Part throttle Vac Issue

[Militant-Grunt]

I forever get the most difficult and unique problems to diagnose with my car..

http://www.youtube.com/watch?v=EADSSPOs41A

Under part throttle the car constantly feels like its pulsating on / off / on / off.

Its not running lean. Not the tune, not the EGT's (coded out), no issue under WOT. Only part throttle. Replaced MAP sensor. No Boost leaks. Running Greddy Profec Spec 2 ebc, tried a mbc (no difference). Sometimes I just want to crush this car.

Ideas?

[Rated S]

Anyway to throw just the n75 on (you run in parallel or standalone for the ebc/mbc)? I'm leaning towards your ebc/mbc being the culprit even when changing to another one; as those tends to make a really shitty part throttle buck.

[Militant-Grunt]

Its just plugged in but not connected via vac. It has absolutely no control over the vacuum.

[Raacerx]

Its just plugged in but not connected via vac. It has absolutely no control over the vacuum.

Militant, have you read my endless threads about this problem? I never figured it out before I garaged the car and begin another engine pull.

I replaced literally everything I could think of. The tune, throttle body, MAF, MAP, boost gauge, all the vacuum hoses, as many check valves as I could find, deleted the N249, MBC vs N75, new Diverter Valves, etc etc. I could never log it, I could never get any data showing it. But it was there whenever I cruised on the freeway right between 10 and 20 vacuum, IDENTICAL to yours.

Try to find my old thread about it. A couple other people had/have the same problem and never fixed it. The first thing I would try, is a stock ECU. Even though I still got the problem with the stock ECU, that is always what kept it from coming back the longest (usually a day or two). Each thing I did seemed to help for a bit, then back to normal.

I feel your pain...

[s4nicetry]

This happened to me when I switched to mbc for my 2+ setup. At part throttle (light boost) the mbc would ramp up my boost to qucikly causing my requested vs. actual boost figures to differ to much giving me boost cut. Using a tcd eliminated this problem for me.

[Militant-Grunt]

Militant, have you read my endless threads about this problem? I never figured it out before I garaged the car and begin another engine pull.

I replaced literally everything I could think of. The tune, throttle body, MAF, MAP, boost gauge, all the vacuum hoses, as many check valves as I could find, deleted the N249, MBC vs N75, new Diverter Valves, etc etc. I could never log it, I could never get any data showing it. But it was there whenever I cruised on the freeway right between 10 and 20 vacuum, IDENTICAL to yours.

Try to find my old thread about it. A couple other people had/have the same problem and never fixed it. The first thing I would try, is a stock ECU. Even though I still got the problem with the stock ECU, that is always what kept it from coming back the longest (usually a day or two). Each thing I did seemed to help for a bit, then back to normal.

I feel your pain...

lol Yes!!!, I'm not the only one. It disappeared for 3 weeks at one moment. Its unbelievably fucking annoying. Initially I thought its the idle over airflow issue (tune related) but we recalibrated the throttle body and the issue remained. Flashing back to stock didn't help either.. Thanks for letting me know, that lets me skip a number of steps on diagnosing this issue.

I have a slight feeling its either one of the injectors or potentially the fuel pump. I have an 044 in my car, and I hear it dipping in pressure at idle and at the same time I see the vac jump. I'm not running lean, but somehow Im starting to think it could be related to this. Did this happen before you went stage 3 or after?

I'll look for your thread and link it here.

[Mattgfx]

hey man! i have the same problem.. this has been pissing me off for the past month or two !!!!! i cant figure it out either....

The only thing i can think of is maybe my MAF is dirty, or my airbox is full of crap. Maybe a super small vacuum leak somewhere..

Someone figure this out!!

[shadwarr1]

OP I have this similar problem, but not very noticeable, and only when the engine's cold. When warmed up, it's gone.

Dunno, I've just assumed it's my plugs because I haven't changed them since I've owned the car, only pulled, cleaned, and regapped.

