Got an Octavia SLX TDI 110BHP. It generally runs fine but on longer journeys the turbo runs a little rough and seems to sometimes pop into limp home mode.

Day to day driving causes no bother and the car generally behaves as it should.

However, on longer journeys (say Yorkshire to Cornwall for example) I've noticed that the turbo gets a bit sluggish, particularly after a long stint at motorway speeds. Boost is there but not as it is when fresh. Once it's reached this stage I just need a bit of a hill and the foot down and it's fairly easy to put it in limp mode. The succession of hills on the A30 was not kind...

Got it on a fault code reader and 1 fault was logged: 17958/P1550/005456 - Boost Pressure Control Valve (N75): Control Deviation

http://wiki.ross-tech.com/wiki/index.php/17958/P1550/005456.

So what's do you think? Issue with the N75 that gets worse when warm? Slight leak on a hose? Sticky vanes?

I might be tempted to stay sticky vanes, thinking a long stint on the motorway it gets warm but does not really move due to the fixed motorway speed.

Easiest thing to do initially is when the car is cold is to disconnect the vacuum hose from the n75 and unclip it a bit. Then stand on the drivers side of the car, and stick your arm down past the anti-shutter assembly on the EGR and feel the turbo. Locate the actuator arm, and then suck on the vacuum pipe, the arm should move in, freely and quietly. If there is sticky-ness, or it does not return smoothly the source the problem is probably either sticky vanes, or a rust build up on the control ring side of the turbo.

I bought a few metres of vacuum hose from the dealers and replaced all my pipework, as a lot of it had seen better days. Start with the cheap/free tests and work on from there. I am now torn between n75, MAF and MAP replacements, all of which are not cheap :-(

Here is mine from yesterday: http://briskoda.net/forums/topic/200068-17965-charge-pressure-control-positive-deviation/, although I am logging a slightly different code.

Unsure if I've found the problem. Found a problem anyway. The N239 had come loose from its clips and was sitting on the metal pipe below (I think EGR pipe). It's partially melted and doesn't look too healthy. Melted together with the clip on the electrical connector. I'm lucky it didn't short out. Probably would have done if not spotted now. Maybe the heat caused a leak or blockage?

One of its vacuum hoses is connected via a 't' junction to the N75 so may have had some effect on it. Going to clip it somewhere safe with cable ties for now and then check if the anti-shudder is working. If not - how quickly do I need to get the N239 replaced?

Limp mode hasn't recurred and a re-scan picked up no fault codes. No long motorway journeys since the earlier scan though.

Edited 4 May, 201115 yr by Uriel

Unsure if I've found the problem. Found a problem anyway. The N239 had come loose from its clips and was sitting on the metal pipe below (I think EGR pipe). It's partially melted and doesn't look too healthy. Melted together with the clip on the electrical connector. I'm lucky it didn't short out. Probably would have done if not spotted now. Maybe the heat caused a leak or blockage?

Mine did that too (melted to the EGR pipe) - I could smell melting plastic and then I spotted it, however no electrical meltage just the side of the n239! The clip is broken on mine, so I used a tie-wrap to hold it in place.

Well, the antishudder works but got limp mode too yesterday on a shorter journey. Maybe I disturbed something...

Mine limped on the way in to work this morning, got to about 4000rpm in 3rd and the power backed off and the limp occurred. I will go and scan it shortly and see what is has to say for itself.

Good job the anti-shudder still works, as that could potentially save your engine if ran away.

My error code for limp is at least consistent. 17958 every time.

http://65.110.12.166/showthread.php?t=3922

Interesting. User got same error code with limp mode on a VW Caddy. They then lost antishudder function. A vacuum pipe was replaced (owner unsure which one) and it restored ASV plus solved limp problems.

It does seem to suggest that a problem on the ASV side of things could cause boost pressure control deviation. Ordered a used replacement N239 off ebay and I'll take it from there. If that doesn't solve it, pipes and connection checks next then checking N75 and turbo actuator.

When test driving the other day I found limp would either be produced when accelerating to high revs up hill but also at sustained high RPM and speed uphill (all approaching motorway speeds).

