1.4 TSI boost/power delivery problems

[warhead]

For a while now I've always had the inkling there was something off about my car. Only today did I actually see and feel something quantifiable.

The story starts yesterday. I had some time so I took the car for short run to try and iron out what was bugging me and after having read a few threads about the N75 valve being a common part that failed and the symptoms were lack of turbo boost, I tried to see if my car had the same problems.
Did a few easy laps around my area to warm the engine up and then did a few WOT bursts down long straight roads in 2nd and 3rd gears and the engine was boosting and holding boost just fine. I felt no surges or drops in power delivery and everything felt smooth. Boost guage climbed steadily and as long as I held my foot steady the boost needle went all the way to maximum and stayed there until I let go of the accelerator.

Now today, half way to work I noticed that the boost would drop out ( 1/3 - 1/2, needle near the middle ) as soon as I hit 3500rpm in 1st or 2nd gear. At 3500rpm you feel a jolt as if you've just suddenly decelerated for a fraction of a second and then the rpms continue to climb smoothly but the boost gauge would remain at 0 ( far left position ).
Sometimes in the higher gears like 4th and 5th I'd struggle to get any boost and I can't hear the turbo spool.

I pulled over and switched the engine off and back on again and I continued to have the same problem.

However, about 3/4 of the way to work, it was fine again! I couldn't reproduce the condition where boost would drop @ 3500rpm and I didn't feel the associated jolt when the turbo was supposed to kick in.

Now I'm really quite confused on what's going on and if I'm having such a hard time getting the fault to appear, I can't imagine trying to convince a mechanic that it's not my imagination.

Has anyone had similar experiences with their 1.4 TSI engines? And can anyone recommend someone in the Sydney Liverpool area that could take a look at my car.
I bought the car in Melb at Barloworld Glen Waverley so not very practical to go back there. Plus the car is out of warranty anyways.

[Gambit]

I am sure someone more experienced in this area may reply, but my laymens understanding is that in some cases dependent upon driving inputs, such as throttle positioning, ambient and intake air temps, load etc etc, boost control via electronic wastegate duty control may in some instances limit or reduce boost. You wont know until you log via an OBD port.

[dionysiac]

Has anyone had similar experiences with their 1.4 TSI engines?

Sounds like the N75 to me. To demonstrate it to mechanic i had to put the engine under load in 2nd. At 3.5K i lost boost.
Problem was intermittent at first but eventually was consistent.

[triode12]

Could be a faulty PCV or DV. Had the PCV on my TSI GT replaced under warranty - it was affecting boost from the supercharger. They replaced the DV at the same time to be sure.

[Paul_OH]

Curious to see what the outcome was for this? Experiencing the same thing currently. At this stage I'm assuming N75 valve but will be getting a scan done tomorrow to see if that highlights anything further.

[BarneyBoy]

I had this problem last year. It turned into quite an epic due to the stupidity and ignorance of the head mechanic at the dealership.

They originally diagnosed it as a faulty Turbo Recirc Valve - N249. They replaced this but it didn't fix the problem (a road test would have shown this )

They then said it was a faulty turbo and replaced that. Why a faulty turbo would deliver boost at all other RPM but 3200 RPM was a question they kept avoiding). Problem still there (once again a road test would have shown this).

Third time I insisted they look at the Solenoid Valve N75 - Turbo Charge Pressure control valve. I had asked whether it could have been this right at the start and it was suggested that they knew what the problem was, and it wasn't the N75.
I was quite unhappy by this time (as you can imagine). They said that since the problem was clearly very complex, they would bring a VW specialist (from VWA) over to look at car.

After checking the car, he proclaimed a faulty N75 valve and it was replaced. The fault disappeared.

There's quite a few instances for this valve being faulty if you spend some time on Google. There's cases in the UK, Cyprus and elsewhere. With the symptoms you have described, personally I would proclaim it as the N75 being faulty.

[Greg Roles]

I wonder if there is a upgraded or revised part number, as it seems like a common problem to the 1.4: The DPF pressure difference valve on the GT TDI is now an upgraded part number from VW as the earlier one was causing issues in comparison.

