Vrs overboosting

[Delcac]

In a MKI vRS?

Yep, look at the sig pic.

[martziniuk]

Ok i would like to try this fix. I need to first find the vacum canister any pics?

How much did they charge you for the pug cannister.

Edit: do you mean the vacuum reservoir which is the flat plastic tank.You know the one that gets in the way when changing the spark plugs.

[idontbelieveit]

yeah, its the black vac store on the head. monted on top of a coil pack with a nut and black pipe leading to it. mine cost a quid. I'm now looking for something that will be a better fit. I suggest paying your local scrappy a visit. sure you will find something.will be intersting to see if it helps.I found it also worthwhile to pressurise the system to 15 psi and check for leaks. I did this with a tyre inflator fastend to the vac pipe which connects to the intake manifold. once pressurised i checked it held the pressure for a few seconds or so. if there is a leak it will show up as a hissing as you apply air or it won't hold the pressure. I chose 15 psi since this is around my max boost pressure.

[martziniuk]

Its certainly worth a try.

Did it completly cure it or just make it better?

I will certainly have a look.I take it your engine cover won't fit over the reservoir any more.

[Angry Dog]

this guy reckons the N75J valve cures the stuttering.

N75J Valve

The black box ontop of the engine - thats the same size as mine - trying to find what cars may have a larger one.

[Angry Dog]

VWvortex Forums: Bypassing the N249 - An Easier Approach

more info - why is this not recommended?

[martziniuk]

this guy reckons the N75J valve cures the stuttering.

N75J Valve

The black box ontop of the engine - thats the same size as mine - trying to find what cars may have a larger one.

I thought it was generally accepted that n75j version usually does not cure the problem and most then go back to the f version.

Mike i have ETKA parts system for the audi,skoda,vw and seat.I looked this morning at the s3s cannister which looks completly different to ours also does not say the size of the cannister.

So i am inclined to go to a scrappy and get one then maybe remount as it prob won't go under the cover

[Angry Dog]

I thought so too Mart.

Same about the N249 valve - seems its possible?

Ah, the S3 is different! Though it too suffers the issues..

[idontbelieveit]

VWvortex Forums: Bypassing the N249 - An Easier Approach

more info - why is this not recommended?

OK... here is my explanation

Revision 1 engine does not have n249

Revision 1 engine uses N75 and wastegate acctuator to regulate boost at the desired level, as specified in the ecu map.

Revision 2 is different:

Revision 2 also uses the n249 to regualte boost via the Diverter Valve, that is why it must be recirculating type and not atmpospheric dump. The n75 is important too, since when the turbo starts to produce pressure which are in excess of what the DV pressure regulating system can handle, it will open and divert the exhaust gas....

Revision 2 uses the n249 because it can keep the turbo spinning at higher revs thus less lag. Thats because exhaust gas is not wasted bypassing the turbine. How does it do this?

Well, when the pressure sensed by the pressure sensor on the intercooler top reaches the requested pressure (eg 14 psi) it then pulses/opens the DV.

This releases pressure back into the intake after the air mass meter, with turbo producing higher boost than requested continuely. Thats is until it pulses the n75 and wastegate actuator.

Instead of regulating ALL the pressure using the turbo wastegate, it regulates some of the pressure using the DV.

During boost, the N249 puleses on and off, like the N75. But it has no vacuum to lift the diaphram or piston in the DV, since the inlet manifold is at 14 psi. Thats where the vac cannister comes in.

When the n249 pulses, there is obviously x volume of air at 14psi between the n249 valve connections and the top of the DV. The air contained within these pipes has to go into the vacuum chamber. Thus, when whoever designed the canister, they were given x volume of gas at 9PSI (standard).

By adding an extra 4 psi, you will need 1.5 times minumum of vacuum capacity. Hence bigger cylinder.

Try this first, this will determine if the fault is with dv regulating system:

See if your car surges when the ecu beahves like revision 1:

to do this, you have to trust physics and laws of pressure.

Remove the vaccum feed from the DV, pull the pipe off the top on a standard DV

Insert an object into the removed rubber pipe that will prevent a leak, eg a bolt.

Take the car for a spin. Not far and dont hammer it too much. take it all the way to red line and see if you surge still.

This trick is how i proved it was the N249.

Explanation:

Since removing the dv vacuum tube will fool the ecu, it will "know" the DV is not working, and will regulate only using n75...like revision 1. The ecu will store a fault code, mechanical failure of dv. it will then initiate a backup plan, regualting boost soley through n75. This fault code will NOT appear if you bypass the n249 valve! Your surging bacuse the n249 is trying to open to regulate boost! when your ecu sees it didnt have enough vacuum, n75 opens and it all goes lumpy, all the ways through the rev range.

what happens when i let off, where will the pressure go in the system?

