# Losing boost around 3500 rpm with stock 1.8t



## vphuynh (Jul 8, 2008)

Hello everyone, I am new to this forum and I was wondering if I could get some help with my mkiv gti's boosting problem.
I've had the car since July of last year; it's a 2005.
As far as I know, its stock. I have not done anything to it. It had a boost gauge already installed when I bought it.
Here is the problem... 
When the car is first warmed up, it peaks at about 17lbs. and drops to about 10lbs. and holds throughout the revs with WOT. After about 20 minutes of normal driving, when accelerating past 3500 rpms, it would lost boost completely (drops to 0 - 2 lbs.) and holds until I shift into a higher gear where the rpms drops below 3500 and would continue to lose boost pressure as I accelerate past 3500 rpms.
Could someone please provide me with some feedback?
Thank you everyone ahead of time.


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## 16plus4v (May 4, 2006)

*Re: Losing boost around 3500 rpm with stock 1.8t (vphuynh)*

thats a big boost spike for stock... check all of the boost hoses from turbo to intake mani


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## vphuynh (Jul 8, 2008)

*Re: Losing boost around 3500 rpm with stock 1.8t (16plus4v)*

If any of the hoses were loose, wouldn't it be losing boost all the time?


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## 16plus4v (May 4, 2006)

*Re: Losing boost around 3500 rpm with stock 1.8t (vphuynh)*


_Quote, originally posted by *vphuynh* »_If any of the hoses were loose, wouldn't it be losing boost all the time?

not really.. could be torn or half way connected.
been through this many times myself with stock and BT setups


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## vphuynh (Jul 8, 2008)

*Re: Losing boost around 3500 rpm with stock 1.8t (16plus4v)*

Alright, I'll try and take a look tonight. It's raining here in Washington so I don't know when I will be able to check it out. I'm hoping it will stop raining this afternoon.
In the meantime, what else do you think it could be if the hoses are fine?


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## bzflag (Apr 27, 2003)

*Re: Losing boost around 3500 rpm with stock 1.8t (vphuynh)*

Sounds like it could have some kind of diode mod done by the previous owner thats putting it into soft limp.


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## vphuynh (Jul 8, 2008)

*Re: Losing boost around 3500 rpm with stock 1.8t (bzflag)*


_Quote, originally posted by *bzflag* »_Sounds like it could have some kind of diode mod done by the previous owner thats putting it into soft limp.

How could I tell that this mod had been done?


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## -Khaos- (Dec 22, 2003)

it does sound like soft limp... and 17PSI is way high for "stock."


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## vphuynh (Jul 8, 2008)

*Re: (-Khaos-)*

I did some digging around during lunch and I found a phone cord type cable tucked into the driver side dash. Here are some links to the pictures I have taken of them.








Cable in dash 1








Cable in dash 2


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## -Khaos- (Dec 22, 2003)

that just looks like a phone charger.....


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## vphuynh (Jul 8, 2008)

*Re: (-Khaos-)*

odd place to put a phone charger cable don't you think?


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## -Khaos- (Dec 22, 2003)

*Re: (vphuynh)*


_Quote, originally posted by *vphuynh* »_odd place to put a phone charger cable don't you think?

Probably just hard wired, or maybe it goes to a GPS unit, I dunno... But it's got nothing to do with the boost issue... If anything trace the wires back from the MAP sensor and look for oddities.


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## vphuynh (Jul 8, 2008)

*Re: (-Khaos-)*


_Quote, originally posted by *-Khaos-* »_
Probably just hard wired, or maybe it goes to a GPS unit, I dunno... But it's got nothing to do with the boost issue... If anything trace the wires back from the MAP sensor and look for oddities.

Alright, I'll see if I can get to it tonight.
Thanks.


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## -Khaos- (Dec 22, 2003)

no problem, looking at the intake manifold,follow the 90* rubber elbow down to the intercooler (down the side). You'll see a sensor on top of it, just grab the wires coming out of it and follow them up a bit.


