GT Sport TwinCharger Shunting

[kane9522]

Hi Guys,

I assume this will be the same problem as some of the other guys have been having with the n75 valve for the GT Sport 1.4 twincharger engine, but I thought I would ask just to see if there is anything I should be looking at.

Anywho, my GT Sport has just had it's second major service (120000km) completed, after which the car has started shunting when under load, but not at the 3500rpm point that most people complain about. It does seem to happen less when the car is cold.

I took it back to the place that did the service a couple of days later and they said they had seen this before and said the turbo was faulty and would need replacing (all $3000 of it), which is why I want to know if there is anything else that needs to be looked at. There was an issue leading up to this service that I was getting a constant error for in the VCDS logs in regards to the n80 valve, which turned out to be faulty wiring.

This had happened once before around the time of the first major service, which I thought was spark plugs because the dealer failed to recognise that they did not have the required spark plugs in stock and did not "get them in stock" till 3 weeks later after I called them a few times, but after the spark plugs being replaced, this problem went away until now.

Any advice that people can shed on this would be greatly appreciated.

[grtuned]

Mate first needs a fault check!!! No point saying turbo needs replacement
It's rare for the turbo to be no good. Many other items are associated with controlling boost pressure so they should be ruled out first. Did the service centre find any faults???

[Mattlock]

The N75 fault drops the boost at 3500rpm as it dumps all the turbo pressure to the exhaust (and the S/C clutch disengages at this point). However, it tends to fail in tandem with the n249.

The N249 doesn't report anything in to VAGCOM and the only way to know it has failed is to address it directly and run a diagnostic. VAGCOM will tell you the N75 is dead though.

In my GT, the N75 was replaced and it didn't fix it until the N249 was replaced on a second visit.

Get the dealer to directly access and run diagnostics on the N249 before agreeing to any other work.

[Transporter]

It's always good to seek a second opinion.

[kane9522]

Mate first needs a fault check!!! No point saying turbo needs replacement
It's rare for the turbo to be no good. Many other items are associated with controlling boost pressure so they should be ruled out first. Did the service centre find any faults???

To be honest, not sure. The guy had taken it for a 20min drive before coming back and proclaiming the turbo was at fault. Was going to get a mate who has a VAGCOM to have a look and see if there actually are any fault codes

[kane9522]

The N75 fault drops the boost at 3500rpm as it dumps all the turbo pressure to the exhaust (and the S/C clutch disengages at this point). However, it tends to fail in tandem with the n249.

The N249 doesn't report anything in to VAGCOM and the only way to know it has failed is to address it directly and run a diagnostic. VAGCOM will tell you the N75 is dead though.

In my GT, the N75 was replaced and it didn't fix it until the N249 was replaced on a second visit.

Get the dealer to directly access and run diagnostics on the N249 before agreeing to any other work.

Rightio, I will see if the guys who did the service can do that for me

[Mattlock]

Has the N75 been replaced before? If not it'll most likely have the original design. VW updated the design to use a different diaphragm which works much better and has less chance of failing and doing weird stuff.

[kane9522]

Has the N75 been replaced before? If not it'll most likely have the original design. VW updated the design to use a different diaphragm which works much better and has less chance of failing and doing weird stuff.

As far as I am aware it has not been replaced. I guess doing 70000km's+ in 2years will expose issues in a 7 year old vehicle...

[Mattlock]

My hunch would be that it's more likely to be the N75 and N249 combination than a failing turbo (I'm not a mechanic).

[kane9522]

My hunch would be that it's more likely to be the N75 and N249 combination than a failing turbo (I'm not a mechanic).

That would be my guess as well...

[piper26]

Hey mate at what rev range is it shunting. You mentioned not around the 3500 mark..
You mentuoned 120 000km service.
Were the plugs and fuel filter definitely replaced? If the plugs were replaced did they move/ dislodge the vacuum lines around coil #4?

Alot of questions I know but just trting to build up the mental image of whats going on. My gt has 140 000km on the clock and is still going strong.

[kane9522]

Hey mate at what rev range is it shunting. You mentioned not around the 3500 mark..
You mentuoned 120 000km service.
Were the plugs and fuel filter definitely replaced? If the plugs were replaced did they move/ dislodge the vacuum lines around coil #4?

Alot of questions I know but just trting to build up the mental image of whats going on. My gt has 140 000km on the clock and is still going strong.

It seems to be shunting around the 2000rpm mark, maybe 1600-2400rpm

Spark plugs were definitely replaced. Fuel Filter, no, I did not think it was part of that service?? I would have no idea if anything would have been moved/dislodged. Further to this though the DSG did have it's service/readaption done as well, silkiest gear changes i've ever had from it since owning it.

And this shunting did not exist before the service, only after. However with the 60000km service it was showing up before hand, then a lot more in between part 1 of the service (stupid dealer "forgot" that they did not have the required spark plugs in stock) and part 2 (spark plugs going in) after which, no issues...

For those following, did manage to get a hold of a VAGCOM cable, and could not find any errors, which is not surprising. We did go for a drive and tried to use the VCDS graphing capabilities to see if there were any noticeable drops in boost or engine load, which certainly did appear.

Is there any harm in getting the N75 and N249 valves replaced??

[piper26]

There is no harm in getting them replaced. Just a cost that may or may not be the cause.. worn plugs and poor fuel supply can cause issues. If there is a fault with n249 then it is usually complete loss of boost

[Mattlock]

There is no harm in getting them replaced. Just a cost that may or may not be the cause.. worn plugs and poor fuel supply can cause issues. If there is a fault with n249 then it is usually complete loss of boost

Interesting. My fault still gave me full boost from the S/C even though the N249 was also at fault. The N75 only failed once the car had warmed up and therefore gave boost from cold.

[piper26]

It seems to be shunting around the 2000rpm mark, maybe 1600-2400rpm

Forgot to mention this range is under supercharger and not the turbo. Faults with n75 or n249 dont rely ahow themselves in this tange as the supercharger is before the turbo.

I can only suggest before you spend money on valves that you buy a fuel filter. Also check the glue rib in the air filter as if this is broken the filter may suck flat 'choking' the engine slighty.

Without driving or seeing the car this is the best advise I can give