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

[BTB]

I hope everyone, wherever you may be, is having a pleasant Sunday. I myself just got back from the ole COVID grocery run in the Ultimate Driving Machine, and after having another truly glorious drive disturbed by this little rev hang issue, finally felt compelled to share with the group.

I'm having a rev hang issue. It's pretty simple; when starting from cold and driving off, the car maintains a slight rev hang which requires slowed shifting to be smooth. The issue doesn't change once warm, and it gets in the way of driving very spiritedly. However, every time I shut the car down (like at the store today) and start it up warm, the car behaves perfect, no rev hang at all and I get maybe a few miles of slick shifts and a wide grin before it comes back. Seems to be 're-activatived' by coming to a stop, when the revs drop to idle and engine speed is 'caught' at 1200 rpm before slowly settling at ~850 (presumably this is the function of the ICV?). After pulling away, the revs are again slower to drop, and I don't think it's ever gotten better without shutting the car off again. You can also feel when off throttle coasting in gear, the car doesn't slow as much via engine-braking, like the throttle is slightly open.

Any thoughts on this? I just pulled codes today (no SES light) with Peake reader:

-1b
-8d (fuel level, plausibility)
-b4 (tank leak detected)
-50 (apparently this is related to clutch switch. Mine is currently bypassed, so that explains this code)

I've had the 8d code since I replaced my fuel pump a few years ago, I never correlated it to a running issue so I've just ignored it. The fuel gauge sometimes takes a few mins to register when filling up, which never happened with the original fuel pump, but I've just taken it as a quirk (lmk if you think this is at all related ... ). Just some background, rod bearings were done in late 2019 with WPC treated OE (thanks Kaiv) and full Beisan VANOS suite a couple years before that. I've replaced an O2 sensor here and there but otherwise the motor has ran perfect.

I appreciate your input. I was thinking I need to change my ICV or check for vacuum leaks, but my symptoms don't really seem to match the things I've read about those systems. My idle can be a tad shaky on warm starts but not excessively so. I did my best to clean out the ICV a while back, ~2.5 years ago. I don't remember ever seeing the tank leak code before. I'm happy to take a video of the rev hang if it helps, it's not craaazy excessive but it is very much noticeably different from how it should be, where revs drop almost instantly. It requires a whole different driving style which is annoying because I'm never positive if revs will drop quick or slow, I have to actually watch the tach.

Thanks
Brendan

[BTB]

Bump. Anyone?

[Ramps]

Only things i can think of is slow throttle actuator or ICV...working well enough to not throw a bunch of codes but slow to adjust. It’s an odd problem...update us when you figure it out.

[nextelbuddy]

what do you mean you bypassed the clutch switch? how did you "bypass" it? did this issue start somewhere around the time when you performed this bypass?

This sounds like the result of a bypassed your clutch switch you also need to delete gear position sensor and the associated DTCs in your tune file much like when somebody does an SMG to manual conversion and doesnt install or wire in the GPS and clutch switch.

[BTB]

Only things i can think of is slow throttle actuator or ICV...working well enough to not throw a bunch of codes but slow to adjust. It’s an odd problem...update us when you figure it out.

Huh, I did the two engine TPSs a couple years ago, car seems to drive great otherwise, just on a closed throttle it’s almost like throttle doesn’t close completely, only I’m pretty sure it does because idle speed isn’t affected.

You think ICV stuck (open..?) could cause this? My buddies e36 325 rev hanged like a b**** so I wonder if the same issue is at play. Engine controls aren’t identical but must be similar. Never heard of this on an M3 though myself

what do you mean you bypassed the clutch switch? how did you "bypass" it? did this issue start somewhere around the time when you performed this bypass?

This sounds like the result of a bypassed your clutch switch you also need to delete gear position sensor and the associated DTCs in your tune file much like when somebody does an SMG to manual conversion and doesnt install or wire in the GPS and clutch switch.

If I remember right I used a paperclip to short the circuit. Was driving cross country and having clutch switch issues (had to pump clutch when starting) but it turns out it’s just my clutch stop was out too far ... I can reinstall the switch and see. The rev hang thing has been an intermittent issue since late 2019 I think, don’t remember it coming on with clutch switch thing but it’s possible it’s related.

Any idea what would happen if you don’t ‘delete gear position sensor’ as you say? You think rev hang is a possible result?

[BTB]

Given that the rev hang occurs between shifts, when clutch is depressed anyway, I wouldn’t think the clutch switch bypass would be related (though my understanding is the clutch switch is only used when using cruise control and when starting up. Just my guess though). No doubt I could be wrong

[EthanolTurbo]

I'm subscribed to this because it could happen to anyone. In for results!

[Arith2]

Look at the AFRs of the rev hang. Does it get lean? Does it stick around stoich? Doing data logs of what your engine is doing compared to what is being commanded to get to the bottom of this. You could just have a simple mechanical failure or the tune from your swap could be botched somehow. Shorting anything that provides direct feedback to the DME isn't a could idea. Who knows what heat you've introduced to the circuit, or possibly screwed a chip up. It could be an ICV but I believe it's at 100% even between gears so your engine doesn't stall. We can speculate all day, but data logs are your best friend. Go use testo and get us some PIDs.

