I started experience very rough 1st to second gear shifts. I finally realized this was happening because My RPM's were hanging and actually reving up slightly between shifts. If I got more than 50% throttle above 3k or so and press in the clutch, the RPM will go up an extra few hundred RPM's and hang there for a few seconds and drop VERY slowly.

A friend of mine suggested unplugging the IAC valve and driving it. Assuming the car would be ok since it was already warm and at operating temp, I unplugged the IAV valve. The idle did not change. And the car also drives 100% perfect without any DTC's throw while driving it with the IAC unplugged. I plug it back in and the revs start hanging again.

This time I unplug it again, start the car up once it's cooled down, as much as it can on a 90 degree day. The idle stumbles a bit, but isn't bad (280 cams). It eventually starts to idle perfect with the IAC unplugged and again drives perfect. Plug it back in and it RPM's hang.

I ordered an IAC valve. But the point of this thread is.. why is my car running so well WITHOUT the IAC plugged in? I expected it to idle very poorly and maybe even drive poorly, and at the very least, throw a DTC. But it didn't do any of those.

I've been searching and it seems the IAC valve is pretty common issue, but the symptoms most people have don't match mine.

 

I've read a bit more, finding that RPM hang can be caused by a setting in the tune/ECU. I had my car tuned years ago and have not done any mods since then. Also, the RPM hang is eliminated when the IAC valve in unplugged. So I'm wondering if my hang really is due to the ECU or IAC valve... If it is caused by the ECU, and I had a tune years ago, why is it suddenly plaguing me now? And again if it is the ECU, why does unplugging the IAC fix it? It seems pretty black and white to me that the IAC valve was the issue. Now second guessing myself.

Luckily I didn't pay much for a new IAC so we'll see.

 

Well I think I figured it out with some knowledge from others.

Prior to this happening, only a day actually, I had adjusted my clutch so the pedal would grab just a touch sooner. By doing this it was no longer making significant contact with the second clutch switch. By readjusting my pedal back to where it was, the RPM's stopped hanging..

This makes sense as some people have stated that there RPM's hang after the have their clutches replaced. Check the pedal adjustment and it's contact with the upper clutch switch.

I'm not sure why no contact with the switch causes the revs to hang, but I'm guessing that it does it by way of controlling the IAC valve, hence, the reason that unplugging the IAC valve would prevent this from happening.

 

so ive been doing a little reading about the clutch switch and what not and i guess its been a thing since the dsm to disable that second switch for various reasons. also people claim that if they give their clutch a little dab (not enough to come out of gear of course) it'll stop the hanging so maybe instead of worrying about where the clutch makes the car happy, you could disable the switch and place it back where you like it? worth a try imo, would be interesting to see

 

For whatever reason, having the switch unplugged alone did not solve the problem, because I did try that at first. My thought was that the ECU sees the switch as NOT engaged if it's unplugged as opposed to simply being eliminated. Maybe the Evo's and DSM's are slightly different in this regard, as to simply unplug the switch.

Has simply unplugging it fixed this issue for Evo's in the past? I can say for certain that, this rev hang only started happening after I adjusted my pedal. And I tried unplugging the switch and this did not solve the issue. I adjust the pedal back so it was making proper contact with the switch and it was instantly fixed.

I'd like to adjust the pedal back to where I prefer it, and also just adjust the switch along with it. Only trouble is that upper switch is a pain to reach. Simply unplugging it was a bit of a struggle.

 

interesting hardware relationship. I never associated my adjusted pedal with the problem, but it has plagued me and and many others for a long time. There is one thread about a million years old and has even more posts. I would drive that a read. I have and it makes my brain hurt. good luck, please post any fixes.

 

I was one of the early contributors. Listing problems but unable to offer solutions. There is definitely a relationship between the hang and that secondary clutch switch. When you tap it slightly it cuts fuel. Otherwise there is still fuel being delivered completely off throttle. I'll revisit adjusting the pedak if need be. The original threads make my head hurt as they offer various fixes but not a single "hey do this and it will fix"

 

This is interesting. My car does this and it can be irritating . So how do u check if it's the switch ? Just look under and see if the switch is touching the pedal?

 

Pretty much. I had rpm hang too and it drove me insane. I changed the iac, found quite a few boost leaks and then just adjusted the tune since it wasn't leaking anymore and all was cured.