Hello
I have a problem with my A3 1.8t AUM stock. Recently I noticed that the car doesn’t run as usual and low rpm pull is missing. I went out to log the car (3rd rear WOT) and saw both requested and actual boost doesn’t meet the specs –  peak boost comes late (above 3000rpm instead of 2500rpm) and is low - max boost was 1450mbar (21psi), should be 1500-1600 (22-23psi), including atmosphere pressure. Then I checked timing retardation and saw massive time pull – sometimes up to 12 degrees KW.

So far I’ve checked the following:
MAF – swap from good car, no change. Also values are up to spec.
Cooling temperature sensor – brand new, OEM
N75 – swap from good car, no change.
Fuel pressure regulator – new 3 bar Bosch.
Fuel pressure measured – 3 bar.
Diverter valve – brand new 710N.
Injector duty cycle – under 80% so that also seems ok. Injectors are all refurbished and tested recently.
IAT – usually 25-30C (77-87F), up to 60C (140F) when drive it hard and then stuck in traffic. Even with lower IAT car still pulls time.
Lambda – actual and requested meet up fine. Did the 034 block in basic settings to check lambda aging and got 1.8 (minimum is 0.8 and a new sensor reads 2.5).
Smoke test shows no vacuum leaks.
Fuel trims are around 0% (+-2) for additive and multiplicative. But then the map is only a few days old too.
Brand new plugs NGK PFR6Q (gap checked), brand mew Bosch coils.
I am in the process of changing gas station, to rule out low quality fuel. Typically run 50:50 RON95 and RON100.

logs:
https://log.tunezilla.com/s/u9nqFyIr

after adding half tank RON100: https://log.tunezilla.com/s/COeWUd35

second gear: https://log.tunezilla.com/s/rKFf5QK9


My suspicions are excessive carbon buildup in fuel chambers and piston heads, or maybe Lambda probe going bad and shows incorrect values, causing the AFR to go lean (the probe is wide band, so I think the car should run in close loop even in WOT).

Any help is highly appriciated.

No, the car hasn't been tuned at this point - everything is completely stock. Only thing added is oil catch can before the PRV valve. When i bought the car, i made the same 3rd gear pull and everything was good - peak boost was 1650mbar(including atmosphere pressure) at 2500 rpm, no timing pull. Recently the car started to feel sluggish at low rpm (below 3500). When the problem is sorted, i'm considering to make AUQ flash (since the hardware of AUM and AUQ is almost the same and i like to stay as much as close to factory setup).

I am the second owner of the car, the previous owner insisted that the car has no tune and is completely stock. I am sorry - the values that i wrote in the first post was incorrect - i reviewed the data from when i bought the car and peak boost in VCDS was actually 1610 mbar (23.3psi)  with atmospheric pressure of 960mbar(14psi),  so the boost that i had when the car was running  good was about  650 mbar (about  10 psi). The MAF at WOT was maximum of 122 g/s, so I assumed that the car indeed is stock with no tune.

Update: The compression is checked and is exacly 12 bar (174 psi) on all 4 cylinders. After carbon clean with hydrogen and fuel additive (the workshop that i go to assured me that it will clean any carbon deposits in the fuel chamber), the problem persists. After the road test, i noticed that when revving in neutral, the engine still retards timing, especially when i press the throttle hard. When i press it gently, there is 0 timing pull.
Recently i changed engine mounts and dog bone with aftermarket SWAG. Today checked the alignment, seems as per the repair manual.
I started to doubt if spark plugs or coils are genuine, the market in my country is full of fake autoparts.
Any ideas or suggestions of what could cause the problem are welcome.

I'm having the same problem. 03 Jetta Wolfsburg Edition 1.8t AWP on Maestro. Stock turbo, 550cc injectors, 3in downpipe + magnaflow cat, SAI and EVAP deleted. Stock otherwise. Always on 91 oct (the best commonly available in my area). Replicated the stock map in Maestro except for the injectors and it exhibits the same issue.

I've tested nearly everything I can and all is good. I've done pretty much everything short of disassembling the motor itself trying to solve this.

When you say "fuel chamber" do you mean combustion chamber? Inside the cylinder? Mine's high mileage, could definitely have lots of carbon in there.

Thank you for the detailed response and product recommendation. Doesn't look like I can get that easily in the US though. I'll have to do more research on it. Your results look great.

I did one seafoam hot soak not too long ago and let it sit for ~1 hour. Made a huge smoke cloud but zero visible difference looking at the piston. I don't have a scope but all I see is black when I shine a light inside. I have 2 more cans and I'll run them when I'm close to my next oil change.

I got my hands on some of that Carbon X stuff and ready to begin in the next few days. I have a vacuum brake bleeder that I plan to use to vacuum the product out.

You say you got 5 full cleanings out of it. Did you set the piston to a specific position to make it last that long (TDC/BDC)?

Thanks again. Your help has been amazing.

I performed the cleaning last week. I manually set each piston about halfway up the bore on the compression stroke for its cleaning. The starter was not run between cleanings. At the end of the 5th cleaning, I added a small bit of oil into each cylinder and sprayed compressed air inside to clear it. I then added another bit of oil and ran the starter with the plugs removed to give it a final clearing before the first start. I drove it immediately after starting and it smoked for about 5-10 minutes.

First run extracted a lot of dirty fluid and created small holes in the buildup where the piston shone through. The second run showed the most dramatic improvement with massive chunks removed and large holes visible in the buildup. Third run was the same, though slightly less effective. The fourth showed little improvement and the extracted fluid was clean. By this point there were only small flecks of carbon still visible on the piston. Fifth run showed essentially no improvement so I stopped there with 20-30% product left over. Overall, the vacuum brake bleeder I used did a great job and removed all visible fluid from the cylinder each time.

The results are unbelievable. It went from about -6 to -10KW timing pull (on a good day) to 0 under WOT even with 50+ degree Arizona IAT! It no longer feels sluggish or hesitant. Revs quick and smooth and boost hits hard. The motor feels alive and wants to go instead of fighting with the ECU. Idle is much smoother as well and my poly bushing idle vibration has been noticeably reduced.

ONLY Cyl 3 still shows ~3KW of pull under light load sometimes (~50-60%) but generally not under high load. I suspect a fouled plug but if that's not the case I still have more than enough product to give cyl 3 another cleaning. I'll edit this post when I get that sorted.

Seriously, thank you so much. This engine feels brand new again.