FYI, I'm running K04's, piggies, and vibrant catback. Everything else is stock, including the ECU.

EDIT: Well not everything else is stock, I'm also running a VAST LWFW and stage 3 clutch, but I don't see that making any difference. But after reading about the N249 bypass thread I'm wondering if that'll solve my issue.

I also seem to have a slight buck between shifts wether it's cold or warm, but always thought it was from me not being use to a LWFW + stage 3 clutch.

[Mattgfx]

i cleaned my MAF which might of helped a tiny bit.. but the problem still is very noticeable . I am going to try and pressure test.. but i have a feeling nothing will come up. I also checked all my vacuum lines... they all seem to be okay.

Maybe the PCV system isn't working correctly?

[BITRBO]

Militant, have you read my endless threads about this problem? I never figured it out before I garaged the car and begin another engine pull.

I replaced literally everything I could think of. The tune, throttle body, MAF, MAP, boost gauge, all the vacuum hoses, as many check valves as I could find, deleted the N249, MBC vs N75, new Diverter Valves, etc etc. I could never log it, I could never get any data showing it. But it was there whenever I cruised on the freeway right between 10 and 20 vacuum, IDENTICAL to yours.

Try to find my old thread about it. A couple other people had/have the same problem and never fixed it. The first thing I would try, is a stock ECU. Even though I still got the problem with the stock ECU, that is always what kept it from coming back the longest (usually a day or two). Each thing I did seemed to help for a bit, then back to normal.

I feel your pain...

Same here dude!! My car has been plagued with this problem for at least a couple years now, and have spent countless hours changing/replacing/testing everything I can think of... It just like you describe too: A pulsating, chugging sensation while at part throttle. I can actually see it registering on my boost gauge (in VAC).

edit: http://www.audizine.com/forum/showth...TED?highlight=

So Racccerx, did you ever fix the problem after all your engine pulls? Or do you still have your car apart from the build?

edit:

"...Its unbelievably fucking annoying."

^I couldn't have summed it up better myself

[BITRBO]

Anyway to throw just the n75 on (you run in parallel or standalone for the ebc/mbc)? I'm leaning towards your ebc/mbc being the culprit even when changing to another one; as those tends to make a really shitty part throttle buck.

IDK, cause I've never done either and I've always had this problem... Raacerx, what about you?

This happened to me when I switched to mbc for my 2+ setup. At part throttle (light boost) the mbc would ramp up my boost to qucikly causing my requested vs. actual boost figures to differ to much giving me boost cut. Using a tcd eliminated this problem for me.

IDK about this either... This problem only seems to happen at very slight/partial throttle i.e. 10-20 in Hg of VAC. If I give it just a little more throttle (e.g. 0-10 in Hg VAC) or more, it goes away completely. This is actually worse, cause 90% of the time your cruising and not in boost!!!

[Johann2.7t]

add my name to what seems to be a growing problem among what I can gather as only 2.7t engines. Probably the most annoying problem I have ever run across from my many many cars.

I have no CEL and no codes according to vag-com, going to try an n249 bypass to see if that helps. Some folks seemed like they had similar issues in this thread...

http://www.audizine.com/forum/showth...9-valve-bypass

good luck everyone, please report back!

[BITRBO]

add my name to what seems to be a growing problem among what I can gather as only 2.7t engines. Probably the most annoying problem I have ever run across from my many many cars.

I have no CEL and no codes according to vag-com, going to try an n249 bypass to see if that helps. Some folks seemed like they had similar issues in this thread...

http://www.audizine.com/forum/showth...9-valve-bypass

good luck everyone, please report back!

Don't get your hopes up... It felt like it worked for a while, but that may ahve just been the placebo effect. Now I just have CEL light I never had before.

[Johann2.7t]

Don't get your hopes up... It felt like it worked for a while, but that may ahve just been the placebo effect. Now I just have CEL light I never had before.

did you keep the valve plugged in?

