# Rev hang... what actually causes it?



## ScubaStevo87 (Aug 9, 2010)

So I have done a search on this topic and only found the solution of a bad clutch sensor. (For those that don't know what rev hang is, it in the hanging/increase in rpm when the clutch is pushed in to shift gears).
My cars cruise didn't work when I got it so I installed a new one and cruise worked afterwards (which if the sensor was no good, it wouldn't). 
I'm sure I'm not the only one with this happening to them, so what are other people doing to solve it?

Thanks in advance!


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## groggory (Apr 21, 2003)

Just an idea...but try a throttle body alignment.


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## schwartzmagic (Apr 29, 2007)

The Uni 830 file I have gives me rev hang. Not too bad, but it is annoying. Its gotten worse lately but that might be because of my exhaust leak :banghead:


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## groggory (Apr 21, 2003)

schwartzmagic said:


> The Uni 830 file I have gives me rev hang. Not too bad, but it is annoying. Its gotten worse lately but that might be because of my exhaust leak :banghead:


I hear people blame rev hang on exhaust leaks too. And I noticed that as my exhaust leak got worse, the rev hang got worse. I'm fixing my exhaust leak SOON and so I'll report back if the rev hang goes away. Maybe the engine is getting bad data from the O2 sensors, and as such doesn't run all that great.


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## Bacon11 (Jan 20, 2010)

Depends where the exhaust leak is. If it's after the first o2 sensor, it doesn't really matter as much. I've been told the rear o2 is there for nothing other than the OBD2 scan to tell e-check if your cat is working/there.

Front o2 does the engine management stuff as far as air/fuel.


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## elRey (May 15, 2001)

vagcom block #66. check second(or third) value while pressing/depressing clutch pedal. one of the bits should toggle 0/1


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## herb2k (Jul 29, 2007)

elRey said:


> vagcom block #66. check second(or third) value while pressing/depressing clutch pedal. one of the bits should toggle 0/1


I had that problem. Replaced clutch switch F36 ($50 part from dealer) and all was good!


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## ScubaStevo87 (Aug 9, 2010)

My car is all stock and there is definately no exhaust leak. I will have to see what the stealership wants for a TB allignment as I don't have Vagcom (or Ross Tech or whatever its called now).

I guess my next question, is there somewhere to pick up the Vagcom/Ross tech program for a cheaper price? I see its 249 on Ross techs site. 
Or is there a different program I can get that will allow me to diagnose and adjust the computer?


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## 01TornadoWolf (Jun 18, 2009)

elRey said:


> vagcom block #66. check second(or third) value while pressing/depressing clutch pedal. one of the bits should toggle 0/1


look at block 66: http://www.ross-tech.com/vag-com/m_blocks/060-069.html

if u dont have vagcom, check around. i guarantee someone local has one.

i had nasty rev hang but my cc still worked. block 66 showed clutch signal at ecu wasnt changing. clutch switch was fine, actually turned out to be my ecu!


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## Three3Se7en (Jul 2, 2007)

My eurodyne 630 flash has nasty rev hang.


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## ScubaStevo87 (Aug 9, 2010)

So I got lookin a little further... I have 3 PCV hoses that are completely $%@#ed. Ordering some new ones from 034Motorsport this morning. Hopefully this is my problem.


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## ScubaStevo87 (Aug 9, 2010)

Got all my hoses changed but still no luck gettin rid of the rev hang. I'll be getting chipped by APR in 3 days so I will have to try that throttle body alignment after that and post my findings


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## ScubaStevo87 (Aug 9, 2010)

Tried the throttle body adaption and still no luck. The hang is definately less noticable with the apr program running, but it is still there. 

Any other ideas?


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## schwartzmagic (Apr 29, 2007)

Have you tried checking for exhaust leaks or pressure testing your charge pipes? Just a thought......

I had all kinds of issues because of a boost leak right off the coupler on the compressor housing. My axle was rubbing on the tbolt clamp that tightens the coupler on the compressor exit and eventually got eaten away by the axle, no more clamping = boost leak. The boost leak was throwing off my idle and AFRs. 

Sent from my MB860 using Tapatalk


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## groggory (Apr 21, 2003)

schwartzmagic said:


> Have you tried checking for exhaust leaks or pressure testing your charge pipes? Just a thought......
> 
> I had all kinds of issues because of a boost leak right off the coupler on the compressor housing. My axle was rubbing on the tbolt clamp that tightens the coupler on the compressor exit and eventually got eaten away by the axle, no more clamping = boost leak. The boost leak was throwing off my idle and AFRs.
> 
> Sent from my MB860 using Tapatalk


I second this. I hear people saying on here all the time that they 'checked' and have no leaks. But to know you gotta test. Not just eyeball it or feel around.

Turbo cars do not like air leaks. They live on the air. They get angry when you take or give air at the wrong times.


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## RafaGti (Jul 25, 2006)

How much would each of those things cost?

The throttle body alignment and the pressure test on the charge pipes?


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## burkechrs1 (Dec 1, 2008)

RafaGti said:


> How much would each of those things cost?
> 
> The throttle body alignment and the pressure test on the charge pipes?


