**REV HANG** - EVAP, ICV, PCV/vacuum etc. experts wanted

[BTB]

Hey y'all,

I uploaded a video showing the effect between gear changes when driving. Probably shoulda done more high rpm shifts in 'free-revving mode' but I think you get the idea, you know how the M3 is supposed to drive. This was taken after I put the clutch switch back in yesterday. Car ran the same, no codes at all when I got home after a long couple drives. It stayed in 'free-revving mode' for a good 30+ mins which seemed unusual, but the rev hang always seems to come back at some point. A/C off

One other thing that I find odd: I had my dad rev the engine in the garage a little while I watched the throttle runner/shaft. Can anyone answer, how is it that on low rpm throttle blips (blip to ~2k rpm from idle) the throttle shaft pushrod from actuator doesn't move AT ALL. Only on larger blips does the shaft visually open up the throttles a little. What's controlling the inlet air at low rpm/low throttle angle blips, if not the ITBs? Don't understand it, wondering if there is a secondary throttle (like ICV) that meters air for small changes.

Thanks

[nextelbuddy]

I'm not really seeing a difference between rev hang versus no rev hang in your video..

Overall nothing looks like it's hanging unless you're talking about a possibly slow RPM drop? When you say hang to me that means the revs stay at a specific spot for any given number of time and then it drops whereas in your entire video rev hang versus no rev hang I'm not really seeing anything hanging.

It's there and somebody else sees it I feel really dumb.

[heinzboehmer]

Hey y'all,
One other thing that I find odd: I had my dad rev the engine in the garage a little while I watched the throttle runner/shaft. Can anyone answer, how is it that on low rpm throttle blips (blip to ~2k rpm from idle) the throttle shaft pushrod from actuator doesn't move AT ALL. Only on larger blips does the shaft visually open up the throttles a little. What's controlling the inlet air at low rpm/low throttle angle blips, if not the ITBs? Don't understand it, wondering if there is a secondary throttle (like ICV) that meters air for small changes.

Thanks

The DME uses the ICV for low throttle input and then switches to the throttle bodies. Totally normal. A bit more info here: https://nam3forum.com/forums/forum/s...position-table

[BTB]

I'm not really seeing a difference between rev hang versus no rev hang in your video..

Overall nothing looks like it's hanging unless you're talking about a possibly slow RPM drop? When you say hang to me that means the revs stay at a specific spot for any given number of time and then it drops whereas in your entire video rev hang versus no rev hang I'm not really seeing anything hanging.

It's there and somebody else sees it I feel really dumb.

It is the slow RPM drop between shifts that I'm talking about. I'm glad the video was able to help out, but yeah maybe rev hang isn't the best descriptor, it seems to be a pretty popular term especially amongst the VW/Audi community, who's cars (some of them, not all) have torrentially slow to drop revs.

The DME uses the ICV for low throttle input and then switches to the throttle bodies. Totally normal. A bit more info here: https://nam3forum.com/forums/forum/s...position-table

Huh, well that makes sense, thanks for confirming. In that case, seems like ICV could definitely be my issue ... ?? If it is stuck open while driving I imagine the car would behave as if the throttle was cracked slightly open all the time, as it does.

Can anyone say if a bad ICV (if intermittently) would be likely to not cause any codes or SES light ?

[sapote]

Can anyone say if a bad ICV (if intermittently) would be likely to not cause any codes or SES light ?

You might have a slightly sticky motorized ICV. Remove and clean it using gasoline and stiff brush. It has a spring loaded rotating half moon rotor act as a control valve. I should oscillate freely.

[BTB]

You might have a slightly sticky motorized ICV. Remove and clean it using gasoline and stiff brush. It has a spring loaded rotating half moon rotor act as a control valve. I should oscillate freely.

I clean it a couple years ago, given that the car has 207k on it I'd be more inclined to just replace it. I wonder if it's possible to have ICV issues with out having poor idle quality. Regardless of the rev hang, my car idles well, doesn't seem to idle higher when rev hanging. Just seems odd as you'd think unmetered air into the engine would increase idle speed.

[sapote]

Just seems odd as you'd think unmetered air into the engine would increase idle speed.

ICV provides metered air.

A slightly sticky ICV will not cause bad idle, but it adds more delay to the final idle which seems what you have.

[BTB]

FYI, seems like this went away when my valve cover issue was sorted. I stripped one of the valve cover studs when replacing it during a valve adjustment. Finally had the shop drill it out and replace, the rev hang seems to have been eliminated now that the valve cover seals well. I’m not an expert on the vacuum system but seems to make sense that a leak in the valvetrain, which is isobaric w inlet manifold due to crankcase vent tube, would cause intake vacuum/rev down issues. I ran the car like this for a few months, it leaked a tiny bit of oil here but now sorted out with proper seal via all 13 cover bolts.