Hi folks, this is my first post so appologies if this post is in the wrong place.

I have just bought a 2002 Octavia 1.9TDI with 85k on the clock, with FSH. Great car, however, I have noticed that there is a problem with the turbo. When the car is started for the first time in the morning, the car appears to lack power and the turbo does not fully kick in. Before the engine gets to optimal temperature, if i floor the pedal, limp mode occurs. This is then sorted by recycling the ignition. After this, the car runs perfectly for the entire day and, the only time this problem occurs again, is when the car is left to go cold again. The same sequence then applies. The car often judders (like a misfire) and slowly (relatively speaking) reaches towards the redline. I may also be worth poiting out that, only for the first time in the morning, when starting the car, a funny 'squark' sounds for around two seconds - it does not appear again for the day. I have checked many posts here and everywhere and many point to the MAF. I changed this and it does run better but has not solved the problem. Sticky vanes have also been mentioned as well as NG75 or EGR(?). I have my own diagnostic kit which shows no error code. Does anyone have any idea what is going on here? My brain is starting to ache now and may well soon turn to mush.

Thanks folks.

If it only does this when cold, I would suggest the CTS may be at fault. EGR itself won't be the problem, but does nothing to help (ie. putting dirty air back into the engine).

The n75 controls the vanes on the turbo, but if this only happens when cold, I think that can safely be ruled out. The CTS has two sets of feeds - one for the gauge on the dash, and one set for the ECU. The older CTS were black in colour, the newer ones are green and cost about £20-£25 from a dealer. If changing it get a new o-ring. If you are skilled/and quick in your change, then you will only have a few drops of coolant come out, so need not worry about topping the coolant back up.

The squark is probably the starter motor failing to disengage quick enough. You can drop the starter motor out and clean it up to resolve that issue.

When I first had the sticky vane problem on my car, it was worse with a cold engine and less frequent when hot. Typically, it would go into limp mode as I accelerated down the on-ramp onto a motorway style A road in the morning (cold engine) and later in the day would quite happily go to 4000 revs. It took about 2 to 3 months before it got to the point where uphill acceleration would also trigger limp mode and from then on got worse quite quickly.

I hope yours is something simpler. Does anyone know what else is likely to cause overboost trip to limp mode other than sticky vanes?

Mike

I hope yours is something simpler. Does anyone know what else is likely to cause overboost trip to limp mode other than sticky vanes?

I started a whole thread on this subject a week or two back....., under the title of "Overboost - how does it know" (or something similar) to try and work out what causes it.

In essence the MAF and MAP sensors along with the duty cycle of the n75 will be the inputs into what might trigger an "overboost" being detected. I have a feeling a slight vacuum leak can also cause problems, as then the duty cycle of the n75 won't actually apply the vane position the ECU is requesting, which then causes an ECU knicker-twisting moment.

Also the return air feed to the airbox if blocked can cause problems, as then the n75 can't release the vacuum quick enough, so the vanes don't move back as quickly as expected, which can potentially cause a problem.

I started a whole thread on this subject a week or two back....., under the title of "Overboost - how does it know" (or something similar) to try and work out what causes it.

In essence the MAF and MAP sensors along with the duty cycle of the n75 will be the inputs into what might trigger an "overboost" being detected. I have a feeling a slight vacuum leak can also cause problems, as then the duty cycle of the n75 won't actually apply the vane position the ECU is requesting, which then causes an ECU knicker-twisting moment.

Also the return air feed to the airbox if blocked can cause problems, as then the n75 can't release the vacuum quick enough, so the vanes don't move back as quickly as expected, which can potentially cause a problem.

did you manage to get to the bottom of your power concerns?

Thanks for the advice folks, however, how would the CTS cause the 'probable' overboost which causes the limp mode? Also, how would recycling the ignition sort the problem out? Sorry if this sounds dumb....I am just not of the mechanical mind! Sometimes I am convinced that it is the vanes sticking, however, once limp mode has come and gone, the turbo works great for the rest of the day when the car is in regular use. Other times, I think it might be the NG75 not working correctly as (I think) this can cause the turbo to overboost somehow. I'm very bewildered by this, particularly as I have only had the car three or four weeks.

Cheers folks for any advice.

Not really sure how the CTS fits into the equation to be honest, but it is often mentioned as a possible cause for lack of power. Otherwise maybe once you turbo has warmed up it changes shape slightly and then is no longer sticky.... However if that is the case, it will only get worse!

The normal overboost issue which then causes limp-home mode is a temporary fault, even though logged by the ECU. Thus when the ignition is turned off and back on again, the limp is forgotten about till it happens next. I could quite happy get several limps in one drive home from work - I would cycle the ignition as soon as it happened and keep on motoring till it next happened.

