I thought this connecting diagram for charge pressure control could be of use to people when hunting down the source of their 17705 fault.

The diagram relates to a 20v 1.8T vRS (electronic throttle control).

It is clear to see that if the leak is not from the turbo to intake pressure hoses, intercooler, or throttle body then it must be related to a faulty D.V./N249/N112 or N75, or even leaks in the hoses connecting these items.

I have a problem with abnormal D.V. actuation, and the 17705 error code. I have renewed the D.V. but the fault remains. I have occasional poor performance and load change jolts, leading me to think that the N249 is providing the D.V. with too higher vacuum. (My car is not remapped). Any thoughts?

Hope this has been of assistance to anyone with this problem.

I have exactly the same symptoms as you mate. The garage did a smoke test today but said they couldn't find anything.

Yeah it's a mystery to me!

Last weekend I took the bracket off the front of the inlet manifold and had a good look at all the hoses and connections, and checked all my turbo pressure hoses aswell. Everything looks fine to me!

I haven't changed the N249 yet as I'm skint, but I'm beginning to think its just the ECU being a norman.

Where is the boost pressure sensor between the turbo and the throttle body? Wonder if it's dippy.

Went for a drive last night. Seemed insanely quick, as normal! Love the power delivery so much. Let the plonker in his overpriced bmw pull away off the line while you gracefully slip it into 2nd and wait for the enormous surge of power to hammer you past and into the sunset. A lovely skoda styled two finger salute to the world of image obsessed motorists!

Map sensor is sat on top of the intercooler.

cheers mart, any history of them getting tired with age or just being spazzy?

I'll have a poke....

The actual error codes I have atm (no EML light showing though):

17705: Connection Charger-Throtle valve. Pressure drop.

17536: Long term fuel trim multi bank 1

NB these were not done on VAGCOM, but some other generic garage diagnostic kit, but i assume they mean the same thing.

As I stated, the smoke test didnt reveal anything, and I had the throttle body cleaned, but to no avail. Boost pressure is fine rarely, but mainly not.

I have noticed that the large "Y-shaped" pipe coming out of the right of the rocker cover is very soft though. I read in another thread that its recommended that it is changed. Thats the next step ill go for. Agree?

Are there any other things it could possibly be? I'm not brilliant with engines, especially turbos.

Oh and what are exactly these n75's and n249's? Could someone please explain to me. Cheers

Edit, the car is not remapped, but well it wasn't stated to me when I bought it a few weeks ago anyways.

The Y-shaped hose coming off the rocker cover is just an engine breather hose. Mine had perished and fell apart so i replaced it, but i cant see it affecting the car in any way since it is just a breather.

The N75 controls the wastegate, and is fed positive pressure from a line between the turbo and intercooler. The N75 is a solenoid valve that the ECU controls. The wastegate allows the exhaust gases to bypass the turbine wheel, and reduce boost levels.

The N249 controls the D.V. (Diverter Valve/recirculating dump valve). It is controlled by the ECU again, and can switch between two vacuum feeds for opening the DV. Standard state takes the vacuum feed from a tap in the inlet manifold, and when the ECU decides it can take vacuum from the vacuum reservoir. The DV allows the charge pressure to be instantly dropped during gear changes, but allows the turbo to keep spinning.

Hope that makes sense! It's a bit of a learning curve owning a modern turbocharged car but I think I'm getting there.....

I have the exact same symptoms,

occasionally the car will drive like there is no turbo what so ever, other times ill be booting it along and then all of a sudden there is a small surge in power and then things return to normal. I have replaced various hoses and pipes and i have a new forge DV, but it still happens!

It does not happen that often and normally stopping and starting the engine will solve it.

Since it's an intermittent problem I'm thinking it's a sensor or valve fault. Bla

Its so frustrating because there are no VAG specialists around here (Morayshire).

Anyone any recoomendations for what to change next? How much do these N249s and N75s cost?

I have never heard of a map sensor causing problems. I would not be looking at the n75 as a possible problem as this controls the boost while accelerating by bleeding excess pressures off, not when lifting off.

Has anyone tried a new n249 or even the n249 bypass mod?

http://briskoda.net/forums/maintenance-performance/boost-vac-leak-tester/104469/

Maybe one of these will help find boost leaks

I have never heard of a map sensor causing problems. I would not be looking at the n75 as a possible problem as this controls the boost while accelerating by bleeding excess pressures off, not when lifting off.

Has anyone tried a new n249 or even the n249 bypass mod?

I've read threads on Seat owners bypassing the N249 sucessfully, but I'd rather keep it in as it's there to protect the engine i think.

i'll try replacing it when i have a spare 45squid. I don't think it the N75 either. And a faulty MAP sensor wouldnt cause the 17705 fault code, unless it was loose or smashed and letting air out... dunno. Or actually maybe it would if it opens the DV! urg whatever! yeah ill do N249 and revised pressure hose when i have the cash in a week. They look like easy jobs.....

gosh! i want an exhaust not some poxy hoses and a valve! not cool.

Does MAF = MAP?

Map & Maf are generally different !

COuld you elaborate please, noob here

Also, what sysmptons does having a dodgy MAF have? COuld it theoretically redust boost pressure?

I'm certainly not the best person to go into detail on this. (Ken o'Neil would be though )

But MAF = Mass Air Flow, and MAP = Manifold Absolute Pressure.

I'm sure a Google search would be best as i'm not tech.

Sorry.

MAF is by the airbox (Air mass meter -G70-).

MAP is on the intercooler (Charge pressure sender -G31-).

They are totally different things.

The reason I was talking about the MAP sender was that if it incorrectly senses a higher than normal pressure it would tell the ECU to activate the N249, which in turn would open the DV to dump the excess pressure. Seems to make sense to me....

See...i knew someone more techy would explain it !

MAF is by the airbox (Air mass meter -G70-).

MAP is on the intercooler (Charge pressure sender -G31-).

They are totally different things.

The reason I was talking about the MAP sender was that if it incorrectly senses a higher than normal pressure it would tell the ECU to activate the N249, which in turn would open the DV to dump the excess pressure. Seems to make sense to me....

or would it open the n75 which is what it should do to release excess boost, exept for foot off pressures which the n249/Dv does.

This really confuses me. If the problem is intermittant, I'd guess it not to be a leak, but a dodgy electrical connection. Hence a sensor or component like n249. Am I right?

I'm an avionics tech by trade so I'm finding hard to get to grips with things that don't come in black boxes.

Snap! i have the same fault code dam 17705! Going to replace n249 and n75 and new dv see what happens.

or would it open the n75 which is what it should do to release excess boost, exept for foot off pressures which the n249/Dv does.

Yeah good point, I'm getting so confused!

After reading a bit about MAP senders I'm kinda thinking it wouldn't do that anyway. As that's the N75's job I recon (assuming the N75 has a pressure sensor in it). Think the MAP is just one of the components to control the A/F ratio, and gathers pressure information for the calculations. As does the MAF and the air temperature sender in the inlet manifold.

Or am I wrong?! Everything is linked to everything bloody else urg!

I'm gonna crack on and change some valves and wotnot like the guy up there says!

Now i'm confused !

Can we get a list of possible causes of 17705 together, arranged by likelyness etc??