Zu wenig Ladedruck <2000U/min, aber keine Ahnung warum | Posts 16+

"I'm sorry, but isn't it logical that at low RPM, there is also low pressure and a small amount of air mass?"

It's a vicious cycle: At low engine speeds, not enough air can flow in because the exhaust pressure isn't high enough to drive the turbocharger. Consequently, the weakly powered turbo only draws in a small amount of air, which means that the full fuel injection quantity won't be released, otherwise verschandeln combustion wouldn't be possible (keyword: particulate filter). But even with the reduced fuel injection, there still isn't enough exhaust pressure to drive the turbocharger, and so on. Eventually, at around 2000 RPM, the exhaust pressure finally provides enough power to the turbocharger to build full boost pressure, causing the airflow to exceed the target value, and it then switches from being limited by particulate emissions to being limited by torque.

"With a diode, the maximum possible air mass is always reported, and fuel injection starts at its maximum level from the idle speed. This means the turbocharger engages earlier because the chain reaction described above doesn't occur. However, this results in excessive soot buildup at low RPMs because it's impossible to achieve verschandeln combustion when there isn't enough air..."

Sven@G4H wrote:

When the engine is idling, the EGR valve is fully open, allowing it to draw in uncompressed air and exhaust gases. If the exhaust gases are reduced, the engine naturally draws in more uncompressed air, which increases the mass airflow sensor (MAF) reading.

Basically, you're right about the idea of "taking more space" or "allowing for more breathing room."
However, this is because the turbocharger spins at a higher speed due to the deactivated EGR system, which results in more air being drawn in, and not because the engine automatically demands more air. The cylinders are already filled with excess pressure, so they don't draw in any additional air.

BeKli wrote:

However, to my knowledge, there is no sensor that measures the amount of exhaust gas being recirculated.

There is already an indirect measurement available through the mass airflow sensor (MAF) signal - see relevant articles on exhaust gas recirculation (EGR).

Quote:

Excuse me, but isn't it logical that at low RPM, there is also little pressure and a small amount of air mass? This is a vicious cycle: At low engine speeds, not enough air can flow in because there isn't enough exhaust pressure to drive the turbocharger.

Basically, yes, but Sven is complaining that his car has significantly less low-down torque than other ALH engines.

@Sven:
Depending on the year of manufacture and vehicle model, there are different versions of the Modular Service Generator (MSG) and software levels available for the Audi Longitudinal/Lateral Hybrid (ALH).

Regarding the ASZ engine in the Golf/Bora and Ibiza 6L models, I am certain that the amount of soot produced varies by approximately 10% at low RPMs, depending on the software version.
Translation: The Ibiza noticeably provides more low-end torque and power than the Golf, without any defect necessarily being present.

Perhaps your "problem" is also related to the software?

Hi,
I'm not just complaining that my ALH has too little boost compared to others, but more importantly, it doesn't even reach the specified target value below 2000 RPM. It should achieve this value regardless of the software used, because the value is set and therefore should be reached. But it simply isn't! I think the problem might be a vicious cycle: too little fuel injection leads to lower exhaust pressure, which in turn means less turbo boost, resulting in low air mass, and so on. The question now is, what exactly is limiting the whole process?

I believe that the target values for the mass airflow sensor and boost pressure are predetermined (or calculated) by the software in such a way that they should be achievable during operation?

I'm not sure if it matters, but with the EGR valve open, I don't hear the turbocharger whistling as much. When I disabled the EGR valve, I suddenly started hearing the turbocharger whistle clearly from around 1300 RPM – not a hissing sound, but a whistling! Oh yeah, yesterday I was on the Autobahn and it reached 195 km/h according to the speedometer!

How would you proceed from here? I'm going to try the diode trick tomorrow, but even if it gives me more bite then, it's not a permanent solution for me!

Sven@G4H wrote:

I believe that the target values for the mass airflow sensor and boost pressure are predetermined (or calculated) by the software in such a way that they should be achievable during operation.

