I have been having trouble with a hunting idle problem on my modified 406 for some time and I have no idea how to cure the problem. My engine is a 406 with a ZZ-409 cam and Superram intake, and 52MM throttle body. The idle will surge between 600RPM and 1200 RPM only when the car is warm and in park. If the car is in drive the idle is stable. I originally thought the surging was being caused by my varying timing over the idle RPM so I burned a new chip that fixed the idle timing at 25 degrees. The car still surges and when I scan the car at idle, the timing is steady at 25 degrees. The only item that is varying is the IAC position and that bounces from a count of 10 to as high as 80 when this surging is taking place. If I put the car in gear the idle will stablize and the IAC count will also hold steady at 30. It appears to me that the IAC is too slow to respond to the idle changes and the IAC keeps chasing the correct idle RPM. The idle RPM is set for 850 RPM when all of this is taking place. How do I fix this problem?

 

set minimum air to match des. idle speed in Prom and try to adjust TPS.

I had similar problem before and that helped me out, also search for a post I wrote about my Idle tuning......

/N.

 

Did the search for you bjankuski as I was also interested:-

Idle tuning with cam, results.
https://www.thirdgen.org/techbb2/sho...ht=idle+tuning

Idle problems $6E
https://www.thirdgen.org/techbb2/sho...ht=idle+tuning

Dfi Help
https://www.thirdgen.org/techbb2/sho...ht=idle+tuning

Will I go lean with a 383?
https://www.thirdgen.org/techbb2/sho...ht=idle+tuning

The first of the listed threads is one you really want to read.

 

Here is the list..

vacuum leak
old IAC
throttle blades not opened enough
bogus "idle speed error" corrections


Let me know if you need help.

 

When you say bogus idle speed error corrections what are you refering to. What should the idle speed error corrections be set to?

There are no vacuum leaks.

The min throttle blade opening is set correctly

The IAC is not new but it is clean and it appers to be working correctly.

 

Email me one of your log files and I'll show you, if it's picking up an error..

How did you do that?

 

another thing is that you can clean the IAC a million times and it will not matter if it's worn out.

 

I've had three different camshafts, two different intakes and two different brands of 58mm throttle bodies and been MAF and SD on my 383 (see below) since I built the engine three years ago. I've NEVER had trouble with idle. But..., read the rest of this first.

I set my idle air with my laptop hooked up. I've had it set from 650 to 800 depending on which cam I had at the time. I just watch the IAC counts, adjust the blades until it reads about 20. I let the ECM do it's idle thing with the timing but minimum is set at 20° in my bin. I tune the VE to get about 14:1 and it smells rich with the CC306 but when I had the smaller camshafts in there I got a really decent idle with not much smell and no surging. I've got a pretty radical cam in it now and it still idles rock solid.

Now, for the part you may be interested in, or it may not be your problem at all: My pal spent three years fighting the surging idle thing with his 355 TPI in his Monte Carlo and one day we fixed it in about 5 minutes. He had NO passage between his throttle body and the plenum for the idle air to go through because it had a ROCK in it from having it tumble cleaned.

Now, you may not have a rock in there but, there are throttle bodies and plenums that do not match for TPI cars. Basically, some had a hole in the center of the plenum and some didn't. Some throttle bodies had a groove around the ports for the idle air to get into the plenum and some didn't.

Make sure you have no other way for air to enter the engine, PCV valve that doesn't work right, brake booster leaking, vacuum leak you missed, etc. Now, I'm just pulling out all my wild guesses here but if the IAC counts are raising and lowering then it should be able to idle stable... Unless, your bin is screwed up, IAC valve is too slow, timing is too low for setup, injector duty cycle way out of range for proper idle, idle blade not set even close to right, not enough stall converter for desired idle RPM (not likely).

I'm done guessing... hope you find an answer in here somewhere.
I've probably forgot another dozen reasons but I'm tired of typing and can't think of any others right now.