The n75 controls the amount of vacuum sent to the actuator on the turbo to move the vanes. If the vanes are sticky, then then duty cycle of the n75 is will set by the ECU given then requested amount of boost based on the throttle position, gear/speed and air intake (MAF). These values via some magical ECU calculation should then give then the MAP (Maninfold Absolute Pressure). If this calculated value differs too much from what the MAP is reporting then something is amiss, the ECU can engage limp-home mode if too much boost is detected.

Steve: I think my turbo issues are now down to a vacuum leak (now with improved bodge!). I booted it a few times on the way home last night, and she was a good girl (even made some nice black exhaust for the poor suckers behind me when I pulled onto the A303 from the A37 north of Yeovil (sorry to the 4wd who was behind me at the lights )

Sounds like it could be the dreaded 17705 code. :( I know Deano79 states he has diagnostics, but what he describes is exactly what I experienced on a MKiv Golf Anniversary which kept throwing 17705. Is your diagnostics Vagcom?

Edited 14 February, 201115 yr by pauldazzle

Beggars belief....my car today has now encountered the same problem with the engine WARM and the car ignition having already been recycled. Turbo spluttered and jerked and made a funny sucking/squarking noise for a few moments, only corrected with the ignition recycled (again!). Does this now sound like a fault with NG75 or sticky vanes? My hoses all look great (the ones that I can see) and I cannot hear any air escaping under the bonnet. My hand-held RAC diagnostic meter shows no error code either before or during a 'turbo or limp home moment'. Suddenly, my old car and mountain bike look more appealing ;-)

Again folks, does this point to anything more specific? Also, just out of curiosity, how much does an NG75 cost?

n75 not ng75.

Before buying any replacement valves, disconnect the vacuum pipe to the actuator on the turbo and suck down the pipe and see if you can get a nice smooth movement on the turbo. If there is no movement, or very sticky then you have sticking vanes[1] in the turbo. When sucking on the pipe, if you can get your hand down to the turbo it would be a bonus, but only did they when the turbo is cold, otherwise you might burn your fingers.

When the turbo stutters and you hit limp mode is that when you are booting the car up a hill (this is the best way to cause it)? Or does it do it on the flat under gentle acceleration?

[1] either a build up of rust on the control side, or a build up of carbon/exhaust on the vanes.

Beggars belief....my car today has now encountered the same problem with the engine WARM and the car ignition having already been recycled. Turbo spluttered and jerked and made a funny sucking/squarking noise for a few moments, only corrected with the ignition recycled (again!). Does this now sound like a fault with NG75 or sticky vanes? My hoses all look great (the ones that I can see) and I cannot hear any air escaping under the bonnet. My hand-held RAC diagnostic meter shows no error code either before or during a 'turbo or limp home moment'. Suddenly, my old car and mountain bike look more appealing ;-)

Again folks, does this point to anything more specific? Also, just out of curiosity, how much does an NG75 cost?

You need to get someone with Vagcom on to this as a problem that puts the car into limp will throw a code. Your generic RAC obviously can't read your ecu fully.

Doh! I did mean N75 valve, honest :-)

I can get limp mode if I floor the pedal uphill or on the flat when there aren't too many revs and the turbo has to build up speed, it then limps out towards the end of the rev range. Again though, this will happen only once if i then recycle the ignition and continue to drive the car; I can floor it in any gear and it just will not happen reoccur, unless the car has been turned off for several hours. I had a feeling that mt diagnostic tool was a but of a simpleton. I actually have AA 5 star 12 months warranty on the car, however, i have to foot the bill for any diagnostics or bench work and many posts claim that a code is often not shown.

Thanks for the time guys, it's very much appreciated.

n75 not ng75.

Before buying any replacement valves, disconnect the vacuum pipe to the actuator on the turbo and suck down the pipe and see if you can get a nice smooth movement on the turbo. If there is no movement, or very sticky then you have sticking vanes[1] in the turbo. When sucking on the pipe, if you can get your hand down to the turbo it would be a bonus, but only did they when the turbo is cold, otherwise you might burn your fingers.

When the turbo stutters and you hit limp mode is that when you are booting the car up a hill (this is the best way to cause it)? Or does it do it on the flat under gentle acceleration?

[1] either a build up of rust on the control side, or a build up of carbon/exhaust on the vanes.

See my post below pls - also, it never happens under gentle acceleration but can cause the turbo to jutter/misfire feeling, initially. Cheers

I have not really seen mine limp on the flat. I think you need to to check to see if you have a smooth vane movement - with the test I described above.

Then find someone local to you with vagcom and get them to scan car for you. There is a list of members with vagcom who are willing to help/scan.

Cost of the n75 is probably around £35, the last one I bought was about 4 years ago.