Hello All, just wanted to share something I discovered. I've been chasing down a rough idle / slight misfire issue I've been having on a my car for a while. About 2 years ago, I noticed a slight misfire at idle after I've driven the car for a while. At some point after this, the misfire had gotten bad enough where I started getting misfire codes.

First, I cleaned out the IACV and the MAF sensor. This caused the idle to get better but it was still bad enough to get the misfire codes. Then, I replaced the spark plugs (NGK), and then I replaced all of the coil packs (Denso). This caused the misfire codes to stop, but I still noticed an off-beat slightly rough idle.

I continued to look into, but it sort of fell by the wayside. Then, a few months later, the idle had gotten bad again, to the point where it felt like the engine was going to choke and die when I was sitting at a stop light. I looked at my engine again and I noticed on my valve cover there was a small seepage of oil around the PCV. I replaced this with the OEM part from Honda, and the idle is now nice and smooth. In fact, the car has never been this smooth at idle during my ownership (I've had the car since July 2020).

So based on my anecdotal experience, it seems that a bad PCV can cause an irratic idle when the engine is at temp and the idle is below 1000rpm. Hopefully, this helps others who are trying to figure this out.

 

This is called a vacuum leak. Classic symptoms. Rough idle, wants to die, etc. Virtually disappears once on throttle, as the vacuum leak becomes such a small percentage of total airflow.

What is happening is the fuel mixture is wrong. An amount of air is entering the engine unmetered. So ecu doesn't know to add fuel for that air.

This happens when pcv is jammed ooen. Also happens when a vacuum hose is disconnected, misconnected, or old and cracked.

 

Manifold vacuum or crankcase pressure is pulling the valve open. The valve closes if there's positive manifold pressure.

OP's PCV seemed to be jammed closed.

Question is...why did the PCV go bad?

 

​Age? All I know is, my idle is really good now.

​​​​​​As for vacuum leaks, I imagine as our cars get older all of those rubber vacuum lines may start to get old, brittle and begin leaking. I think I may attempt to replace them, just in case Honda stops producing them, if they haven't already.

 

When replacing vacuum hoses, always do them one at a time, avoid mixing up what goes where.

So far, none of m vacuum hoses is showing signs of age. '06, 165k miles

 

Thanks for the advice. Mine is similarly aged, 05 141k miles. I was mostly thinking out loud, not sure how vigorously I'm gonna pursue replacing them. The thought process is to avoid the scenario where by the time my rubber lines do need replacing, they would become unobtainable.

I don't have a smoke machine or anything to help me check my lines for leaks, but like you, mine look like they're in good condition.

 

You can buy rolls of hose that will work perfectly for the PCV system. Needs to be oil resistant but you easily find oil/fuel resistant hose of the correct size at a parts store or online, so no need to worry about proactively purchasing Honda ones in case they go out of stock. If they fail later it is an easy thing to fix.

 

The Honda hose fits so perfectly though, it's just the right size and it lasts over a decade easily.

 

Welp, my misfire is back. Sheesh, sorry all, I guess my anecdotal evidence is no longer valid. Car ran great for almost a whole week, but now I'm back at the drawing board. I'm getting multiple misfires codes after a full week of smooth sailing with my new PCV. I think the next thing I'll do is check valve adjustment, then after that I'll look into either replacing or refurbishing fuel injectors. Tracking down misfires is not fun.

If anyone here is a mod, please feel free to change the title of this thread as to not confuse others in the future. Thanks and sorry for the misinformation.

 

I bet it's the valve lash.