[BITRBO]

did you keep the valve plugged in?

yep - I followed the DIY guide to the "T".

[Militant-Grunt]

Im going to get ECUx on my laptop soon, log the car, and see whats what. N249 bypass wont help squat, I don't have any of that crap on my motor.

Its got nothing to do with PVC, I did notice though, the problem comes in different intensities. For the most part its the worst in hot weather and with the AC on (tho sometimes its just as bad or worse during cool nights). I thought about it possibly being IAT's, but with a meth setup that shouldn't be an issue (and it is..)

[Mattgfx]

^ i have ECUX i will try and log the problem.. but sometimes its hard to catch

[Johann2.7t]

does anybody else notice its speed sensitive?

mine doesn't seem to do it over 60mph, it seems to be most low speed cruising around town.

[BITRBO]

Im going to get ECUx on my laptop soon, log the car, and see whats what. N249 bypass wont help squat, I don't have any of that crap on my motor.

Its got nothing to do with PVC, I did notice though, the problem comes in different intensities. For the most part its the worst in hot weather and with the AC on (tho sometimes its just as bad or worse during cool nights). I thought about it possibly being IAT's, but with a meth setup that shouldn't be an issue (and it is..)

Yup, same here. In the cooler [winter] weather here in SFL (50-60 deg. F) it will hardly ever to it (sometimes it will though), but once it gets warmer or during the summer it almost always acts up. The a/c definitely makes it worse, which tells me that the problem is load-dependent.

Militant, as soon as you get ECUx let me know what you're logging/checking so I can try it as well... From my experience WOT/FATS runs don't really tell me anything, and I've logged at cruising speed and during idle (when it's acting up) but I wasn't really able to notice anything "out of the ordinary". I've always had my suspicions about the IAT's (which I did swap a new one in temporarily), but never logged them in VAGCOM or anything.

does anybody else notice its speed sensitive?

mine doesn't seem to do it over 60mph, it seems to be most low speed cruising around town.

Nope. I used to think it was speed/gear related, but after watching it closely for a while I'm 100% positive it's associate to throttle input and VAC.

[BITRBO]

On a side note: Has anyone with these symptoms tried replacing their spider hose? Cause I know mine has ALWAYS had a small leak that leaves a small amount of oily residue all over the hose itself and around the immediate area, and I think this hose is part of the VAC system? Someone told me once (maybe FlaS4, but I don't recall) that they replaced theirs and it helped, but IDK for sure.

[Johann2.7t]

On a side note: Has anyone with these symptoms tried replacing their spider hose? Cause I know mine has ALWAYS had a small leak that leaves a small amount of oily residue all over the hose itself and around the immediate area, and I think this hose is part of the VAC system? Someone told me once (maybe FlaS4, but I don't recall) that they replaced theirs and it helped, but IDK for sure.

i saw a DIY where some replaced the hosing with some 3/4 heater hose type stuff they found at home depot, might give that a shot as I know mine has a small pinhole in it as well.

[BITRBO]

i saw a DIY where some replaced the hosing with some 3/4 heater hose type stuff they found at home depot, might give that a shot as I know mine has a small pinhole in it as well.

I may have seen that thread too, but that kinda sounds like a jerry-rig to me... As functional as it may have been for that guy, I don't want my engine bay looking like a hardware-store mix & match if you know what I mean. I seem to remember someone selling an updated version of the spider hose (i.e. made from newer more resilient materials?), but it was like $300-400 IIRC so I've been putting it off to say the least...

The bottom line is: I'm not necessarily convinced that the spider hose is the problem (although it is a possibility), so I'm curious to know if someone here has already changed theirs and if they noticed a difference...

Anyone? Anyone?

[Militant-Grunt]

I have a new spider hose in my car lol.... So there goes that theory. If anything I have a tiny leak right at the Y pipe, but its small. Doubtful that would be causing it regardless tho.

