Here's what fixed the idle problem on my corolla

[badbrakes]

I have been having an idle problem on my car and I finally managed to fix the thing. Hopefully this can be of use to someone out there... once I managed to find the problem, the fix was fairly easy.
This thread will be of interest to you if you're an occasional DIYer... if you're a pro, this will be obvious stuff to you.

Car:
96 corolla 1.6
standard
210K kms

Problem:
Idle erratically - between 600 to 700 rpms
Idle drops when depressing brake pedal at a red light
Idle erratic to the point where the car would almost stall

Cause of the problem:
Very badly gummed up intake manifold.

Here's how I fixed the problem.
I started by dismantling the throttle body and cleaned it up. Also dismantled the IAC valve which is attached to the throttle body. I cleaned it up nicely being careful not to spray any carb cleaner onto the IACV motor itself. However these two parts weren't too badly carbonized.
When I got the surprise though is when I looked through the hole (from where the TB was and into the intake.... oooohhhhh not pretty... I was fairly confident I had then found the problem. I removed the air intake plenum (cover for the intake manifold) and then I got a good look. It was near totally gummed up... the air chambers and passages to EGR valve, brake booster, etc were blocked. No wonder the thing wasn't idling nicely... I was surprised the car could run at all. So I cleaned up everything with carb cleaner, tootbrush, qtips until all the gui stuff was gone. While at it, I also dismantled the EGR and cleaned it up. I bought from the dealer 3 gaskets (plenum to intake, intake to TB and EGR gasket)... the gaskets are about 6$ a piece and there's no point reinstalling the old ones. About 20$ of gaskets + 10$ of carb cleaner (not counting the toothbrush cause I will reuse it... no not true) and the car now runs like absolutely new. Problem is gone 100%
All in all, easy job.... just label the heck out of every hoses and connectors and take your time. I am no mechanic, but the way that thing was gummed up, I can't imagine that any chemical just sprayed into the TB would have rectified the problem... had to open the thing up.

[badbrakes]

Forgot to mention 2 things in the above.
1) I also replaced the Idle Air Control Valve to Throttle Body gasket... the IACV is coolant cooled so if u open it up, do replace the gasket - otherwise you're running the risk of coolant ending up in your throttle body... which probably wouldn't be a good thing.
2) Make sure you torque everything to spec and in star pattern.

[roadrunner2]

Good to know, Badbrakes.

Thanks for posting that.

[Brian R.]

Yes, thanks for posting your cure. Now, the $10,000 question: What caused the deposits in the intake?

[badbrakes]

Hey Brian R.

Right on with your question. I think it came from the 2 hoses that connect from top of valve cover to the intake manifold plenum. So I suppose over time, small mists of oil vapor accumulates in the intake air chambers. That may answer where it came from... as to the more important why, I have no idea... is it normal or poor design or something else broken... just don't know. I'm kind of taking the approach that if it took 210K to gum up that badly and I've just cleaned it up thoroughly, it may take at least another 100K to get as bad... so I would just add this item to regular maintenance and do while doing timing belt type of thing. HOWEVER, if someone has the answer as to why the gumming happens, I would sure like to know....

[famlycar]

I'm a little confused but doesn't sea foam or techron gas prevent all that poor idling stuff and all? My corolla is approaching 200k but it still runs great with excellent mpg.

Here in California it doesn't rain too often but I keep my engine clean at every oil change by wiping down with baby wipes (I've never used water or chemicals) all the hoses, wires, engine parts,....everywhere that's reachable but I've never actually dismantled anything as you have described. But if I ever have to, then your info will help.

A carload of thanks for your help and any other advice in future.