Providing you have soemthing simular to a standard dv in dv spring terms, the system is designed for this kind of fault. you will hear the wastegate fluttering when you let off.

If your car runs smooth during this setup, you've cracked it, then get a bigger vacuum can.

then try this:

1. locate a compressor for inflating tyres

2. locate vacuum hose on underside of intake plenum, its a short black tube leading to the valves, NOT the fiberous tube leading to fuel pressure reg.

3. inflate system to 14PSI, check for hissing, especially around the DV. leave 30 secs.

4. If hissing found fix fault.

please let me know your findings.

jay

[Angry Dog]

frankly, why VAG decided this was a good method is beyond me!

[Angry Dog]

im gonna give it a go this weekend.

man i cant wait to junk the VAG ECU!

Thanks Jay.

[idontbelieveit]

im gonna give it a go this weekend.

man i cant wait to junk the VAG ECU!

Thanks Jay.

Try the above tip. takes 5 mins. make sure its standard setup, by that i mean ur not bypassing the n249. block the original fitted DV vac feed by removing it from top of DV and instering tight fitting object (no jokes please! lol) make sure there are no loose pipes, eg if you did bypass n249 you will have to seal off your bypass pipe.

Drive

did it work?

[Angry Dog]

if the dv isnt under vacuum, surely its bleeding off boost! the vacuum keeps it closed.

[richm]

something is botherig me about all this...

If the dumpvalve is used to a degree as a boost regulator in normal operation, and the shortcoming is the lack of vacuum that is capable of being stored for DV operation under positive manifold pressure conditions, this would imply that the DV would be lazy opening which should cause an excessive boost spike ABOVE the nominal target figure.. a lot of people seem to report boost DROP during the "surge" condition which doesn't make sense.

During measurements I have made, comparing target boost with actual boost, I've never seen exccessive boost spikes - does anyone have any measurement evidence to support the idea of getting excess boost spikes above the expected level?

[idontbelieveit]

if the dv isnt under vacuum, surely its bleeding off boost! the vacuum keeps it closed.

Nope, other way round. vaccum lifts the psiton/plunger up.. this realeses gas from turbo.

Its only under vacuum when you let off the gas, or when n249 wants to lift it. When you press the gas and boosts hits in, and x psi is in the intake, this pressure also goes to the DV to push it down, so it doesnt leak.

Plunger up (vaccum = idle, overrun, let off gass, n249 regulating boost using vac can.) is turbo pressure out

Plunger down (x psi, boost condition, n249 shut) pressure is held in.

[idontbelieveit]

something is botherig me about all this...

If the dumpvalve is used to a degree as a boost regulator in normal operation, and the shortcoming is the lack of vacuum that is capable of being stored for DV operation under positive manifold pressure conditions, this would imply that the DV would be lazy opening which should cause an excessive boost spike ABOVE the nominal target figure.. a lot of people seem to report boost DROP during the "surge" condition which doesn't make sense.

During measurements I have made, comparing target boost with actual boost, I've never seen exccessive boost spikes - does anyone have any measurement evidence to support the idea of getting excess boost spikes above the expected level?

It drops suddenly because the n249 is for fine adjustment. when the ecu realises it cant adjust boost this way, it quickly opens n75 and wastegate, hence boost drops. I should have started this thread with a different title.... underboosting?

like this:

foot down 2nd gear, revs climb, boost climbs , hits desired boost level x psi, cant regulate using n249, open n75, boost drops. looped from 3 thousand rpm upto redline.

The n249 can drop quite alot of boost if it wanted too, eg if wastegate failed. thats because the intake pipe to the turbo is a vacuum, when the DV opens it "sucks" the boost into that vacuum faster than the turbo can produce it... Hence dump to atmospher is no option. Relies upon vacuum in intake to rid itself of boost..

[martziniuk]

something is botherig me about all this...

If the dumpvalve is used to a degree as a boost regulator in normal operation, and the shortcoming is the lack of vacuum that is capable of being stored for DV operation under positive manifold pressure conditions, this would imply that the DV would be lazy opening which should cause an excessive boost spike ABOVE the nominal target figure.. a lot of people seem to report boost DROP during the "surge" condition which doesn't make sense.

During measurements I have made, comparing target boost with actual boost, I've never seen exccessive boost spikes - does anyone have any measurement evidence to support the idea of getting excess boost spikes above the expected level?