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## vphuynh (Jul 8, 2008)

*Re: (-Khaos-)*

I haven't had time yet trace the issue you were talking about yet. It's been raining here and it gets dark around 4:30 and I don't get off work till 5.
Today on the way home, I pulled over and restarted the car as soon as it started happening and it performed fine again, but would then go back to losing boost after awhile (20 or so minutes).
I don't know if that helps anymore or less.


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## ypsetihw (Nov 20, 2008)

*are you really losing boost or are you just watching your boost gauge?*

when your car is warmed up its running more efficiently and when you're just driving around town its smart enough not to spool your turbo all the way up all the time so you get better gas mileage and drivability. I drive a stock tune engine like yours and commonly get readings of 1-2psi just driving around, 5 maybe when passing somebody. you're not really gonna push 11psi for long stretches unless you're wide open. 
your boost gauge is also reading vacuum (the scale labelled in/hg below 0 psi) notice how vacuum drops (the needle goes up) when you depress the throttle? that's showing you that your throttle plate is opening and sucking in air. 
vaccum is a good indicator of your engine's overall performance. at idle and warmed up you should be pulling about 15-20 in/hg and around 20 cruising on the highway in 5th not giving it much throttle. if you're not getting good vacuum readings, or the needle is all over the place, then you might actually have a problem. 
check here for a list of common problems and vacuum indications: http://www.iwemalpg.com/Vacuum_gauge.htm
your turbo isn't boosting until you're above 0 and it takes you a few seconds to spool up. just driving around you can shift below 3000 just fine and never boost! gets great mileage!!! also, for the love of god, don't shift at redline when you're pulling hard, you make power more efficiently a little lower in the band. if you drive it around all wound up like a honda it'll drive like crap. 
get on some open road and try flooring it from 25mph in 3rd and pull, throttle wide open and hold it there until the needle settles in peak boost, hopefully around 10-11 psi. it should peak and settle quickly once you're spooled up, and then hold steady as you accelerate. again if it doesn't act like this, or the needle is all over the place, or you actually see the psi reading drop pulling at wide open throttle, then you might actually have a leak or other issue
its common to spike 14 and hold 10-11, but i've never spiked over 15 so i might look into that. 










_Modified by ypsetihw at 8:56 PM 1-13-2009_


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## vphuynh (Jul 8, 2008)

*Re: are you really losing boost or are you just watching your boost gauge? (ypsetihw)*

Thanks for the driving tips. I'll make sure to apply your suggestions on my drive up Interstate 5 every morning during traffic hour. I'll be sure not to spool my turbo up all the way all the time.
By the way, thanks for the explanation of how force induction works with turbos and also how to read my vacuum/boost gauge.
Here is a picture (tonight) of my engine's vacuum at idle. As you can see, it's where you say it's suppose to be (below).

Below is a video of the boost spike I was talking about. 5th gear @ 65mph. Spikes like that from 2nd to 5th gear. In lower gears the boost drops to a constant ~10lbs.
It's clearer than it looks.

Maybe you should read my post again








Thank you for your input.









_Modified by vphuynh at 9:35 PM 1-13-2009_


_Modified by vphuynh at 9:50 PM 1-13-2009_


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## ypsetihw (Nov 20, 2008)

*Re: (vphuynh)*


_Quote, originally posted by *vphuynh* »_Today on the way home, I pulled over and restarted the car as soon as it started happening and it performed fine again, but would then go back to losing boost after awhile (20 or so minutes).