OK,

Replaced N239 and took for a test drive. Long hill at 70 in 4th brought limp on. Stopped, looped round and tried the same hill again in 4th. No limp. Looped round and tried again in 3rd, nearly red-lining. No limp. Car feels nice and sprightly.

Will have to see how it goes. Might have been slightly sticky vanes and a bit of an italian tune up has helped. We'll see.

Edited 7 May, 201115 yr by Uriel

Right - started searching for a vacuum leak. Not much kit so I'm using the old 'suck it and see' method.

Started with link from N75 to actuator. Held on my tongue for a couple of minutes and on release I could hear the actuator moving (couldn't hear it when sucking initially though). Think that bit's fine.

Then went to other end. First small vacuum line that comes off near the vacuum pump. Can give it a good suck but vacuum doesn't hold on the tongue for more than a few seconds. Should it? Worth tracing it in the general direction of the n75?

According to this: http://www.myturbodiesel.com/1000q/multi/limp-mode-TDI-fix.htm boost leak intermittent probably means a vacuum leak.

Edit: Well I think something helped. Car very sprightly this morning. Maybe tightening up a few vacuum connections helped. Maybe giving the acutator a bit of excersise helped. It would be too optimistic to say problem solved but I'll give it some beans up hill next time the car is properly warmed up and see how it does.

Edited 10 May, 201115 yr by Uriel

Meh! Have limped again.

However, turned up something else. When applying vacuum to turbo actuator pipe and releasing, it takes a long time for the actuator movement to stop and it's not always smooth. Suggests sticking actuator or vanes to me. :(

Have had a bit of email diagnostic help from Greg at Small Engines in Nottingham, who I think posts here sometimes. He thinks the non-specific code readers I've been using might be logging fault code incorrectly.

Slow return sounds like sticky vanes, or a rusty control ring in the turbo. If you are mechanically minded you can pop the turbo off and clean it up....

Images for when I did mine: http://www.matthewames.co.uk/briskoda/index.php?loc=./20101112_turbo_clean

Well - exercised the actuator until it wasn't sticking.

I have a 'cunning' plan but don't want to give anyone ideas in case it doesn't work very well.

Will report back when I have results...

<awaits the outcome>

Still no comment on the 'cunning' idea. Actually managed to get in to see and feel the actuator ARM though.

Movement occurs around 13 to 18 mm hg on my vacuum pump. Can move it below 13 position with my fingers but it's stiff and creaky. I gather it should start motion at around 3-5. Essentially it looks like I can get mid to full boost position on the vanes but low boost position not available.

So either the actuator or the vane mechanism is sticky. Any way to narrow it down without taking off the actuator?

Stiff and creaky does indicate sticky vanes, or rust on the control ring. It should have a smooth movement for the whole distance of travel.

I wish I had a vacuum pump to play with

In for a service today. The mechanic is going to check out actuator and VNT movement. If it's VNT sticking I may just go for the turbo off and clean option. Tired of the limp-mode lottery every time I put my foot down at speed when going uphill.

Have tried a few non-intrusive things that have given partial / temporary improvement to limp issue but nothing that seems to really sort it out.

Edit: in progress. Car back tomorrow but mechanic has found loose boost pipe near intercooler. Still to find out about VNT / actuator. Loose boost pipe alone would be a lot cheaper...

Edited 3 June, 201115 yr by Uriel

Still limping when pushed but the pipe has stayed on. Mechanic reckons VNT is probably fine.

Somewhat mystifying...

Update on this:

Further diagnosis did reveal a definite sticking VNT issue. Realtime logging of requested boost indicated that it was a bit all over the place but mainly overboosting. Remedy was going to be turbo off and remanufacture or replace - plus I wasn't going to DIY it. Unfortunately the whole 'to do' list was longer (needed thermostat, wheel bearing, brake discs + pads replacing sooner rather than later). Tax was coming up (in the engine size rather than CO2 bracket so nearly double a 2001 car)and I had the offer of a long-term interest free car loan from a family member.

To cut a long story short, I'm no longer a Skoda Octavia owner. PXed for a 2010 Honda Insignt ES at the weekend.

Will copy this info into threads where discussion of the car's limp mode was active to bring a little closure. I guess I'll be a lot more scarce around these parts from now on.