Login | Register
SearchMy Stuff
My ProfileMy PreferencesMy Mates RSS Feed
2 3
Reply to Topic
Author Discussion

Ipswich Jules

Original Poster:

13 posts

52 months

[news] 
Tuesday 20th July 2010 quote quote all
Hey people, just signed up for these forums as they seem pretty useful

I've got a bit of a boost problem with a Audi A3 1.9 TDi Sport (110bhp, W reg, 2000) I recently bought, it seems a fairly common problem from a quick Google search but one which can have many diffrent solutions!

The car is fine and boosts as (I think) it should, but once at about 70mph (3k rpm or so in 5th) the boost seems to vanish which can make for some pretty tricky moments on the motorways!
If I stop, turn the engine off and back on again the boost is there again but it doesn't stay and is soon back to it's astmatic ways

I'm not getting any lights on the dash to indicate a ECU problem - but I wonder if the ECU light even works? Should it come on as part of the ignition sequence when most of the other dash lights come on/go off before getting the OK to start, maybe the seller had no morals and rather than fix the problem (which I got money knocked off for anyway) decided to try and hide it? Personally I don't mind buying a car with a fault but would rather be told about it, but that's by the by......

Any help you chaps/chapettes can give would be great!

Cheers

Jules

galenthe2nd

59 posts

64 months

[news] 
Tuesday 20th July 2010 quote quote all
To me that sounds like you could have a faulty temperature sensor in the exhaust. Its telling the ECU that the turbo is getting too hot, and in turn opens the waste gate and you loose all boost. Turning off the car resets this and all will work fine again.. This is the first thing id look at.
This used to happen on my old 1.9tdi which was rempped and would happen when i had the revs up close to 4k on a hard trashing. It's there to save the turbo, but yours does sound like a fault..

Tame Technician

2,358 posts

91 months

[news] 
Tuesday 20th July 2010 quote quote all
What you are experiencing is limp home mode. There are a number of things that can cause it, so you need the fault codes read to give you a clue what it is.

Could be an N75 or vacuum fault - underboost

A turbo fault - overboost

Or a number of other sensor issues such as boost pressure sensor.

Reading the fault memory is the first thing.

The glow plug light and engine light should be on at the ign on bulb check, so maybe someone removed the bulbs, it does happen.

Ipswich Jules

Original Poster:

13 posts

52 months

[news] 
Tuesday 20th July 2010 quote quote all
Thanks people, will do a bit of investigating about the possible missing ECU bulb on my days off

Tame - these possible sensor faults I guess would cause the ECU light to be on all the time I would assume aslong as the sensor (whichever one) is reading incorrectly?

I guess the best way to check this out if via the VAGCOM that everybody seems to talk about?
That's something else I'll investigate, I downloaded the free shareware version and will get a lead - or will I need the full monty version do you think?

Cheers once again for the help - got more response in one day here than on a certain well known Audi forums....

Cheers

Jules

Tame Technician

2,358 posts

91 months

[news] 
Wednesday 21st July 2010 quote quote all
Any half decent garage will be able to read the fault memory for £30 or so. Dont know if thats cheaper than the VAG COM lead.

Your local audi centre will also be able to do this, and with there specialist knowledge and a bit of further investigation no doubt pin point the fault quickly.

Most audi centres will read the codes and do a preliminary investigation for half to an hours labour. About £50 - £100

You will have to pay for the diag, but Once they tell you what it is, you dont have to have it fixed at there labour rate and could repaire it your self etc.
Advertisement

Ipswich Jules

Original Poster:

13 posts

52 months

[news] 
Wednesday 21st July 2010 quote quote all
Thanks again Tame Tech, just to clarify - the ECU light SHOULD come on (for a second or two?) and then go off before the big "OK" comes up to start the car?

