T4 Syncro 2.5 TDI: Power Loss Troubleshooting

Hello,

I'm currently troubleshooting our T4 Syncro. The problem is a significant lack of power; there's very little power at low speeds, and the maximum speed is nowhere near what it should be (around 140 km/h is the limit, whereas it used to be around 160 km/h).


Vehicle data:

- T4 Syncro, manufactured in February 1999 (already with the facelift).
- 2.5 TDI ACV engine with 75 kW.
- Engine, chassis, and drivetrain are all standard.


The new ones were actually installed as part of the maintenance anyway.

- Timing belt
- Water pump
- Clutch
- Start date of VEP funding reviewed and approved >> OK.



Now, due to the lack of maximum speed and the troubleshooting related to it, a faulty N75 valve was discovered, therefore:

- N75 valve replaced.
- All tax hoses replaced.
- Intercooler circuit visually inspected >> Leak-free.


Previously, it had some issues with low-end power, but the same vmax (maximum speed) problem. Now, since the N75 valve has been replaced and the turbo is being properly controlled, it has lost some low-end power, especially in the range below 2000 rpm. According to the specifications, it should be producing the full 250 Nm of torque at 1900 rpm.


All the other readings, such as the air mass sensor, look good, so I'm a bit puzzled... do you have any other ideas?

I didn't install the N75 myself, a workshop did... but you can't really mess it up too much, usually.


What I wanted to do today was to log the boost pressure again and re-check the mass airflow sensor (MAF) values... What else can be done?

And it's definitely an ACV, not an AXL, which was very common in the Syncro. The AXL, unlike the ACV, has a VTG turbocharger. However, both have 102 horsepower.

definitely an ACV... the AXL wasn't released until after 2000, and ours is from 02.1999 I checked it specifically, and it has a wastegate.

Octavia_4x4 wrote:

definitely an ACV...

Gut. Typische Fehlerquellen schon geprüft? Ansaugschlauch? AGR?

It's best to start by checking the logs.

Are you referring to the intake hose, the hose that goes into the air filter box? No, that one hasn't been checked yet , at least not by me.

"The AGR system is okay according to the workshop... and no, nothing will be deactivated."

Octavia_4x4 wrote:

By "intake hose," do you mean the hose that goes into the air filter box? No, that one hasn't been checked yet

, at least not by me.
Yeah, everything else are just pressure hoses.

Octavia_4x4 wrote:

... and no, nothing will be shut down there...

The matter isn't over yet, my friend!

The missing clamp on the aforementioned hose at the connection to the turbocharger is something to note for ACV engines, as they tend to leak there. So, installing a clamp would eliminate a potential cause of leaks.

What is the diameter? I'm curious to see if it's even possible to install it given its complex design.

matthiasTDI96 wrote:

The missing clamp on the aforementioned hose at the connection to the turbocharger is something interesting for ACV engines, as they tend to leak there --> install a clamp and eliminate a potential cause.

It usually doesn't do much. The material is so hard that a clamp can barely exert any pressure. Only a new part can really fix the problem.

More likely the opposite. AXL is very rare, while ACV is usually more common.

AXL = structurally similar to the AXG, even the software is almost 1:1 and throttled at specific addresses.

What to work on: http://www.t4forum.de/wbb3/index.php?page=Thread&threadid=84828

Okay,

My feedback:


- Error memory is empty.
- Log data: Mass airflow values were slightly too low; boost pressure was only 1720 mbar, minus the 920 mbar ambient pressure, which in my opinion is too low.
- Clean air filter.
- Ensure the air filter intake is clear.
- No oil-soaked hoses, which suggests the charge pressure line is likely sealed.


The workshop replaced the EGR control valve, not the L-pressure valve. Neither valve is being activated when I run the actuator diagnosis; at least, nothing is happening.

If I disconnect it, there's an error in the FSP (Field-Specific Protocol), so the wiring is probably okay. This is a bit confusing, as it shouldn't be happening.


The MAF sensor was slightly oily, so I cleaned it, which brought the MAF sensor readings back into the normal range, but it didn't improve the performance.


Since the LD (likely referring to a pressure reading) of 0.7 - 0.8 bar seems a bit low for the ACV (likely referring to a specific component or system), something must be wrong with the turbo control system.


a) How can a fault in the LD valve and EGR valve be detected by simply disconnecting them, but b) why is nothing activated during the actuator diagnosis? Especially since the issue is the same for both valves, and one of the valves is even new.


Unfortunately, I don't have any logs to upload because, as a standalone tester without a PC, I didn't have my laptop with VAGCOM.

Greetings.
Georg.

Octavia_4x4 wrote:

Since the LD (low-pressure setting) of 0.7 - 0.8 bar seems a bit low for the ACV (air conditioning valve) in my opinion...

Okay, that part produces 102 horsepower from a 2.5-liter displacement. A boost pressure of 0.7-0.8 bar is probably sufficient. The boost pressure is fine like that. The engine only has a power output of 40.8 horsepower per liter. A 1Z also only has 0.8-0.9 bar of boost, and a higher power output per liter.

What exactly does the idle speed control system indicate? It's not that there's a lack of compression, or that the injectors are completely faulty.

Do the idle speed adjustments affect the differences between the individual injectors?

"One had a value of -0.68, another had -0.42, and others had values of 0.10, 0.20, and 0.10, if I remember correctly..."

Okay, he just lacks power. Now I have another suggestion from a VW master mechanic friend... let's say the injectors are dripping (it's producing soot, but it's been doing that for a long time, even when it had good power), but it now has an aftermarket DPF. He says he's seen cases where the exhaust flow gets too hot due to the dripping injectors, which causes the DPF to melt and clog on its front side. This can be identified if the exhaust coming out the back is not the normal flow, but rather too little. Of course, I haven't checked that.

Sure, here's the translation:

"First, like this:"

Octavia_4x4 wrote:

- Engine, chassis, and drivetrain, all standard equipment

.
Okay, here's the translation:

"Now like this:"

Octavia_4x4 wrote:

...now has a retrofitted DPF...

Okay, please write something like "always" first, so that people know what to expect! That completely changes the situation. If you say it's smoking a lot, it's quite likely that the darn DPF is clogged. So, let's try taking it out for a test! Then we can talk more about it...

Yes.

I had forgotten about my debt... I suspect it might be related to that, but it will be checked today...

Okay, so...

The problem likely stems from the fact that neither the LD valve nor the EGR valve are being activated, even though they are technically functioning correctly.

What's strange is...

- They are not controlled or monitored during operation or during actuator diagnostics.
- No error codes stored.
- If you connect them, you immediately get the familiar "Short circuit to ground" error message.

Okay, in my opinion, the main power line is definitely interrupted. I think the -15 terminal might be damaged or disconnected from the valve(s).