This may be related to your "vibration" issue;

"Balancer Shaft Buzz or Rattle Is Normal Currently Applies To: 03-07 Accord L4, 02-09 CR-V, and 03-11 Element EDITORS NOTE : This article replaces the one issued in June 2004. Got a buzz or rattle when the vehicle is sitting in Park or Neutral and the engine is winding down from
3,000 to 2,000 rpm? What you're hearing is just a harmonic resonation coming from the oil pump and balancer shaft assemblies, and its a normal characteristic of the vehicle. Don't bother replacing engine components; it won't change a thing.

 

Where are the EGR passages located, and whould it be better to clean or replace the EGR valve/sensor?

Yes, foot is off the gas. I'll look at the IACV and TB again but I cleaned them really good when I did the head gasket almost six months ago. The engine idles beautifully when the vehicle is in neutral or park. I tried putting in a replacement IACV and flashed the ECU but it made the idle go totally BONKERS. All kinds of bouncing like it was a huge vacuum leak. Defective unit, maybe?

Also, I think the pulsing stops whenever I put the tranny in neutral. I can't remember for sure, so I'll have to try to get it to do it again and see what happens.

Another random thought: could the absence of the plug over the idle air adjusting screw cause a slight vacuum leak? I know it's kinda out there, but just a thought, maybe?

 

What do you mean when you say you "flashed the ECU" after replacing the IACV? Alternate maps for fuel or spark?

 

I had the battery disconnected the entire time I had the vehicle apart (nearly a week). What are alternate maps for fuel and spark?

I confirmed that the bouncing does indeed stop when I put the transmission in neutral. Torque converter acting wonky? Should I test the clutch solenoid?

 

Update: I pulled the codes again and now, along with the P0420, there's a P0705 and a P0706. I don't understand the service manual's instructions on how to do the continuity test. Help please!