It really happens at any speed, even at 80mph =/ (well, not really in the triple digits)

[BITRBO]

^damn. So I guess it's back to the IAT's or some other type of VAC leak (although I've leak-tested the CRAP out of this car, so IDK where else it could be coming from)...

What about injectors? Has anyone had theirs cleaned and/or replaced? I haven't done that yet, but have been seriously considering anyway for my eventual upgrade to stage III.

[Johann2.7t]

has anyone done seafoam or something similar? Maybe not as good as a bench flow but maybe would give some insight if it improved the problem even a little?

[Militant-Grunt]

^damn. So I guess it's back to the IAT's or some other type of VAC leak (although I've leak-tested the CRAP out of this car, so IDK where else it could be coming from)...

What about injectors? Has anyone had theirs cleaned and/or replaced? I haven't done that yet, but have been seriously considering anyway for my eventual upgrade to stage III.

I have 120k injectors on the motor, and one of them is LOUD. So it could potentially be that. I'll be upgrading to 60lb when I switch over to an E85 K03 tune.

[BITRBO]

has anyone done seafoam or something similar? Maybe not as good as a bench flow but maybe would give some insight if it improved the problem even a little?

Yup - tried that too... A whole bottle in fact, and I even put some in the tank. Nothing I even took it to the dealer very early on to change the fuel filter and do an injector cleaning (not really sure what that entails though, so it may have been a waste of money). I know there was another guy on here (Unispeed) that had the same problem and he had his injectors set out for ultrasonic cleaning... He said it may have helped some of his misfires a little (but not completely), but the problem still existed. I know he mentioned to me that he messed up and only had them clean up to a certain pressure (don't recall which), and that he could've gone much higher...

http://www.audizine.com/forum/showth......?highlight=

I have 120k injectors on the motor, and one of them is LOUD. So it could potentially be that. I'll be upgrading to 60lb when I switch over to an E85 K03 tune.

Yeah, but I still have the stock injectors and I only have ~36k on the clock... But then again, the car used to sit A LOT from the previous owner (it only had ~12k on it back in 2007!!), and I know that can be just as bad for these cars...

I definitely think that injectors may be at least partially to blame, but I'm not sure if anyone else has changed theirs out and still experiences this issue...

[AudiSportB5S4]

Yeah, but I still have the stock injectors and I only have ~36k on the clock... But then again, the car used to sit A LOT from the previous owner (it only had ~12k on it back in 2007!!), and I know that can be just as bad for these cars...

HA Holy crap!

[Johann2.7t]

shucks, i was starting to blame my injectors but I guess ill have to look elsewhere. Maybe we should get a list together of our top culprits and each replace one of them and report back.

This issue makes me not want to drive my car!

[The_Jerbel]

I would look at 2 things first: if you have a boost gauge, replace the T fitting with one that has the brass constrictor installed. Second, I would re-install the N75 instead of the MBC. See if either of those things helps. I assume you guys have all pressure tested thoroughly with soapy water? I would suspect something like a leaky injector seal to cause a problem like this, as well.

[Johann2.7t]

I would look at 2 things first: if you have a boost gauge, replace the T fitting with one that has the brass constrictor installed. Second, I would re-install the N75 instead of the MBC. See if either of those things helps. I assume you guys have all pressure tested thoroughly with soapy water? I would suspect something like a leaky injector seal to cause a problem like this, as well.

i dont have a mbc and my n75 is still installed. I have pressure tested a few times and found nothing at all.

[BITRBO]

I would look at 2 things first: if you have a boost gauge, replace the T fitting with one that has the brass constrictor installed. Second, I would re-install the N75 instead of the MBC. See if either of those things helps. I assume you guys have all pressure tested thoroughly with soapy water? I would suspect something like a leaky injector seal to cause a problem like this, as well.

Thanks Jordan... I know you have provided me with a wealth of knowledge and guidance in the past!