Your comment that it performed fine was why I suggested that maybe you just needed a refresher on the gauge reading, didn't mean to make it sound like you're dumb, but it took me a while to understand the boost gauge too, what it meant and what I could learn about my engine from it. 
ignore the gauge for a minute - so you actually feel it crap out and limp? it looses power and drivability suffers? you can feel and maybe even hear that the turbo is not actually spooling, even when you floor it? no other indications such as funny noises or check engine lights? 
wathced your video, that spike doesn't seem abnormally high to me, and it seems to be responding correctly and quickly but it did settle very low. 
was your foot all the way down that whole time or did you let up on the throttle after it spiked? i think I can hear your DV dumping after the peak which would mean you're letting up, so boost is gonna drop, again its boosting in proportion with your throttle demands. Its not gonna spool up to 11 and just hold there if you're not telling it to. 
what about your ESP (electronic stability program)? if your wheels are slipping your engine will take over from you, cut the throttle and limit boost and subsequently power and torque. it is winter - stupid question but are you sliding all over the place or is your ESP triggering all the time unecessarily? 
I agree that you should check all your intake, vacuum and charge hoses, run the engine and feel around in there at all the joints, feel any air blowing out at you? also check the connections on the vacuum hose running to the boost gauge, make sure its all tight. 
one weird thing is that you're getting normal boost readings in the lower gears, but if you had a leak you should be losing boost across the power band all the time, in every gear. 
your vacuum readings look good, and if the needle is behaving at idle then you can infer that your engine itself is running fine. http://****************.com/smile/emthup.gif now you can start working backwards from the throttle body and check EVERYTHING on the intake track between the turbo and the throttle body. http://****************.com/smile/emthdown.gif
one really common problem where people leak boost all the time is the charge hose between the turbo outlet and the pancake pipe. its a pain in the ass to get to it and it sits behind your passanger's side wheel well. search the forums here and you can find lots of detailed information and people bitching about this pipe. 
you've got me all interested now, keep us posted on what you find










_Modified by ypsetihw at 2:54 PM 1-14-2009_


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## vphuynh (Jul 8, 2008)

*Re: (ypsetihw)*


_Quote, originally posted by *ypsetihw* »_ what about your ESP (electronic stability program)? if your wheels are slipping a lot your engine will take over from you, cut the throttle and limit boost and subsequently power and torque. it is winter - stupid question but are you sliding all over the place or is your ESP triggering all the time . 

Although it is winter, it's usually dry when I do this. ESP is not intercepting.
Let me see if I can go into details about what exactly is happening.
Lets say I'm in 3rd gear (doesn't matter what gear I'm in, same thing happens) about 2500-2700 rpm (this is when the problem starts happening, about 20 or so minutes into the drive) and I floor it; the boost pressure will then peak at ~17lbs drop to ~10lbs, then as soon as the rpm passes 3500, boost pressure drops to ~0-2lbs.
When I was driving home the other day (when the problem is currently happening) I floored it in 4th gear going up a slight hill on the freeway and I would have no power once the car loses boost. RPM would stay still around 4000 and car would not accelerate.
As I mentioned about pulling over yesterday and restarting the car... the problem did go away, but would come back around ~20 minutes.
BTW, the video is a quicktime video (.mov)
I don't know if I will have time this weekend to check the hoses and for modifications. I am going to be in Whistler all weekend. I will keep you guys posted.
Again, thanks for all the input.


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## ypsetihw (Nov 20, 2008)

*Re: (vphuynh)*

ugh, i hoped you were just noob and there wasn't anything wrong with your car








ok so here's a related thread on diode mod that was alluded to earlier, check for this it might be causing the limp issues described above: http://www.anthonydanna.com/vw/diodemod.htm
then you start checking off sensors - MAF (although its before the turbo so don't know if this is the culprit and you'd be having other problems), and MAP (aka Boost Pressure Sensor). MAP is an excellent candidate if you find no other problems. 
also, I'd still check for any vacuum/intake leaks. maybe its only having trouble after it warms up because all the moving around loosens up some connections? 
stumped . . .


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## LittleNugen (Mar 18, 2009)

*Re: (ypsetihw)*

Hi Ya'll, my first post on the vortex.
Am running into this same problem, but car is bone stock.
04 jetta 1.8t.
just started today. A friend of mine (has 99 A4) said my blowoff valve sounded funny....has been kind of fluttering in that state where its either boost or no boost.
where is the blowoff valve located.
Oh, i am a vw noob, but not a car noob.


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## LittleNugen (Mar 18, 2009)

After allowing the car to cool, i drove it and it was fine. I found the BOV and upon inspection, there was black goo on the inside. I cleaned the inside for good measure.
I remembered the symptoms happend while engaging in very spirited driving for 20 minutes.
is there a heat soak issue with the stock BOV? could it just got to hot and malfunctioned?


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## SSG_GLI (Jan 12, 2009)

*Re: (LittleNugen)*


_Quote, originally posted by *LittleNugen* »_is there a heat soak issue with the stock BOV? could it just got to hot and malfunctioned?


put your flame suit on man. your gonna need it here.