Cheers

Jules

Tallbut Buxomly

12,254 posts

103 months

[news] 
Wednesday 21st July 2010 quote quote all
Ipswich Jules said:
Thanks again Tame Tech, just to clarify - the ECU light SHOULD come on (for a second or two?) and then go off before the big "OK" comes up to start the car?

Cheers

Jules
I dont think you have a light as i have a 2001 a4 tdi and have never noticed the light though could be lack of observation.
Suspect boost control sensor.

TDIfurby

1,863 posts

62 months

[news] 
Wednesday 21st July 2010 quote quote all
I have vagcom if you need it. But I am down in Devon, end of M5

Is the car going into limp mode when accelerating hard? If so I would suggest this is overboost, i.e the car is requesting for example 1 bar, and for some reason the map sensor is seeing the turbo producing 1.5. It will shut down the turbo totally in this case. A number of things can cause overboost. DOdgy vacuum line - A split. Turbo vanes can stick in open position so not closing when at higer revs/speed and producing overboost. Sticky vanes can be checked by sticking your hand down the back of the engine and wiggling the turbo actuator (there is a vacuum hose going to it) - It should move with a lever on the turbo itself (this is where the variable vane technology is.)

If the car is high mileage, I would suspect sticky vanes first, then a faulty vacuum hose. Sadly after this, you are generally looking at a worn out, about to blow, turbo. frown

Ipswich Jules

Original Poster:

13 posts

52 months

[news] 
Thursday 18th November 2010 quote quote all
Thanks for the previous help with this people - I had the codes read (for free!) by a local VW/Audi indy and they threw up a knackered MAF, so that was replaced and all is fine......kind of!
It pulls fine until about 3k - 3.2k (in 5th) then the turbo seems to stall for a couple of seconds before picking up again and all is fine, but drop below 3k again and try to speed up and it will do it again
It's a consistant problem which hopefully should make it easier to diagnose, I bought a VAGCOM setup (from fleaBay) and after much messing about I finally got it to do something, it did read a faulty part number (I think) but won't tell me what it is or does as VAGCOM wasn't registered, I go and register it and lo and behold it's not a genuine version (or something)

Does the symptom with the boost sound familiar? Any suggestions to what it may be or anything I can try to fix/troubleshoot it?

Cheers for any help

Jules

Tallbut Buxomly

12,254 posts

103 months

[news] 
Thursday 18th November 2010 quote quote all
Boost control sensor or valve knackered and therefore causing the maf failure through dirt build up.

Ipswich Jules

Original Poster:

13 posts

52 months

[news] 
Thursday 18th November 2010 quote quote all
Cheers Tallbut, any idea where I can find the boost control valve please and roughly how much German and Swedish will rush me for a new one?

I'll give it a poke this weekend if I can

Cheers

Jules

Tallbut Buxomly

12,254 posts

103 months

[news] 
Thursday 18th November 2010 quote quote all
Ipswich Jules said:
Cheers Tallbut, any idea where I can find the boost control valve please and roughly how much German and Swedish will rush me for a new one?

I'll give it a poke this weekend if I can

Cheers

Jules
Gods that i couldnt tell you. Boost sensor is roughly 85quid fitted by audi.
There is some sort of regulator which is a round silver thing n top of the engine hard to describe it was pointed out to me on my a4 by my mech when were trying to diagnose a really odd possible altitude related boost issue.

Colonial

11,365 posts

92 months

[news] 
Friday 19th November 2010 quote quote all
N75

Mine (non diesel, but still vag turbo) had similar symptoms.

Replaced that, the diverter valve and the maf which cured it.

Not sure which one fixed it.

Tame Technician

2,358 posts

91 months

[news] 
Friday 19th November 2010 quote quote all
So it had a MAF fault code? and now the MAF has been replaced it doesnt log that code any more??

or the independant just guessed it needed a MAF??
its the norm to try that first with plug and play diag.


When MAF's fail, they often dont log a code at all, if you get a MAF fault, usually means there is a boost leak or the EGR valve isnt working proberly, which makes the MAF read out of spec.