If you know of a VW specific shop (not dealership) you may be able to go there and they can do a throttle body alignment with a VAG-COM. Takes about 60 seconds. The pressure test, shouldn't cost you more than $40 to make one, as long as you have access to compressed air.


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## RafaGti (Jul 25, 2006)

burkechrs1 said:


> If you know of a VW specific shop (not dealership) you may be able to go there and they can do a throttle body alignment with a VAG-COM. Takes about 60 seconds. The pressure test, shouldn't cost you more than $40 to make one, as long as you have access to compressed air.


Thanks for the help. Ill start looking for a place to get this done. Ive already replaced the switch and its still holding/reving between gear changes.


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## David_Tedder (Oct 24, 2005)

I also experience this and would love to see what the outcome is


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## gitman (May 13, 2004)

David_Tedder said:


> I also experience this and would love to see what the outcome is


have you also replaced your clutch position switch? i think the tell-tale symptom for this is if you disengage cruise control by pressing in the clutch, and witnessing the RPMs not only hang, but climb up before finally going down.

edit: i used this DIY: http://forums.vwvortex.com/showthre...h-Position-Sensor(Cruise-Control)-replacement


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## RafaGti (Jul 25, 2006)

gitman said:


> have you also replaced your clutch position switch? i think the tell-tale symptom for this is if you disengage cruise control by pressing in the clutch, and witnessing the RPMs not only hang, but climb up before finally going down.
> 
> edit: i used this DIY: http://forums.vwvortex.com/showthre...h-Position-Sensor(Cruise-Control)-replacement


Yes sir. I tried the pressing the clutch in and the revs did climb before eventually going down.

Its so annoying, specially in the upper RPM range if youre tryiing to shift even remotely fast or hard.


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## gitman (May 13, 2004)

RafaGti said:


> Yes sir. I tried the pressing the clutch in and the revs did climb before eventually going down.
> 
> Its so annoying, specially in the upper RPM range if youre tryiing to shift even remotely fast or hard.


sorry, my question was directed at David_Tedder who did not specify whether he had done the clutch switch.


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## David_Tedder (Oct 24, 2005)

I haven't done the switch because up until today and seeing this thread I hadn't put much thought into what was causing it as there is too many things changed with my build


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## rogerius (Jun 14, 2004)

I had rev hang when I used a 70mm TB with an Eurodyne file for 60mm TB, with a maffless setup.
Once the correct 70mmTB file was flashed, the rev hang dissapeared, in spite of the bug with the clutch switch (a bug in the software disables the switch, this only affects the CC).


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## gdoggmoney (Feb 21, 2004)

rogerius said:


> I had rev hang when I used a 70mm TB with an Eurodyne file for 60mm TB, with a maffless setup.
> Once the correct 70mmTB file was flashed, the rev hang dissapeared, in spite of the bug with the clutch switch (a bug in the software disables the switch, this only affects the CC).


Wait wait.... Eurodyne software... has a bug? :laugh:


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## rogerius (Jun 14, 2004)

check ur clutch switch in MB066 of VCDS. Report here if u see the bits changing 0 to 1 when u press the pedal. I am curious...


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## RodgertheRabit II (Sep 13, 2012)

rogerius said:


> I had rev hang when I used a 70mm TB with an Eurodyne file for 60mm TB, with a maffless setup.
> Once the correct 70mmTB file was flashed, the rev hang dissapeared, in spite of the bug with the clutch switch (a bug in the software disables the switch, this only affects the CC).


Mine does the same thing. Does it a little on the Stock TB and a base 630 file. What did you adjust to fix it?


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## l88m22vette (Mar 2, 2006)

As a general answer, rev hang occurs to let as much fuel as possible burn past once the acceleration/cruising interval is over...you let off the gas, and for a second (or whatever) there is still fuel going into the engine for either acceleration or typical driving, but that fuel might not get burnt because the engine is no longer operating in a partial/full throttle mode. Unburnt fuel causes both increased emissions and damage to the catalytic converter, both big no-nos for OEMs (this also causes the cool-ass flame spitting you see on race cars, which occurs while braking/decelerating).


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## gitman (May 13, 2004)

l88m22vette said:


> As a general answer, rev hang occurs to let as much fuel as possible burn past once the acceleration/cruising interval is over...you let off the gas, and for a second (or whatever) there is still fuel going into the engine for either acceleration or typical driving, but that fuel might not get burnt because the engine is no longer operating in a partial/full throttle mode. Unburnt fuel causes both increased emissions and damage to the catalytic converter, both big no-nos for OEMs (this also causes the cool-ass flame spitting you see on race cars, which occurs while braking/decelerating).


i can only speak for myself but i hope that for the purposes of this discussion everyone is referring to rev hang with an abnormally long duration, and even rev climb. what you're referencing should be considered normal behavior.


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## RodgertheRabit II (Sep 13, 2012)

Yes, this is beyond the "normal behavior"


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## Brake Weight (Jul 27, 2006)

Do these cars suffering from this have a cable TB? Or are they 'fly by wire?' My GLI is the latter, and compared to my cable operated Tacoma it's a bit different. I have to adjust to the two differences between them. As in, in my VW I lift off the gas prior to shifting. In my Taco, I do it simultaneously. 

