Vrs overboosting

[idontbelieveit]

Hi
I have a ocativa Vrs it has a surging and overboosting problem and its driving me mad!.

The symptoms are during acceleration the car seems to surge all the way through the rev range from 3000 up to redline.

When looking at the boost gauge, boost shoots to 14psi and back to 9 psi in a continous loop.

After hours of teadious testing and fault finding, the only way i can cure the fault is by removing the vaccum hose from the diverter valve.
This throws a fault on the car ecu. (DV mech failure). It seems the car only works propley with this fault active!

I have tried bypassing the n249, which is working fine, with no improvement, tested the N75 and air pressure sensor. no leaks on any hoses.

I'm lost as to what to check next!

Your help is much appreciated.jay

[Beretta]

Just use the search button, and see the threads already running about this problem and stuttering.

[idontbelieveit]

I ve found a few discussions, but these are all chipped. My car is totaly standard (well least i hope it is!)

I can't seem to find a scenario wherby removing the vaccum hose to the DV is the only (temporery) fix!

I'll keep searching!

Thanks

[ethos]

If it's peaking at 14psi it's not standard

9psi is standard boost.

[idontbelieveit]

When the car hits 14psi it then drops to 9 psi

This causes a surging sensation through the rev range

When vaccum pipe to DV is removed, it boosts to 9psi and it doesnt surge at all. It boosts up and holds it at 9psi, all the way through the rev range.

I just wish it did this without having to bypass the DV!


A question
If a vrs has been remapped does it still keep the facory settings for limp mode? im wondering if it has been chipped and this is causing my problem. when i disconnect the vac line to dv, it causes a fault code on ecu.
Fault is "DV mech failure". Will the ECU continue to operate normally, or will it revert to another "map" to work around the "dv failure"?

thanks

[Beretta]

Originally Posted by idontbelieveit

When the car hits 14psi it then drops to 9 psi

This causes a surging sensation through the rev range

When vaccum pipe to DV is removed, it boosts to 9psi and it doesnt surge at all. It boosts up and holds it at 9psi, all the way through the rev range.

I just wish it did this without having to bypass the DV!


A question
If a vrs has been remapped does it still keep the facory settings for limp mode? im wondering if it has been chipped and this is causing my problem. when i disconnect the vac line to dv, it causes a fault code on ecu.
Fault is "DV mech failure". Will the ECU continue to operate normally, or will it revert to another "map" to work around the "dv failure"?

thanks

Are you running the OEM dump valve ????

If so then it may be this that is now playing up as you say it boosts correctly when you disconnect it, have you tried another dv ???

[idontbelieveit]

I have checked the DV

I've swapped it and it has made no difference.

It did seem like that kind of fault, boost escaping from DV but ive proved that not to be the case.
Its almost like disconecting the dv causes an ECU fault condtion, and whilst the ecu operates to compensate for the loss of the DV the car works properly.

Does anybody know where the ecu is located?

[martziniuk]

Ecu is under the window wiper covers, about middle way across.
Are you sure there are no leaks in the pipes from the turbo through the intercooler to the throttle body?

[richm]

do you have access to vag-com?
If you monitor the target boost value and actual boost as comparison, that will tell you if it has been remapped
If it is standard, it sounds like a N75 valve problem to me, or possibly blockage in one of the n75 hoses which is interfering with boost control?

[idontbelieveit]

Ok
I have checked for leaks on the hoses leading from the turbo to TB
No leaks there

I do have vag com, im yet to get a full version. I am unable to view the group for requested turbo pressure.

I read somewhere that the N249 valve is also used for regulating boost as well as the turbo actuator (via n75).

I am going to re test the n249 along with the vaccum store. I am wondering if the extra boost (if remapped!) is to much for the n249 and vaccum store to handle, as it switches between positive pressure from the inlet manifold and vaccum from the storage canister.

I will post results up here.

I think by removing the vaccum line to the DV, the car uses the N75 to regulate turbo pressure via the acctuator. Hence it all works ok when the DV vac line is disconnected.

[idontbelieveit]

Ok i have found a fix for my overboosting, surging and generally annoying problem.

I calculated that the vaccum canister mounted on the cylinder head is not capable of delivering enough vaccum during boost.
This is because the cannister is designed to work upto 10 psi (the available vaccum it can hold is overcome when boost is higher than the standard 9psi, since it has to "absorb" the boost leading to the DV as well as provide a constant vaccum for boost regulation via the N249 valve)


I wonder if this will help anyone who also has a surging issue. Having spoken to friends who don't have this problem, it is evident they all have revision 1 1.8t's with no n249 or vac store. i'm yet to find someone with a revision 2 engine, like mine in my VRS, who has a surging issue who could possibly also try this to see if it helps:
Go to a scrap yard, buy a vaccum canister double the size of the standard OEM, fasten the vac pipe to it. Now you have a much larger "resivour" of vaccum. Now, when the n249 valve switches over to the vaccum store, it will "absorb" the air in the DV system, plus provide a vaccum. Unlike the OEM which absorbs the air in the system, then has no vaccum left to regulate boost with, via the N249. Hence, i overboost, my n75 kicks in, i loose boost, turbo kicks back in, i overboost, n75 kicks in, i loose boost... in a loop!


