Rough Intermittent Idle After Warmed Up @ Stop - 500-600 RPM

[NPVibe]

2003 Vibe, 1.8L, 5 speed, 96k miles. Recently replaced cat after getting ongoing low bank code, etc. Replaced cat and code went away, improved idle slightly. 5k miles ago I replaced the spark plugs (but not the coil packs) (plugs were ashen, etc., car ran better after that, some power came back).. Heres the thing:

Once car is warmed up, when I come to a stop and put it in neutral, idle will sometimes stay at 750, all good, or will dip down to 500, idling rough.. If you watch the tach: sometimes it will be idling fine at 750 and then will rev up to 1000rpm on its own for a second and back down to 750.. I checked for vacuum leaks and there doesn't seem to be any. So, here is where I'm at:

Wife's car.. We waited too long to do some basic maintenance and then did it all at once: Brakes/Rotors(cuz we waited too long), Tires, Shocks/struts & then the clutch started slipping so we replaced clutch assembly and flywheel. Grand total - $2300. So, I'm not too keen on spending more money on this heap right now. Its running splendidly except for the random idle issue. I have seen this mentioned in various threads but the issue seems case specific and annoyingly, could be a number of things. SO, being that my automotive knowhow/tools etc. is around basic to intermeidate, what can I do before bringing it to a shop.

Replace PCV valve? Clean MAF sensor? How complex is the throttle body cleaning? Any other ideas.

Thank you and much appreciated. I have the Chilton manual for the car as well so I can def do some basic troubleshooting stuff before paying somebody $80 an hour to 'troubleshoot' for me.

THANKS

[zyzyix]

Have you checked Youtube. i think there was a video similar to your issues, but it happened when the car was cold.

[dgekiere]

If you had lean codes before then the cat shouldnt have fixed the lean code. The 1.8l had a bad problem with the intake gasket cracking and causing lean codes. The gasket will also cause rough idle. The gasket is not expensive and is easy to change. I have xone a few myself for other vibe owners. The parts are only about 10 to 15 dollars and only takes about 30 minutes to do.

[sideshowalan]

I'm gonna second the intake manifold gasket. Get the replacement from a toyota dealer as they have been redesigned.

A leak there would cause idle problems and the factory ones were known to have problemes over time.

[NPVibe]

Think I may have fixed it, or at least, improved it. Since this forum doesnt allow you to embed pictures w/out using a URL, and since Ive forgotten my Flickr login info (never use it), pictures I took won't be attached.

Went and got a can of CRC Throttle Body Cleaner today. Started by pulling off the top of the airbox and disconnecting the hose from the throttle body. Just one clamp, phillips screwdriver, easy. Pulled that aside and had a look. The inside of the TB was filthy and lined with caked on carbon buildup. I sprayed liberally and with a toothbrush and rag, gave it a thorough cleaning. I then removed the MAF sensor. Disconnected the plug.. And extremely carefully, used a cotton swab and distilled water and removed the caked on black soot on the tiny little sensor bead.. This is risky because its really easy to damage. It was ashen and black so I cleaned and reinstalled. I put everything back together and started the car no problem. Rev'd the crap out of it and then went for a hard drive, pushing 4k RPMs+.. After it was fully warmed up, I came to a stop and let it sit and it was locked in at 750rpm. No dipping lower than that and idling pretty steady.

Im going to keep an eye on it. Good be the gasket, which Ill do next, but this is a good start. Regardless, it needed to be done. I don't know how much someone would have charged me to do this but anyone keeping cars past warranty should really know how to do preventative and basic maintenance like that. Saves a lot of money.

** My only concern is the following and maybe someone can shed some light on this:

When I come to a complete stop at a light or sign and put it in neutral, it now idles steadily right at 750. But I noticed that if I am moving and I throw it into neutral, that the tach drops down to about 1200rpms, but no lower. Shouldn't it always drop down to 750 or so when not in gear?

[Harojake]

I was experiencing this same thing on my vibe. When I went to replace the intake gasket I noticed a hose going from the intake to the air box was disconnected. I reconnected the hose and the rough idle was gone. Hope this my help.