Heres mine as you can see the boost flies over the target figure then drops and eventually levels out.

ImageShack - Hosting :: block115kt2.png

[richm]

I don't buy the idea that the n249/DV is the primary boost regulator.. the n75/wastegate is the primary regulator mechanism, with the n249 capable of opening the DV whilst under boost purely as an overboost protection mechanism. The n75 provides a proper pulse width modulated pressure control mechanism to the wastegate, to allow for mapped boost characteristic under varying operating conditions.

The dumpvalve + n249 + vacuum canister mechanism is much too crude and coarse for target boost control.

Also the turbo intake is at essentially atmospheric pressure, NOT a vacuum - apart from the small potential restriction attributable to the airfilter and MAF meter at high flow rates incurring a barely measurable pressure drop.

The dumpvalve needs to be recirculating rather than atmo venting to avoid losing pre-metered air from the system and make a fuel-rich transient condition.

[richm]

Mart, I wouldn't call that "flying over" the target boost... the one largest individual reading right at the point where the boost is rising fastest is only 7-8% higher than it should be, to the tune of 160mbar, apart from that it all runs pretty close to what it should be.

What gear was that run done in out of interest? i'm guessing it was third maybe? I suspect it would see less variation than that in higher gears when it's loaded fully.

[martziniuk]

Yes it was done in third and it is hardly noticable in that gear, 4th is worse and fifth feels worse still.

I just blocked up the dv vac hose and took for a spin.Holy cow the hesitation has completly disapeared and no engine light came on.The car did feel slightly down on power though is this normal?

[martziniuk]

Mart, I wouldn't call that "flying over" the target boost... the one largest individual reading right at the point where the boost is rising fastest is only 7-8% higher than it should be, to the tune of 160mbar, apart from that it all runs pretty close to what it should be.

What gear was that run done in out of interest? i'm guessing it was third maybe? I suspect it would see less variation than that in higher gears when it's loaded fully.

I have boost plots through the gears which show 560 mbars over target boost:eek:

[vRSLen]

Yep, look at the sig pic.

Nice one Des, Could not see it on my 12" screen at home

[idontbelieveit]

I don't buy the idea that the n249/DV is the primary boost regulator.. the n75/wastegate is the primary regulator mechanism, with the n249 capable of opening the DV whilst under boost purely as an overboost protection mechanism. The n75 provides a proper pulse width modulated pressure control mechanism to the wastegate, to allow for mapped boost characteristic under varying operating conditions.

The dumpvalve + n249 + vacuum canister mechanism is much too crude and coarse for target boost control.

Also the turbo intake is at essentially atmospheric pressure, NOT a vacuum - apart from the small potential restriction attributable to the airfilter and MAF meter at high flow rates incurring a barely measurable pressure drop.

The dumpvalve needs to be recirculating rather than atmo venting to avoid losing pre-metered air from the system and make a fuel-rich transient condition.

The n249 theory i have given is just that, a theory, it has fixed my issue. I started looking deeper into the n249 because people were bypassing it and still having the same issues. It was when i removed the DV vac connection that i noticed a change. The car stopped surging. The n249 was apperently the answer to preventing the car blowing the DV diaphram, since there is only a limited vacuum rather than a constant as produced when you let off the gas. I dont know if you can check the duty cycle of the n249 on vag com, but mine was receiving a pulsed voltage during boost periods, measured with dmm.

as for DV dumping to a vacuum, i totaly agree, it is infact atmospheric pressure. Equally, the contained gas pressure is still drawn from the DV since the pressure of the atmospher is less than that contained in a turbo system.

still, will be interesting to see if it cures anybodys surging issues.

[richm]

I have boost plots through the gears which show 560 mbars over target boost:eek:

hmm.. fair play, that's a bit different :confused:

with the DV vac pipe disconnected, it's quite feasible that the boost pressure on the bottom of the valve is enough to overcome the spring preload and start lifting it and effecting a boost leak. Normally there is equal (boost) pressure applied to the top of the valve as the bottom, PLUS the spring pressure holding it shut.

The fact that disconnecting the DV and taking away the peculiarities of n249 behaviour, improves boost stability noticeably, is an indication to me that the DV/n249 isn't the primary boost control, but more of an interference at the wrong time.

We are probably all agreeing I think that the n249 behaviour is responsible for the boost instabilities, which become more acute as boost levels rise with remaps etc.

[martziniuk]

I must say its good to have two sides to this thread.You both seem to know an awful lot about the boost issues with these engines.

Hopefully we can help an awful lot of people who have this problem,not just on this forum but the entire 1.8t vag range.

I will certainly be trying a larger cannister.