I haven't switched out my brass T-fitting, as I still have the one that came with the AWE gauge... I guess I'll give it a try, but I'm not sure if you can just buy 'em at Autozone or somewhere like that.

I can't speak for everyone else, but I have p-tested my car up the wazzu, and that last two times I used soapy water like crazy! I didn't check around the injectors or intake manifold, but those would be the only other places cause all my hoses are tight (read: no leaks at all).

You know, the more I think about it it seems plausible that there is some kinda of VAC leak somewhere, cause when the car is acting up, the surging almost seems rhythmic... Not necessarily associated with the engine speed, per say, rather every couple seconds it will surge and my boost gauge will flutter; Like the car builds up vacuum in the system, and then reaches some kind of limit (21 in Hg) and then releases it... Then builds it up again, and then releases it... Like a hose that has just enough of a crack/hole/leak/whatever that it can hold VAC for a short period of time, but then leaks it out when it reaches some sort of threshold.

Is there anything electronic (aside from the N75 valve) that holds/controls VAC?

[hellrot98m3]

i have the same problem, and it drives me insane. i thought it might not happen when using the cruise control but nope it even happens using it. its only part throttle, at wot there is not one hiccup of a problem. no cel or codes to be reported.

[The_Jerbel]

VAC is created by the engine sucking in air, and is limited by the volumetric constriction of the intake plumbing. There is nothing electronic to control the vacuum generated by the engine.

I have noticed that it is common when there is an injector or IM gasket leak, while not audible unless the leak is huge, they will definitely show up when sprayed down with soapy water while doing a boost leak test.

[BITRBO]

VAC is created by the engine sucking in air, and is limited by the volumetric constriction of the intake plumbing. There is nothing electronic to control the vacuum generated by the engine.

I have noticed that it is common when there is an injector or IM gasket leak, while not audible unless the leak is huge, they will definitely show up when sprayed down with soapy water while doing a boost leak test.

I know P0234 said he tore down a 5.0 once looking for something similar, and turns out it was the IM gasket.... And you can't really see/access the IM gasket or injectors on our car w/ the Y-Pipe in place, can you?

[Mattgfx]

Did some logs... going to send them to VAST but it might be something to do with the Throttle body. might need to be re calibrated or replaced... Will follow up..

[BITRBO]

Did some logs... going to send them to VAST but it might be something to do with the Throttle body. might need to be re calibrated or replaced... Will follow up..

Great, let us know! I have some old logs too if you think that'll help... Did you log it at idle or cruising?

I know one of the first shops I took it to when trying to diagnose the problem claimed that the TB just needed to be "realigned"... I was like "okaaaay'", but I figured I'd let them try it... They did it a couple different times (and probably cleared all the codes too), and although it seemed to help temporarily, that may have just been the car not acting up afterward cause it usually went back to normal the next day. IDK, maybe the TB does go out of alignment or is leaking from somewhere (except the boot, cause I've checked mine like 100 times)...

[AudiSportB5S4]

Last night we logged the car, I tried to have him simulate it and I certainly found what he said above...

Below is a graph of the data I collected, and its showing the problem happening at its finest!

Blue Line represents Accelerator Position, which is constant here, Red represents the throttle plate angle during the problem.



Here's another graph of Blue Accelerator being constant again, and the Red is the MAF Readings, and the Green is the Throttle plate angle.

[BITRBO]

OMG, this is awesome data guys! I guess the question now is: Why is the t-plate moving all over the place at constant throttle??

[Audiophyle]

I was going to ask about the throttle body, glad it looks like somebody found proof of it.

Being a drive by wire setup, it could be as simple as the internal gearing being dirty or worn down. As voltage rises (gas pedal pressed in), the throttle should open more & more. At WOT the throttle pushes completely open, but at partial throttle it is trying to hold at a very specific value. If its internals are dirty it would have difficulty trying to reach that specific position & the car could be trying to compensate for stuck gears, causing the throttle to overshoot & thus fluctuate.

Anyone willing to break open their TB housing? lol