Anyone from VW have an idea?? Umai??

[MariusGT]

Just to clarify (BarneyBoy and others who may know)
- N75 and PCV are not the same valve right?

[BarneyBoy]

Two different things:

The positive crankcase ventilation (PCV) system is for recirculating combustion gases that have blown past the piston rings and ended up in the crankcase. In pre Green days it just vented to the atmosphere. These days its recirculated in a controlled manner back through combustion to burn off the nasty stuff.


"Crankcase Breather

The crankcase breather allows the crankcase to be rinsed out and thus reduces the formation of water in the oil. The breather is in the form of a hose from the air filter to the camshaft housing.

Crankcase ventilation:
Unlike conventional naturally-aspirated engines, the crankcase ventilation system for a charged engine is more complex. While there is a constant vacuum in the intake manifold of a naturally aspirated engine, it is up to 2.5 bar (absolute) in the TSI engine.

Oil separation:
The gases are drawn out of the crankcase by the vacuum. In the labyrinth and in the cyclone oil separator, the oil is separated from the gases and drips back into the oil sump.

Gases are sent to the intake air as follows:
The gases flow from the timing chain case to the check valve for the crankcase ventilation. Depending on whether the pressure is lower in the intake manifold or in front of the regulating flap control unit, the return valve will open and allow the gases to pass through. In the intake manifold or in front of the regulating flap control unit, the gases mix with the intake air and are fed to the combustion chamber. A throttle in the connecting hose to the intake manifold limits the throughput when the vacuum pressure becomes too high in the intake manifold. A pressure regulating valve is therefore no longer necessary." (from VW Self-study programme 359)

The N75 is the Charge Pressure Control Solenoid Valve. It's attached via a Pressure canister to the exhaust manifold (ie between the engine and the turbo. Its main function is to control the boost pressure of the turbo at RPM at or around 3,500. A maximum boost of 2 bar is monitored by the G31 Charge Air Pressure Sender, and regulated by the N75 valve. Incidentally, the supercharger can produce 2.5 bar boost! That's tons!

The 2,400 - 3,500 range is controlled by a flap J808. In this rev range the turbo doesn't quite have full boost, so under high throttle, the ECU switches the supercharger on until the turbo has full boost then switches it off. That's why the GT has such nice mid range grunt.

Download this:
VolksPage.Net - Technik - SSP - Self Study Program VAG
You need #359 (in the 4th link)

It's a good read with some nice diagrams.

[joejoeblogs]

N75 valve replacement fixes boost problem

I shared the boost problem described above and, having read this thread, had my N75 valve replaced. The problem has gone. Very happy.

My car:
Manual 2007 Golf Sport GT (1.4 TSI twincharged), 75,000km. I have had the car for only a couple of months / 2,000km.

My symptom:
Boost gauge would indicate full boost at lower revs.
In the first 10-15 minutes of use of the car, the boost gauge would also indicate full boost at higher revs.
However, when the car had been running for 10-15 minutes (presumably now at running temperature? - hard to tell without a temp gauge), at approx. 3,500RPM, the car would lose power noticeably. Above 3,500RPM, the boost gauge would hover around 50 %, never much higher.

My solution:
Having read this thread, I asked my mechanic (not a VW dealer or specialist) to replace the N75 valve. Sceptical about diagnosis via the web, he was initially reluctant but eventually agreed to replacing the valve. The part cost less than $100. The problem is no longer evident.

Thank you all for your help in posting here.

[edward918]

Hi, apologies for digging up an old thread but I've been having a similar issue with my Volkswagen Polo BlueGT Manual with the 1.4tsi ACT 150ps engine. There's a hesitation between 4 - 5k rpm in 2nd and 3rd gear when I accelerate hard from around 2k rpm. Here's a video to show what I mean: VW POLO 1.4TSI HESITATION WHEN ACCELERATING - YouTube

Could this possibly be a faulty N75 valve? When I bought the car from the dealership, an EPC light came on during the test-drive. The fault was down to the hose for the turbo coming off, so I was wondering if there may also be a slight boost leak?

Thanks again.