I must admit though your description of the fault now sounds like a boost pressure sensor. If the control solonoid N75 fails, this usually means you get no boost at low rpm, the turbo should spool up arround 1400-1800rpm, with N75 not working the turbo wont spool up untill 2200-2500. (you can disconnnect it and check for a difference to rule it out)


ETA

Is it still going into limp home, MAF wont cause that or fix it.



Edited by Tame Technician on Friday 19th November 21:01

Ipswich Jules

Original Poster:

13 posts

52 months

[news] 
Saturday 20th November 2010 quote quote all
Hi Tame

Since replacing the MAF it's gone into LHM once (by LHM I'm assuming you mean where it cuts the boost all together until the ignition is toggled?), but before I changed the MAF it was doing it far more often so that has improved greatly

As I tried to describe, all seems well with the car pretty much, boost seems to come in where you describe (~1200RPM) but at around 3k RPM you can feel the car physically hold back - only for a few seconds - before it picks up again, it's like a massive flat spot, at about 3500RPM it will be back to normal.

Is the N75 serviceable in anyway or is it a replacement job? i.e can I pop it open, spray some contact cleaner (or similar) and hope for the best? Or is it like the MAF and very sensitive to that kind of intrusion?

Tame Technician

2,358 posts

91 months

[news] 
Saturday 20th November 2010 quote quote all
N75 is sealed and cant be opened up. If you get boost at 1200rpm, N75 is working fine.

You could unplug N75 and retest to confirm its working, unplugged I would expect boost to only come higer up.

Not sure what the flat spot is tbh, but it could be the boost pressure sensor mis reading, but its hard to say.

TDI engines going into limp home with N75, turbo or boost problems are quite common and reletivly straight forward to diag and fix, you just need to know if you have to much or to little boost helps figure out why.

What ist doing now however, is unusual and a bit more complicated.

If I was working on it I would want to see the air mass and boost pressure values on the diag computer screen while the car accelerates and look for abnormalities when the flat spot occurs.


nickd01

466 posts

102 months

[news] 
Saturday 20th November 2010 quote quote all
We had similar issues with a Y-reg 1.9 Tdi.
Hills on motorways would see us down to 60mph despite foot flat to the floor! It would be sprightly(ish!) to around 2.5k revs then nothing else happened.

Turns out it was a throttle body issue, and is common on these cars. A reconditioned unit was put in, and now it's like a new car.


TDIfurby

1,863 posts

62 months

[news] 
Sunday 21st November 2010 quote quote all
sticky vanes on turbo causing it to boost fine when open (from standing start) but causing overboost when trying to use boost in higher gears? This would throw up a fault along the lines of "Thrust sensor: positive deviation", which can throw inexperienced people to think the thrust sensor (i.e the map sensor) is up the spout when all it is doing is reading a fault with requested VS actual boost.

Tame Technician

2,358 posts

91 months

[news] 
Sunday 21st November 2010 quote quote all
TDIfurby said:
sticky vanes on turbo causing it to boost fine when open (from standing start) but causing overboost when trying to use boost in higher gears? This would throw up a fault along the lines of "Thrust sensor: positive deviation", which can throw inexperienced people to think the thrust sensor (i.e the map sensor) is up the spout when all it is doing is reading a fault with requested VS actual boost.
That is true if it goes into limp home often, every time you use all the rev range, pulling from low down to up high for example. The code would be as you say positive deviation, also known as overboost.

But the holding back for a bit and then pulling again, dont think you would get that with sticking turbo vains. You need ignition cycle to reset after limp home, it wouldnt just clear its self.

TDIfurby

1,863 posts

62 months

[news] 
Monday 22nd November 2010 quote quote all
I didn't see the bit about hesitation. Ooops. biggrin

Well, I have vagcom should this Audi be able to get down to Exmouth. smile
2 3
Reply to Topic