This started when I replaced the throttle body shaft seals on my Evo 8. I started the car after I finished and it was idling at 1500 rpm at operating temp. I knew that my work was done right, so I started doing some searches about idle topics to try and figure out why the car was idling high. I ran across the "goop" (MolyKote 321) on the throttle plate. I mistook the goop for years of grime and I cleaned the TB and plate until it was blinging. The throttle blade was sticky before and now it transitioned from closed to open to closed like butter. So now I know why I had a high idle. Next question is, how do I fix it.

The first the thing after saying Doh! was let the car idle for about 25 minutes. The idle went up and down and back around but eventually it settled at around 950-1000. I'm assuming the ISCV (idle speed control valve) is at it's minimum travel since there is no reason it shouldn't have lowered to 800-850.

Coming from a 1g DSM, adjusting the idle was as easy as grounding a couple pins here and there, and setting the biss screw to achieve desired idle and then ungrounding the pins. Apparently the EVO needs an MTU (mitsu computer?) from what I have read to reset the ISCV. I have also learned that Evo owners are scared to touch the BISS screw like it's some kind of forbidden fruit.

Now here are questions that hopefully somebody can give me a good answer for:

Is there a difference between air traveling around the throttle plate and air traveling the through the BISS passage? (This is a rhetorical question, the answer is no) In my mental simulation, it seems that one could compensate for the extra air coming around the throttle plate by adjusting the BISS. As long as the ISCV has enough room to adjust in either direction, and the ECU can learn, why do you need an MTU or whatever other fancy stuff to turn the biss screw a little bit?

Why do you need the Dow MolyKote 321? No offense to Mitsu engineers but the goop is a ghetto solution. From what I know about mechanical engineering they probably used the goop because they could not rely on the throttle bore and blade tolerances to be reproducible and precise enough to have one ISCV/BISS setting from the factory. So they used the goop to make that tolerance obsolete. I say that the throttle blade is better off unhindered by goop and that the BISS can be tweaked to compensate for the extra air entering the engine past the throttle butterfly even if it ends up being a different amount of tweaking for thing 1's evo vs thing 2's evo. The ISCV will adjust to the new setting and you are good to go.

What do you guys think?

 

Old thread but the questions are new.

After cleaning it and installing new MIL.Spec seals the idle will go up to 1500 eventually, and stay between 1000 and 1200rpm most of the time. Target idle is 1000 due to Kelford 272 cams.

Never realised that gunk wasn´t dirt, so cleaned it up.