Overboost - how does it know?

[mbames]

you need to seperate the different logs really and run them thru the graph on vagcom. starting at 485.84 you appear to have a reading of 1095 mg airflow (which is high) and 2377mb of boost and continuing to 496.78 which is what has set the limp off.

it is reasonable to say it is not the map sensor playing up as you are seeing a corrosponding spike in the airflow drawn in.

have you checked the vac pipe that goes to the airbox as that dumps the vac from the actuators and if blocked will make the actuator work slowly

I'll go and check the return feed to the airbox now (pretty sure I checked it the other day, but I'll go and take the filter out too and check for a good clear route). If my airflow is high (and the filter in the car is a good 8 months old - changed before my MOT), then maybe my MAF is borked.... But I would have thought a dirty MAF would have read low rather than high?

[mbames]

The returns are fine from the n75 to the airbox. However as I was under the bonnet I thought that might as well change a few more lengths of vacuum hose over for new stuff.

When I came to replace the tap off pipe from the vacuum pump I stumbled across something..... the large pipe that comes out of the vacuum pump seemed quite loose and I was able to rotate it (as much as the other attachments on that pipe allowed). Initially I though it was the metal clip that was loose. However upon closer examination, it appears to be what ever the pipe attaches too which comes out of the metal body of the pump.

I started the car up and wiggled things about and could not hear a massive air leak, but when I turned the engine off and then wiggled that pipe I could make an obvious different in air leaking in/out from this joint. Therefore my new/additional questions are:

- could a loss of vacuum cause my limp homes

- (more importantly) is this normal behaviour for the vacuum pump or is my busted.....? :'(

This might explain why on the odd day my brakes feel better than normal.....more vacuum assistance?

Other pics here: http://www.matthewames.co.uk/briskoda/index.php?loc=./20110122_vacuum_pipes

(edit - to add pictures)

Edited 22 January, 2011 by mbames

[lumac999]

- could a loss of vacuum cause my limp homes

- (more importantly) is this normal behaviour for the vacuum pump or is my busted.....? :'(

This might explain why on the odd day my brakes feel better than normal.....more vacuum assistance?

Good question about the vacuum. But as long as this is only feeding the brake servo and nothing else couldn't really see how it could affect limps.

Dunno about the play in the joint but will check mine tomorrow and let you know.

I'm quite interested in this VVT VNT turbo problem as I have similar issues which I think are not connnected with sticky vanes and maybe more to do with other reason i.e. MAP etc. although I chaned both the N75 and MAP 4 months ago. This seemed to cure limps and didn't have a problem until a couple of weeks ago when it started again but seems to more ok than not.

Nick

Edited 22 January, 2011 by lumac999

[rwbaldwin]

1095 mg/stroke airflow is high. On a standard car it should be about 850mg/stroke at WOT. However, the airflow is only high when the measured boost pressure is also high.

From around 489 you have a problem but I can't identify the exact cause without seeing the N75 duty cycle (IIRC, block 118). Up to this point there is correlation between MAF and MAP, so I would have to say that they are both OK

Intermittant vacuum could cause a problem, but unless the non-return valve upstream of the N75 is faulty, the vacuum should be fairly stable due the vacuum reservoir (looks like a ballcock).

Edited 22 January, 2011 by rwbaldwin

[mbames]

1095 mg/stroke airflow is high. On a standard car it should be about 850mg/stroke at WOT. However, the airflow is only high when the measured boost pressure is also high.

From around 489 you have a problem but I can't identify the exact cause without seeing the N75 duty cycle (IIRC, block 118). Up to this point there is correlation between MAF and MAP, so I would have to say that they are both OK

Intermittant vacuum could cause a problem, but unless the non-return valve upstream of the N75 is faulty, the vacuum should be fairly stable due the vacuum reservoir (looks like a ballcock).

Is the non-return valve the black and white thing? As just to the left of middle in this picture?

I am familiar with the vacuum reservoir, I replaced the vacuum pipe to that when I cleaned the turbo as the clips under the vacuum pump has started to chaff through.

I am wondering if my car has long since had a problem which caused lack of vane movement, and thus caused the vanes to get jammed and make me think that was problem....

If I get a chance I will try and log 10, 11 and 118 tomorrow on a better section of road (ie.- one long blast up a hill).

[ExAudiSi]

If you do need a new MAF, there is a Pierburg unit from a Mercedes which I am told is a worthwhile upgrade for around £50.

Haven't found out yet why it is better, other than it is reputed to be far more reliable than the Bosch version.

I have asked the question, so when I get a reply I'll let you know.

[rwbaldwin]

Is the non-return valve the black and white thing?..

Yes and it is the right way around. When you are testing it remember that air always flows towards the pump.

[mbames]

Good question about the vacuum. But as long as this is only feeding the brake servo and nothing else couldn't really see how it could affect limps.

Dunno about the play in the joint but will check mine tomorrow and let you know.

I'm quite interested in this VVT VNT turbo problem as I have similar issues which I think are not connnected with sticky vanes and maybe more to do with other reason i.e. MAP etc. although I chaned both the N75 and MAP 4 months ago. This seemed to cure limps and didn't have a problem until a couple of weeks ago when it started again but seems to more ok than not.

The vacuum is also used to control the vane movements on the turbo too., there is a small fabric hose which taps off the side of the main hose to the brake servo. It is the right hand of the two marked pipes in this image: http://www.matthewames.co.uk/briskoda/20110122_vacuum_pipes/P1000888.JPG

It would be handy if you could check your joint. I let my engine idle for a minute or two, and then turned it off, and on wiggling the joint I could then hear air hissing.

I don't mind changing parts if it will cure it, but I don't want to just go blindly changing bits on the hope it might work..... I am too tight for that

[mbames]

Yes and it is the right way around. When you are testing it remember that air always flows towards the pump.

