What would cause a 400rpm surge?
My IAC is not the reason. It's staying at a fixed value during the surges.

 

VE table, MAF reading, or timing variation. Flatten both the timing and the VE in the cells surronding the RPM/KPA that your engine is idling at. Also zero out any idle speed spark correction tables in effect.

 

...double check for small vacuum leaks...

 

I know you say your IAC is not the problem. But why?

I have found that if you increase the commanded idle speed, that you usually need to increase the minimum RPM of the IAC from the stock value. I find adjusting the IAC to about 50-100 rpm less than your commanded idle speed usually does the trick.

 

Hmmm.
During the surge I have an IAC position of 1-47. Wherever it is it doesn't move in accordance with the surge. This tells me that the IAC isn't causing the surge with its opening and closing.
I've gone through and haven't found any vacuum leaks.
I've even run the car on a completely stock bin and the surge remained.
I'll try changing the values for the IAC.
What sucks though, in order for the car to run with the IAC disconnected and fully closed, I need to have the idle screw completely screwed in (open).

I've also noticed that this surge happens while on light throttle, cruising DOWN a hill, and sometimes just cruising.

 

Please explain a little more on what table...I'm using TunerPro RT and sometimes experience the same thing.

 

Based on your comment, it very well could be a vacuum leak OR the commanded idle speed is too low.

Set the commaned idle speed higher and re-adjust the IAC as per my PM.

Then, if still exists (or just at certain points), we can look at those. Once eliminated (say it is a vacuum leak), you will need to re-adjust the IAC again.

Also, are you invoking Highway Mode? If so, disable it while working on this. Lastly, if you have an EGR, try turning it off. A weak/bad EGR can really cause problems too. If you turn off the EGR and the car runs great, then you found it.

Last major source of "surging" is the pick-up coil.

The good thing about eprom burning, you don't HAVE to buy an EGR to test it anymore. Just turn it off, and see if it goes away.

You now have a few things to try. Good luck.
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You need to adjust the PHYSICAL IAC. The stock setup, is to set the IAC to 450 rpm (which is 150 rpm less than the stock 600 rpm minimum speed).

I have found that when you bump up the idle speed in the eprom, you need to set the IAC to around 100 rpm less than the desired idles speed. If not, the engine will "hunt & surge" (assuming no mechanical/electrical issues). Many guys seem to overlook this.

 

Not much of an update but here's what I have.
SAUJPV3 8D 730
Tunerpro RT and Ostrich

Yes i'm using highway mode. Apparently EGR is already disabled in V3 so I guess that's out but it is still connected.
My commanded idle speed was 800 and now 900 + the 100 that the other table adds.
I've set the IAC according to the tech article with this commanded idle speed several times with no change.
In order for my car to actually run with the IAC closed, I have to have the idle screw completely open. Is this a problem or at least an indication of a problem elsewhere?
I've looked for vacuum leaks using propane and then MAPP. No luck.
Surging is slight in OL. Surging gets more pronounced in CL.
I've tried tightening up the O2 swing points and lowering the threshold. It helped a bit. My surge dropped to about 250 rpm. I wasn't comfortable leaving it there so I changed it back for now.
Seems that during these surges my BLM cells change from 9 to 5. Spark advance related to TDC also moves 6 degrees. I must be missing something there because at those rpm's and kpa's I had already made them identical.

I'll disable highway mode and start pinching vacuum lines. Maybe something will happen. PU coil? Never woulda though of that.

BTW - If I put the car in gear the surging settles down and then disappears. As soon as I start moving though the surging comes back. Again it surges at idle and light throttle/load. Not nearly as much though while under light throttle/load.

 

Are you sure the IAC is closed all the way? If you can VERYIFY it's 100% closed, then unplug it. If it's an IAC issue you will know right then. Vaccum leaks can be hard to find. I've been in the automotive industry for over 10 years. There are still vacuum related problems that thows me off. Just like the ford explorer I repaired recently. It would only have a vacuum leak when max ac was on. The vacuum leak could not be heard, I had to trace it down. Not saying that is your problem. Just pointing out that while it can be redundant, it's good to repeatedly go over things.

Two other things come to mind. How old is the O2 sensor? If it's older than a couple of years, I would toss it. If you don't have a cat, you don't need the O2 anyways.

TIMING! Yes, timing fluctuations OR too much can create idle problem....

Most of the time that I can't get the idle to stop surging, I'll adjust the throttle blades while the engine is running. It's (one of ) the best way to verify minimum air rate is enough.

 

Well according to my datalogs the IAC is 100% closed. I've gone over so much and can't find a leak. I've completely flattened my spark in the rpm and kpa ranges. It's still there. I have found that it seems to be more temperature related. OL and CL it's there a little. Once it hits a certain temperature though it's fluctuates like a swingset.
Unfortunately I won't be able to look too much more into this. I just floated the valves on cylinder 3. I FORGOT to change the rev limiter in SAUJPV3 to accomodate my setup. I'm going to be tearing it apart today. Hopefully my problem IS a vacuum leak that I just can't find and that I happen to fix it on the re-assembly.
I'll keep you guys posted once i'm back together.

 

Okay. I didn't float a valve. Part of the top of one of my pistons broke off and disappeared. Bad news. On a good note though, I believe I found my vacuum leak. I can't verify it as a leak but it looks like a good possibility. One of the lower runner gaskets didn't look like it was sealed all the way. Different shading of the gasket indicating air or condensation leaking through.
Hopefully that's the problem.
Thanks all for the help. I learned a lot.