Just something to consider.


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## David_Tedder (Oct 24, 2005)

Mine is dbw and at full throttle if you were to do it at the same time the rpm will flare up 500rpm almost , I too have to let off gas just before clutching in


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## Brake Weight (Jul 27, 2006)

After swapping from my truck to my VW, it'll take a run through the gears to get used to it again.


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## rogerius (Jun 14, 2004)

RodgertheRabit II said:


> Mine does the same thing. Does it a little on the Stock TB and a base 630 file. What did you adjust to fix it?


I asked the Eurodyne dealer to flash the correct Alpha-N from a 70mm TB base file. Problem solved, no more annoing rev hang (long duration/high rpm climb).
I only have now the normal rich condition at throttle lift off, for 1-2 seconds, as I see on the wide band AFM.


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## Chickenman35 (Jul 28, 2006)

ScubaStevo87 said:


> My car is all stock and there is definately no exhaust leak. I will have to see what the stealership wants for a TB allignment as I don't have Vagcom (or Ross Tech or whatever its called now).
> 
> I guess my next question, is there somewhere to pick up the Vagcom/Ross tech program for a cheaper price? I see its 249 on Ross techs site.
> Or is there a different program I can get that will allow me to diagnose and adjust the computer?


E-Bay cable for $25 to $30 and VCDS Lite for $99 from Ross-Tech. That's all you need.

Throttle Body Alignment is critical to the proper operation of the throttle. Rev hang is just one symptom of a bad TBA, but it is one of the most common. A TBA is necessary after any battery disconnect or removal of electrical terminal at throttle body. Doesn't hurt after a cleaning either. 

If you go to the Vortex VAG-COM Forum, there is a locator thread there that shows who is nearby that has a VAG-COM and may be able to assist you. Very handy service. Bring :beer:

http://forums.vwvortex.com/showthread.php?4991419-Vag-Com-Locator-searchable-map-style!-Owners-pm-me-your-info-requested-in-the-thread


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## gitman (May 13, 2004)

Chickenman35 said:


> VCDS Lite for $99 from Ross-Tech


$99 gets you a support contract. VCDS Lite is the free version of VCDS.


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## Chickenman35 (Jul 28, 2006)

gitman said:


> $99 gets you a support contract. VCDS Lite is the free version of VCDS.


Sorry, that's not quite correct. VCDS Lite is available as both a Free and Paid version. The Free version is EXTREMELY limited and won't even do Autoscans or Output Tests. The Paid version unlocks many features of VCDS LITE... as well as giving you support of course.

Here is a chart from Ross-Tech showing the differences between VCDS Lite Free version, VCDS Lite Registered Version and the Full version of VCDS:

http://www.ross-tech.com/vag-com/vag-functions.html

I've used the Free version of VCDS Lite and it was next to useless. However the fully registered version of VCDS Lite does pretty much everything I need on my 1998 Audi A4

Edit: Typo in my original reply.


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## gitman (May 13, 2004)

Chickenman35 said:


> Sorry, that's not quite correct. VCDS Lite is available as both a Free and Paid version. The Free version is EXTREMELY limited and won't even do Autoscans or Output Tests. The Paid version unlocks many features of VCDS LITE... as well as giving you support of course.
> 
> Here is a chart from Ross-Tech showing the differences between VCDS Lite Free version, VCDS Registered Version and the Full version of VCDS:
> 
> ...


ahh i stand corrected, i have not seen that page prior to now. thanks for the info!


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## 976-RADD (May 29, 2003)

My experience has nothing to do with throttle bodies, exhaust leaks, or the like. When I bought my car the CC wasn't functioning, I experienced rev hang, and the driver's-side door locks would not automatically lock while pulling away, or when locking the car, via FOB. Little did I know all of these symptoms were related to the clutch switch. After doing research on how to fix the CC, I found that I had a bad clutch switch. I replaced it, and the CC was now functioning. I gave up on the rev hang and door lock issues. About a year later, the "clutch switch signal" code was thrown, via CEL. After that, the light came on and off every few months, until a year ago, when it stayed on for good. I knew I had replaced the switch, and none of the symptoms of a bad switch were present, or so I thought. Last week, for schits and giggles, I pulled the trim off from underneath the steering wheel, and crawled under the dash, to make sure the switch was, mechanically, operating correctly. It was not. If the rod that actuates the switch was being press in, when the clutch was engaged (foot off pedal), then it was _very_ minimal. I took the switch out to see if there was a way to adjust the length of the rod, but found that there wasn't (maybe there is, but I couldn't find it). So instead, I added some backing to the place the rod makes contact. Afterwards, not only did ALL of the rev hang issues go away, when shifting from near-redline, and when pushing in clutch pedal, while in CC, but the CEL went off and the door locks were now functioning automatically. 

I don't understand how a switch can "sort-of-work." I thought what makes a switch, a switch, it's ON and OFF abilities. Maybe some switches are limited by amperage, via switch position?


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