Cool, I give it a suck tomorrow.... that will impress the neighbours

I assume me blocking off the pipe for the EGR vacuum control pipe with a bolt and a jubilee clip should not have caused any problems....

[mbames]

If you do need a new MAF, there is a Pierburg unit from a Mercedes which I am told is a worthwhile upgrade for around £50.

Haven't found out yet why it is better, other than it is reputed to be far more reliable than the Bosch version.

I have asked the question, so when I get a reply I'll let you know.

Cool... and £50 sounds like a better price than a MAF from Mr VAG dealer too.....

[lumac999]

The vacuum is also used to control the vane movements on the turbo too., there is a small fabric hose which taps off the side of the main hose to the brake servo. It is the right hand of the two marked pipes in this image:

It would be handy if you could check your joint. I let my engine idle for a minute or two, and then turned it off, and on wiggling the joint I could then hear air hissing.

I don't mind changing parts if it will cure it, but I don't want to just go blindly changing bits on the hope it might work..... I am too tight for that

Yes the pic explains it.

Thinking about when my limp modes started a couple of weeks later I travelled from Nottingham to Preston.

Had the EML light on and going into limp mode, cycled the ignition and it was ok. I found that leaving it it 4th gear on the motorway (70mph +)

stopped it form going into limp mode but if i put it in 5th gear it would go into limp mode again so maybe the vacuum has something to do with it i.e more vacuum at higher revs, 4th as opposed to 5th gear.

Checked my pipe on the vacuum. spins round a bit but maybe the clip needs replacing (still on original VAG clip) but no noticable hissing. Does not seem loose other than turning round a bit. Did notice that there was some (old) oil deposits there.

[ExAudiSi]

Very slightly off topic, there seems to be quite a bit of oil under my vacuum pump. Anyone know how it is sealed to the head?

Is there a gasket or maybe an O ring behind it?

[rwbaldwin]

Very slightly off topic, there seems to be quite a bit of oil under my vacuum pump. Anyone know how it is sealed to the head?

Is there a gasket or maybe an O ring behind it?

There is an o-ring.

From memory it is part 038 145 345

[felicia16v]

a vac leak will cause underboost so the leak will noy be that prablem..

the outlet on the vac pump is pressed in so take the pipe off and try giving it a gentle tap to reseat it

[stevezuki]

mines suffering from excatly the same symptoms- 3rd gear, 3000-3500 rpm on big hill (flooring it ) the car loses power and drops back to 3000rpm (although dosnt do it on flat at 3-3500??)

just changed n75- slight power increase in lower rpm but, did excatly the same thing (on same hill!!).. so dont think n75 is the answer- map sensor sounds interesting= although im not into buying bits just to try.. might just avoid hills/3rd/3-3500rpm! :S

[ExAudiSi]

There is an o-ring.

From memory it is part 038 145 345

Thanks, thought it might be an O ring. Another job to add to the list

[mbames]

If I get a chance I will try and log 10, 11 and 118 tomorrow on a better section of road (ie.- one long blast up a hill).

Pesky laptop has stopped wanting to play when the engine is running -vagcom drops the connection to the car, so I could not log anything while goiong up a nice hill :-(

However, it has to be said that after fiddling with the pipework and having several days of driving with multiple limps, yesterday it refused go into limp mode however much I tried to provoke it (and I tried quite hard!). It did not seem quite as perky as I would expect, but not proper rubbish either!

I will remove the original vag clip on the pipework and try tapping home the outlet stub.

[stevezuki]

Pesky laptop has stopped wanting to play when the engine is running -vagcom drops the connection to the car, so I could not log anything while goiong up a nice hill :-(

However, it has to be said that after fiddling with the pipework and having several days of driving with multiple limps, yesterday it refused go into limp mode however much I tried to provoke it (and I tried quite hard!). It did not seem quite as perky as I would expect, but not proper rubbish either!

I will remove the original vag clip on the pipework and try tapping home the outlet stub.

are you talking about the outlet on the vacuum pump?

[mbames]

are you talking about the outlet on the vacuum pump?

Yep, as it seems to not be making a perfect seal.

[stevezuki]

just checked mine- excatly the same- quite a bit of movement there :S washed down joint with brake cleaner and a nice helping of instant gasket to seal.. i'll wait for it to dry and see if it improves things..

[ExAudiSi]

Cool... and £50 sounds like a better price than a MAF from Mr VAG dealer too.....

The answer I've had back is that the Mercedes MAF gives a bit more grunt at the bottom end, but can make it a bit more sooty. I'm told that the Pierburg item is more reliable than the Bosch unit too.

Merc part no. A6110960048

Pierburg MAF part number - 7.22684.00 (last two digits have probably changed).

[felicia16v]

if the N75 doesn't fix it , it may still be sticking on the control ring inside the turbo. looking at your pics it looks like you didn't scrape the rust from the casing there.

[stevezuki]

:( just noticed mines still doing it :( looks like im gonna have to strip and clean the turbo :'( really didnt want to get involved in stripping turbo's :( gonna wait for a drier/warmer day..

[PonnyJ]

hi guys, i had a similar problem on my fabia 1 20 mins after i bought it yesterday!! turned out that the diaphragm inside the turbo actuator had split therefore not allowing the wastegate to work propperly (creating overboost as it isnt opening when it should)

just a thought, may be something to check out?

[ExAudiSi]

just checked mine- excatly the same- quite a bit of movement there :S washed down joint with brake cleaner and a nice helping of instant gasket to seal.. i'll wait for it to dry and see if it improves things..

Had a look at the vacuum pump on mine today. That spigot seems to move in and out on mine too, but I didn't hear any sounds of air leaking when I moved it.