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## zeusenergy (Feb 25, 2008)

*Re: (LittleNugen)*


_Quote, originally posted by *LittleNugen* »_After allowing the car to cool, i drove it and it was fine. I found the BOV and upon inspection, there was black goo on the inside. I cleaned the inside for good measure.
I remembered the symptoms happend while engaging in very spirited driving for 20 minutes.
is there a heat soak issue with the stock BOV? could it just got to hot and malfunctioned?


Black goo inside? Might be an indicator of oil in the pancake and possibly clogged up throttle valve. Remove the hose from the throttle body at the front of your motor and look inside. Is it crusty? Is there tons of oily mess inside? Fixing just that may fix the problem but the source of the issue is prolly the PCV system at that point. 
The DV (blow off as you call it) can definitely kill boost above a certain point, had the same issue here and once it was replaced everything was good. 3500 RPM is where the turbo will spool best, and provide the most pressure, so any weak link may be failing only when you reach that point. Again that charge pipe behind the passenger tire is notorious for this issue. Let us know what you find once you use our advice on what to look for. Good luck...


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## zeusenergy (Feb 25, 2008)

*Re: (zeusenergy)*

Oh, and a note:
Your stock boost should be 11-12 PSI. Maybe spiking at 14 initially, but immediately dropping to stock 11-12. If you spike higher, it might mean you have a problem with the stock boost control such as diode mod, broken N75 or wastegate lines, etc. The engine computer is regulating the boost once you hit the 17 you were seeing and tells the N249 solenoid attached to the DV to open, dumping boost around the turbo enough to make your boost drop back to 10 PSI. This issue may have killed your DV since it was being used so much with such a high spike. No matter what you fix, there may be an underlying cause you should be aware to find. http://****************.com/smile/emthup.gif


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## vphuynh (Jul 8, 2008)

*Re: Losing boost around 3500 rpm with stock 1.8t (16plus4v)*


_Quote, originally posted by *16plus4v* »_
not really.. could be torn or half way connected.
been through this many times myself with stock and BT setups

It was a torn hose! Thanks everyone for their input. You guys are awesome.
http://****************.com/smile/emthup.gif


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## airmanboe (May 18, 2010)

*Same issue / Turbo stops spooling and loss of power after B.O.V. install on stock 1.8t*

I have a stock 2004.5 GLI w/ a 1.8t and I'm loosing power intermittently. i just installed a forge motor sport blow off valve (FMDV004) and i think this is what is causing my issue. does anyone have any input on these? i am new to the turbo setup as I've always drove 5.0 mustangs. i was told i need a tune to allow the computer to realize it inst recirculating air back into the engine form the stock diverter valve. Thank you anyone for any input you can provide.


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## BoosTT 315 (Jan 29, 2012)

*Help?!*

I have an 02 TT 225. I put a brand new k & n Apollo CAI in yesterday and took it for a test drive. It is apr tuned also. The car usually peaks around 18psi but decides to spike from 15 down to 2-3 around 3300rpms. It sounds as if installed a bov when I let off but I'm not sure if it's because of the intake. It just feels like the car is starving and the pedal practically goes dead. I'm not sure if it's a hole in the piping or if my inlet hose is collapsing under the force of too much air. I ordered a Samco silicone inlet but am forced to keep it parked for now. Any input would be greatly appreciated.


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## jeeprnovru (Jun 24, 2011)

What you posted there made little to no sense. We need a more clear and concise explanation. And yes a CAI will allow you to hear your DV.


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## BoosTT 315 (Jan 29, 2012)

At 3300rpms the boost drops from 15psi to 3psi. And it feels like it is starvig for air, the car has very little acceleration. I'm under it at the moment lol. I haven't noticed any hoses disconnected.


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## BoosTT 315 (Jan 29, 2012)

*Fixed*

My theory was right, it was the stock turbo inlet hose collapsing. Which is quite a hassle to get on and off but it's fixed and is running like a champ. Apparently the stock plugs weren't gapped enough either so I replaced them with irridiums @ .028. Good luck to everyone else.


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