[sapote]

Make some observation: place a mark on the throttle when they are completely closed with engine off, then check the mark during high idle (1200rpm or so) at operating temp to see if the mark is off. This indicates if the DME commanded to open the throttle slightly for some reasons, or not. If mark is not off then either it has air leak or the ICV is stuck above idle 870 rpm.

To look into air leak, use INPA looking at short term fuel trim when it stuck at 1200 rpm.

[nextelbuddy]

could be throttle actuator, TPS sensors, accelerator pedal box. have you tried using INPA to reset all adaptations

[heinzboehmer]

Your issue kinda sounds like an electrical one, but I had a slightly shaky idle that was caused by a small vacuum leak at the port in the air rail that leads to the fuel pressure regulator. I managed to knock the hose off of it accidentally when working on something in that same area (its just held on by friction, no clamp or anything).

Super simple to check so might be worth a shot to take a look without any prior diagnosis. Remove the cabin air filter housing and make sure there's a hose (that goes down to the fuel pressure regulator) plugged into this port at the rear of the air rail:



Again, just a blind guess, but it'll take all of five minutes to check.

[BTB]

I'm subscribed to this because it could happen to anyone. In for results!

Lol, let's hope I get em

Look at the AFRs of the rev hang. Does it get lean? Does it stick around stoich? Doing data logs of what your engine is doing compared to what is being commanded to get to the bottom of this. You could just have a simple mechanical failure or the tune from your swap could be botched somehow. Shorting anything that provides direct feedback to the DME isn't a could idea. Who knows what heat you've introduced to the circuit, or possibly screwed a chip up. It could be an ICV but I believe it's at 100% even between gears so your engine doesn't stall. We can speculate all day, but data logs are your best friend. Go use testo and get us some PIDs.

To be clear-- my car is a factory 6mt (not an smg swap). I may have fused the clutch bypass, I don't remember. Seems to be a popular mod but I may not have done the most professional job ... I think I'll replace the clutch switch this weekend and see what happens. Hopefully no harm done there

I don't have any way of monitoring AFR, is there any easy way to do this? My dad has a fancy OBD reader, guess I'll look into its functionality. I agree that it doesn't seem like ICV itself would have an impact on engine operation at higher RPM, unless there was a massive vacuum leak near the ICV.

Make some observation: place a mark on the throttle when they are completely closed with engine off, then check the mark during high idle (1200rpm or so) at operating temp to see if the mark is off. This indicates if the DME commanded to open the throttle slightly for some reasons, or not. If mark is not off then either it has air leak or the ICV is stuck above idle 870 rpm.

To look into air leak, use INPA looking at short term fuel trim when it stuck at 1200 rpm.

The car doesn't idle high at operating temperature. Maybe I didn't explain the issue right-- it's only on engine decel while driving. No problems with my idle that I can speak of, doesn't seem like a throttle position issue, I guess I could have someone rev the engine while I watch the throttle shaft to make sure it's not doing weird stuff on engine decel

could be throttle actuator, TPS sensors, accelerator pedal box. have you tried using INPA to reset all adaptations

Don't have INPA, and wouldn't know what to do if I did. Worth looking into you think? All I got is my Peake reader

Your issue kinda sounds like an electrical one, but I had a slightly shaky idle that was caused by a small vacuum leak at the port in the air rail that leads to the fuel pressure regulator. I managed to knock the hose off of it accidentally when working on something in that same area (its just held on by friction, no clamp or anything).

Super simple to check so might be worth a shot to take a look without any prior diagnosis. Remove the cabin air filter housing and make sure there's a hose (that goes down to the fuel pressure regulator) plugged into this port at the rear of the air rail:



Again, just a blind guess, but it'll take all of five minutes to check.

Hey I appreciate the input, I'll give it a look over

[sapote]

when the revs drop to idle and engine speed is 'caught' at 1200 rpm before slowly settling at ~850

Well, it did hang at 1200 instead of 850 at idling and at operating temp. No?
So at this moment, you can do what I suggested.

[sapote]

The car doesn't idle high at operating temperature. Maybe I didn't explain the issue right-- it's only on engine decel while driving.
No problems with my idle that I can speak of, doesn't seem like a throttle position issue, I guess I could have someone rev the engine while I watch the throttle shaft to make sure it's not doing weird stuff on engine decel

When foot off gas pedal and gear is neutral, this is idle rpm regardless it was driving or stand still. So it only happened when rpm was higher and then let off the pedal in neutral and rpm didn't drop down to 850ish at the expected time. Yes?

[BTB]

Well, it did hang at 1200 instead of 850 at idling and at operating temp. No?
So at this moment, you can do what I suggested.

Well, idk about hang-- I guess I mean, when you clutch in at high rpm, the revs dart down and then sort of catch at 1200 rpm, then slowly rev down to ~800 rpm. Not that it stays at 1200, just that it changes deceleration rate so as not to stall.

When foot off gas pedal and gear is neutral, this is idle rpm regardless it was driving or stand still. So it only happened when rpm was higher and then let off the pedal in neutral and rpm didn't drop down to 850ish at the expected time. Yes?

The revs do always settle at ~800 rpm. The phenomenon I'm describing and concerned with is the slowness of the revs to drop from higher rpm, between gear changes. Lemme see if I can take a video this weekend to show specifically what's going on and me help explain