I'm not saying this will fix everyone elses problem, but certainly fixed mine!


It turns out my car is remapped

thanks for your suggestions
jay

[martziniuk]

How do we tell if its a revision 1 or 2 engine?
And what car was the oversized canister from?

[idontbelieveit]

Hi
I'm not sure of all the diffferences, but the revision 1 does not have a n249. The n249 is mounted just under the dip stick on a metal plate. it has lots of black tubes going to it, three i think in total.

The vaccum cannister is the black box mounted on top right of the cylinder head, with a single black pipe coming from it.

I used a cannister from a diesel pug. Not sure which model. I was just looking for anything larger in capacity than the standard.

Hope this helps

[V666 VRS]

Usefull info this. Well done mate.

Darren

[fatty5000]

Originally Posted by idontbelieveit

Ok i have found a fix for my overboosting, surging and generally annoying problem.

I calculated that the vaccum canister mounted on the cylinder head is not capable of delivering enough vaccum during boost.
This is because the cannister is designed to work upto 10 psi (the available vaccum it can hold is overcome when boost is higher than the standard 9psi, since it has to "absorb" the boost leading to the DV as well as provide a constant vaccum for boost regulation via the N249 valve)


I wonder if this will help anyone who also has a surging issue. Having spoken to friends who don't have this problem, it is evident they all have revision 1 1.8t's with no n249 or vac store. i'm yet to find someone with a revision 2 engine, like mine in my VRS, who has a surging issue who could possibly also try this to see if it helps:
Go to a scrap yard, buy a vaccum canister double the size of the standard OEM, fasten the vac pipe to it. Now you have a much larger "resivour" of vaccum. Now, when the n249 valve switches over to the vaccum store, it will "absorb" the air in the DV system, plus provide a vaccum. Unlike the OEM which absorbs the air in the system, then has no vaccum left to regulate boost with, via the N249. Hence, i overboost, my n75 kicks in, i loose boost, turbo kicks back in, i overboost, n75 kicks in, i loose boost... in a loop!


I'm not saying this will fix everyone elses problem, but certainly fixed mine!


It turns out my car is remapped

thanks for your suggestions
jay

If your right your a genius

[ethos]

I'm guessing mine will be revision 1 as it's 2001?

Anyone know what the difference is?

[delcac]

You can tell if it's a faulty N249 by bypassing it. I could tell you how to do this but I would have to kill you.

[idontbelieveit]

mine is 2001 an is revision 2 in 1.8t engine terms so I'm told.best thing to do is to check at the very front of the engine on the intake manifold. below the dipstick hole there is a metal shelf. mounted underneath that shelf there should be 2 valves with black pipes connected to them. you will have to look carefuly as they are mounted in a way you wouldn't necessarily see them. hope this helps

[idontbelieveit]

Originally Posted by delcac

You can tell if it's a faulty N249 by bypassing it. I could tell you how to do this but I would have to kill you.

you mean run a hose directly off the intake mainfold to DV, and block off the exsisting hose?

You could do that, and i did. I still had slight surging as i said above.

By bypassing this valve you run the risk of turbo and engine death... so it might be your engine that you kill!

This is because if your turbo acctuator fails or any of the hoses connected to it, the car opens the dv (Using n249 and vac store during boost) so it doesn't over boost, thus killing your car, turbo and whatever else.

Its a safety feature as well as a DV diaphram saver the old N249.

[delcac]

Originally Posted by idontbelieveit

you mean run a hose directly off the intake mainfold to DV, and block off the exsisting hose?

You could do that, and i did. I still had slight surging as i said above.

By bypassing this valve you run the risk of turbo and engine death... so it might be your engine that you kill!

This is because if your turbo acctuator fails or any of the hoses connected to it, the car opens the dv (Using n249 and vac store during boost) so it doesn't over boost, thus killing your car, turbo and whatever else.

Its a safety feature as well as a DV diaphram saver the old N249.

To be honest the whole set up with the N249, DV and expanion tank cannot cope with big boost levels hence the boost spikes. The expantion tank is just not big enough however, there is a bigger one available from Audi. I would put money on this being the problem.

Oh, the by pass is for checking the N249 only, not for everyday use.