Incorrect. At least in the turbocharger performance charts, the values listed for low RPM ranges and high mass flow requirements often represent figures that are not achievable in practice.

However, the fact that your engine only reaches its target load at 2000 rpm is probably not normal.

Quote:

How would you proceed from here?

Measure the available vacuum pressure in the pneumatic system.
If, for example, the pump is slightly weak on the suction side, the pressure may be too low even when the variable geometry turbine (VGT) is fully open in low load conditions, depending on the amount of air and the engine speed, due to the setting of the LD-TV control.
A partially defective solenoid valve could also produce similar effects, as could a resistance in its circuit.

Well, in my case, the target boost pressure isn't reached until just before 2000 RPM. However, this is probably due to the turbocharger and the two intercoolers.

However, I would also advise you to simply check the entire intake system for leaks, from the turbocharger to the intake manifold. Defects don't always have to be visible; in my case, an intercooler was leaking so badly that the air escaped just as quickly as I pumped it in with a bicycle pump. However, there was no oil or anything else visible on the intercooler.

If the LLK (likely referring to a component in a turbocharger system) has a leak, the full boost pressure may arrive slightly later, and this can ultimately damage the turbocharger over time, leading to undesirable pressure spikes.

Hi,
How can I test the suction and pressure pipes? You mentioned something about using a bicycle pump!

I just performed the diode test, and the error log showed something about an implausible signal from the mass airflow sensor (MAF). It didn't go into limp mode, but it was running noticeably worse than with a working MAF.

How is the vacuum tested? Can it only be tested when idling, or am I mistaken?

-Sincerely-,

Well, the problem description reads exactly like what I experienced with my AXR.

You mentioned that you 'blew out' the vacuum hose, but did you also check it for leaks?

In my case, it was a faulty vacuum sensor in the exhaust cooler, which is also controlled by the vacuum system (but you don't have that). If you have a small leak in the vacuum system, the vacuum may not be sufficient to fully engage the variable turbine geometry (VTG). This results in delayed pressure build-up, leading to reduced responsiveness and slight overshooting or undershooting of the boost pressure.
Disconnect the vacuum hose directly behind the tandem pump and suck on it. If you get a good amount of air coming through, then there's a leak somewhere.

I did this using two drain pipe fittings. Both were 50mm in diameter, and I measured them beforehand. I disconnected the pressure hose after the turbo and inserted a fitting (be careful to roughen up the surface of the fitting and degrease the pressure hose beforehand, otherwise it could explode and fly off!). In the other fitting, I glued in a bicycle valve (it's best to buy a cheap bike inner tube with a Dunlop valve). Cut out a small piece of rubber around the valve, drill a hole in the fitting, and glue the valve in place from the inside, for example using Pattex extreme. Again, roughen up the surface and degrease it!

As an air pump, I used a regular bicycle pump with a pressure gauge (it's not very accurate, but it's something). I inflated it to 1.5 bar. Ideally, the air shouldn't leak out anywhere.

As I said, in my case, I couldn't even reach 0.3 bar. I pumped air into it, but as soon as I stopped pumping and the bike pump came apart, the pressure had already escaped from the intake manifold.


It might sound complicated now, but it's actually simpler than you think and shouldn't be a problem with some manual dexterity .

Hi,
Okay, I'll look into that. But what seems like a strange phenomenon to me is this: why does everything work when idling? Shouldn't the vacuum pump be working much less strongly at idle since the engine speed isn't as high?

Again, I'm asking how the vacuum is measured (is there a DIY method?) and how are the intercooler pipes checked for leaks (is there a trick to it?)

-Sincerely-,

Sven@G4H wrote:

Hi, Okay, I'll check that out.

