T4 ACV: Low Boost Pressure? Solution Found!

Hello!

I'm having a problem with a T4 TDI ACV engine.
Initially, there was a clear lack of performance and error code 17964 (boost pressure control exceeded limit).
I found a damaged (cracked) hose between the intake manifold and the boost pressure control valve, but that didn't improve anything. Several diagnostic runs with VAG Com yielded the following results:

LMM is: maximum 610 mg/h.
Boost pressure: max. 1420 mbar.

All other values are within tolerance.
I've already read quite a bit in the archive, and it doesn't necessarily mean the mass airflow sensor (MAF) is broken. However, I tried a new MAF sensor -> no improvement!

Okay, here's my question: How can I bypass or disable the boost control valve? If I connect the boost pressure directly to the wastegate, that will only confirm that the wastegate opens, but I still won't know if my turbo is still capable of building the target boost pressure. Am I understanding this correctly?

I've already checked the boost pressure control valve; the resistance is 28.5 ohms, and the power supply to pin 1 is also okay. According to VAG-COM, the duty cycle is also within normal limits.

I almost forgot... When I was working on the EGR valve, I accidentally installed a gasket with only 2 holes...


Please help .

Greetings.
Martin

Hello,

Have you ever checked the wastegate yourself to see if it's working properly?

Hello,

Have you ever checked the wastegate yourself to see if it's working properly?

Yes, I do. It uses a small hand pump with a pressure gauge. It opens at approximately 0.6 bar and closes properly as well; the control rod moves freely.

Thank you for the quick response!

Greetings.
Martin.

Hi,
If you disconnect the pressure sensor, the system will no longer regulate. However, BE CAREFUL during test drives. While the EDC will go into emergency mode, there is a risk of damaging the turbocharger. A better solution would be to use an analog LDA (Linear Differential Amplifier) for pressure monitoring.
With the connector disconnected, the LDR valve should be open; when at 12V/100% (e.g., in idle), it should be closed.

Hi,
If you disconnect the pressure sensor, the system will no longer regulate. However, BE CAREFUL during test drives. While the EDC will go into emergency mode, there is a risk of damaging the turbocharger. A better solution would be to use an analog LDA (Linear Differential Amplifier) for pressure monitoring.


Hello!

But if the control unit only detects insufficient boost pressure, then the full amount of fuel won't be injected, and with the analog boost pressure measurement, I also won't achieve a significantly higher boost pressure, right?
Or, if I disconnect the hose from the wastegate, hardly anything will change...
Does the turbocharger already reach its full pressure with a relatively small amount of fuel injection?

greetings
Martin.

But if the control unit only detects insufficient boost pressure, then not the full amount of fuel will be injected, and I won't achieve a significantly higher boost pressure with the analog boost pressure measurement, right?
Or, if I disconnect the hose from the wastegate, hardly anything will change...
Does the turbocharger already reach its full pressure with a relatively small amount of fuel injection?


Hello MartinN,

If you disconnect the hose from the wastegate, the engine control unit (ECU) no longer has the ability to regulate boost pressure or open the wastegate when the boost pressure is too high. Therefore, Christians advises exercising extreme caution during test drives (preferably with an analog lambda sensor) to prevent the turbocharger from over-revving. In other words, the turbocharger will be running at its maximum potential.

Certainly, the normal boost pressure, if it is even achieved, will only be reached and exceeded at higher engine speeds than in a completely healthy engine.
I would definitely check the LDR valve first.

Hello!

Just one more question for general understanding...
The boost pressure measurement, specifically the value displayed by VAGCom as the actual value, or the value processed by the engine control unit, is determined solely by the turbocharger boost control valve. Otherwise, there isn't a separate boost pressure sensor, right? (T4 ACV, manufactured 04/2000)
Is that correct?

Then it would also make sense that the turbo isn't spooling up properly, even with the wastegate hose disconnected, because there simply isn't enough fuel being injected, and as a consequence, the mass airflow sensor (MAF) signal is also too low.

greetings
Martin.

Normally, there are two devices involved, with the pressure sensor sometimes integrated into the control unit. The LDR valve has a cable with 2 wires, while the boost pressure sensor has one with 3 wires.

Hello MartinN,

In your T4, the turbocharger pressure sensor is integrated into the engine control unit (ECU). This ECU is located in the engine compartment on the driver's side, under a cover. Please check the pressure line between the connection point (intercooler) and the engine control unit. In this model, it often happens that the connector nipple on the intercooler becomes constricted or blocked due to deposits. To check this, you must carefully detach the hose (delicate connection, risk of breakage!!!) from the connector nipple on the intercooler and then verify if it is clear.

Best regards, huskydoctor.

Hello!

I've now measured the boost pressure manually with a gauge, and it's really only around 1.4 bar (atmospheric pressure plus approximately 0.5 bar). I then disabled the wastegate, but it produced the same result.

For the sake of completeness, I also checked the boost pressure sensor. In my car, there's a combined boost pressure and temperature sensor. It doesn't have a direct line to the engine control unit, as described above. It seems there was probably a change at some point (the car was manufactured in 04/2000).
I then created an adapter that allows me to manually apply pressure to the boost pressure sensor and read the pressure using a pressure gauge.
Result: The pressure is displayed correctly using VAG Com.

Here's one more attempt...
I then applied pressure to the turbocharger pressure sensor while the engine was running, but I couldn't detect any noticeable change in fuel injection volume, nor any black smoke (due to the actually low boost pressure). However, I suspect that the engine control unit is going into a limp-home mode.

A few more questions...
How do I know if the emergency steering system is activated?

What do you think? Is the turbo defective? Or is there a problem with the ESP's mass airflow sensor? Or could it be the engine control unit (ECU)?

Thank you for the responses!

Greetings.
Martin.

Did you measure the pressure up to 4000 RPM? I suppose you did.
First, I would check the functionality of the wastegate. It might not be closing properly. Unfortunately, if the actuator rod isn't free to move (which varies depending on the turbocharger), it can be difficult to diagnose.

...or the lovely charge air quietly mixes with ambient air between the turbocharger and the intake manifold...

Best regards, Rainer.

Hello!

I measured the boost pressure across the entire RPM range.
I loosened the nut on the actuator rod connected to the wastegate – the wastegate is moving very freely... almost too freely for my liking.
However, there's a drawback to the wastegate... The boost pressure builds up relatively early, starting around 1900 RPM, and then remains at a relatively constant 1.4 bar throughout the entire remaining engine speed range.

I checked the intake system, including the hoses and intercooler, for leaks – there was no pressure loss.

'Thank you anyway!!!'

greetings
Martin.

Hi,

If there is no loss of pressure and the wastegate closes properly, then it's still possible that the turbocharger is damaged or that the fuel injection amount is being limited for some reason, resulting in a lack of drive power for the turbocharger. Are the air filter verschandeln and the intake passages clear?
Please measure the fuel injection amount at full load and compare it to the target values listed in the technical articles.

Best regards, Rainer.

Rainer K. wrote:

Hi, Please measure the fuel injection volume at full load and compare it to the target values listed in the technical specifications.

Hmm, I suspect you can really learn a lot from that table. If I've accessed the correct one, it shows values for full load, not for the maximum power output achievable with different air mass flow rates. The engine doesn't actually reach full load because of the low boost pressure and the resulting limited air mass flow.
Couldn't the value of a functional Leakage-to-Mass Air Flow sensor be distorted during such a measurement? The smoke cloud would certainly become large, but there might be comparable readings.