Cannot lower idle RPMs with BISS in SAS mode
 

Hey everyone, the title pretty much says it all. So, I let the car (2006 Evo IX SE) fully warm up and then used EvoScan v2.9 to enter SAS mode to set my base idle RPM. I was able to get into SAS mode no problem and when I did the RPM's went up from around 1000 RPMs to 1400 RPMs. So I went into the engine bay, got my trusty stubby phillips head screwdriver and proceeded to tighten the biss screw to lower my idle RPMs. Here's the thing, it wouldn't go lower than like 1200-1300 RPMs and that was with the BISS fully tightened!

So, I have previously checked for boost leaks up to 30 lbs (at the throttlebody and at the turbo inlet) and had fixed the couple of smalls ones I had but I did ANOTHER boost leak test tonight and found none. Just a slight air-moving-sound in the valve cover but no leaks at all.

I tried it all again just to make sure and same results: I couldn't lower my idle RPM's to the target of 750-850 like I want. So, I happened to have a new IACV motor lying around (one from a Dodge, I believe, that I purchased from Amazon) and did another boost leak test to confirm that it seals correctly and I didn't have any leaks. It did and I don't.

So, I gave it another try: turned on SAS mode in EvoScan, tightened BISS and it STILL WON'T lower the RPMs lower than 1200-1300 RPMs.

So, I can only assume that something is letting some amount of air bypass the IACV motor but I'm not sure how that's happening. I don't know what else could possibly cause this problem. Maybe it's something simple?

If anyone has any suggestions, please let me know!

Thanks! :beer:

You have probably checked that the throttle cable is not hanging the throttle plate open.

And read this thread about the throttle plate sealing:

https://www.evolutionm.net/forums/ev...e-sealing.html

Have you pulled the biss screw all the way out and inspected it? Sounds like a problem I had, couldn't get the rpms to lower. Pulled the screw only to find out the o-ring was missing.

+1.

Have you recently rebuilt your TB, with new shaft seals? If so, did you clean the backside of the Butterfly off? IF so, there's your problem.

MR ArcticC and Raceghost:

I forgot to mention in my original post that my car has a Buschur Racing ported throttle body and intake manifold. Thank you very much for replying because I think that you both were correct that my issue is the gap between the throttle plate and the throttle body housing.

In preparation to fix my issues I purchased a new OEM BISS screw and o-ring as well as a can of the 842 dry moly lube from Amazon. After I received those things I went ahead and removed my throttle body and inspected it. First, my BISS and o-ring were in great shape and I don't believe they were causing any issues. I had stated in my first post I had performed more than one intake leak test and found absolutely no leaks at or after the throttle body but it was good to replace it just to eliminate a possible issue.

Here is what my BISS looked like:

Attachment 262473

As you can see it looks pretty good but I still ended up replacing it with my new one since I had it and it's cheap. {thumbup}

Next, I inspected the gap between the throttle plate and the housing. This is what mine looked like:

Attachment 262474

I'm not sure how this compares to other people's throttle bodies but it doesn't look great to me. It looks like there is a lot of area for air to get past the plate. So, I spent roughly an hour applying multiple thick coats of the dry moly lube. Unfortunately, I never got it to completely seal like some other threads indicated. It DID improve my idle issues though. I was able to lower my idle to just under 1000 rpm's in SAS mode. It was not a rock steady idle though, it seemed to hunt between 900-1100 rpm's. I assume this is the best I can do with this particular throttle body.

I have considered picking up a used OEM TB and simply rebuilding it with a mil.spec kit and either sending my ported one to mil.spec for them to possibly fix or just run what I have and deal with the 1000 rpm idle I have now.

Does anyone know if this lack of sealing issue would effect the drivability of the car? I would assume it would not because once the throttle plate is open at all the amount of air moving past the plate would not really matter. In other words, this not sealing issue would only affect idle, correct?

Thanks for everyone's input! :beer:

Dry Moly actually doesn't hold up. Literally spray it with light cleaner, and you can rub it off with ease. I laid 5 layers of molly, and rubbed them off on accident when I was trying to just rub some residual residue away. The easiest way, but pain in the ass way, get regular JB weld. Very very lightly rub it around the crack of the Butterfly in the half moon shape on the back, and then do the same for the opposite half moon on the front. You need to open and close the butterfly for about 30 minutes to get it to set right, and not seal or weld the butterfly closed. Fixed mine on the side of Pikes Peak when I was stranded last summer there, fixed it, exercised it, and then never had an issue with it sealing again. JB Weld doesn't rub off. OF course it is now my spare, but...
Carry on.

Final thoughts
 

I decided to keep the ported throttle body for now. On a side note, I went to a stock MAF setup and my idle is now rock steady at about 800 RPM. I'm sure a fair amount of air is still bypassing the throttle plate but for now it seems to be working reasonably well. Thank you everyone for your input and hopefully this thread helps someone in the future.

That is a LOT of space, around the butterfly. I'd assume that's the problem, since you mention you have no vacuum leaks and the BISS looks good.

+1