However, what seems to me like a small phenomenon is this: why does everything work in idle, when the vacuum pump should be much weaker since the engine speed isn't as high?
Yes, but as the engine speed increases, the solenoid valve receives less current, even if the target load demand has not yet been reached (that's just how the load demand management system works in TDI engines).
If the vacuum pressure is slightly low, the valve travel indicator (VTG) may already retract from its maximum position, whereas it would still be at the maximum position with normal vacuum pressure.

However, your car shouldn't perform significantly worse than other similarly heavy ALH engines (with the same RPM-to-speed ratios and software) once the torque curve starts to decline.

Weakness and insufficient boost pressure at low RPMs lead me to suspect restrictions in the fuel system (which would also reduce turbocharger drive), but that should primarily become noticeable in the maximum power range... quite puzzling

Quote:

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Again, the question: how is the vacuum pressure measured? (Is there a DIY option?)
1. using a suction-type LDA (Laser Doppler Anemometer).

2. with a water canister and approximately 25 meters of transparent aquarium tubing or similar (including some extra length).
Connect the ends of the tubing to the vacuum to be measured or attach them to the canister.
Hang the middle of the hose approximately 10 meters high, start the motor, and measure how far the water is sucked up from the canister.
Should be approximately: 8 meters equals 800 millibars of negative pressure.

Make sure that air is never drawn on the canister side, otherwise water can be sucked through the highest point of the hose and into the pump, and then likely further into the engine .
The same applies if the highest point is lower than the height to which the water is drawn up (it cannot exceed 10 meters, which corresponds to a water column of 1 bar).

Otherwise, the method works reliably, and is certainly accurate to within mbar (which is equivalent to 1 cm of suction height). After all, you wanted something for DIY enthusiasts to read... or would you rather buy a vacuum-assisted lifting device (LDA)?

Quote:

and how are the intercoolers checked for leaks (is there a trick)?}Take a look at my previous answer!

Hi,
Sorry, I hadn't read the previous thread yet! I think I'll just buy a naturally aspirated LDA instead, or try to borrow one somewhere.

I've also been thinking about the fuel supply, but as I said, the diesel filter is new and the line from the tank to the filter has been blown through with compressed air. Could there be a strainer inside the tank?

-Sincerely-,

Sven@G4H wrote:

I think I'll probably just buy a naturally aspirated LDA, or try to borrow one somewhere!

Unfortunately, I had secretly hoped for a photo with the hose gauge .

Quote:

I've also been thinking about the fuel supply, but as I said, the diesel filter is new, and the line from the tank to the filter has been blown through with compressed air. Could there be a strainer in the tank?

Hm, the records show no strainer as a separate item... but I can hardly imagine that all the dirt that could get into the tank should also be allowed to enter the thin pipes .

Hi,
"So, I connected an LDA (Linear Differential Analyzer) today. I installed it in the line that goes to the pressure sensor, so I can directly measure the pressure at the sensor! When the car is stationary, it's around 600 mbar. When I give full throttle in 4th gear at 1200 RPM, the vacuum increases to 700 mbar and only drops to 400 mbar at 2000 RPM! So, it seems like everything is working correctly!"

The behavior is still the same: it only starts to provide power or build pressure above 2000 RPM. What else can I check now?

-Sincerely-,

Sven@G4H wrote:

The behavior is still the same, it only starts to pull or build up pressure from 2000 RPM onwards! What else can I check now?

For example, a clogged catalytic converter.

Hi,
I've suspected the catalytic converter for a while now, but checking it is quite involved, so I want to rule out everything else first! However, the fact that the speedometer reads 195 km/h also argues against a clogged catalytic converter; wouldn't that be noticeable?

"I've now connected the LDA (Lambda-Sonde) to the intercooler. When the engine is idling, it shows 0. The needle only starts to rise when I give it full throttle, and around 1800 RPM. At 2500 RPM with full throttle, it shows exactly 1 bar! If I drive at half throttle below 2000 RPM, the needle doesn't move at all. Could this indicate a leak in the intercooler area? What do you think? I'll definitely buy two more caps on Monday and check that."

-Sincerely-,