AHU 66kW: Troubleshooting & Repair Guide

Hello,

For a long time, I've been struggling with my car due to its lack of power, or rather, its lack of agility. I first noticed the problem at the beginning of last winter (as far as I can remember, shortly after the timing belt was replaced and the winter tires were installed ).

Here's the translation:

"This manifests itself as follows: the acceleration is sluggish, and the maximum speed is reached slowly, but eventually reaches around 180 km/h (according to the speedometer). There's a noticeable lack of power, especially in 4th and 5th gear. When I fully accelerate at 100 km/h (5th gear, approximately 2200 RPM), not much happens. Subjectively, I would say that the engine starts pulling reasonably well, or even better, from around 3000 RPM."
"I'm hearing a faint noise (a hissing or sucking sound, it's hard to describe) somewhere between 1400 and 3000 RPM. The Bosch technician noticed it too. He said (before the dyno test) that the car was missing about 10 horsepower. So, it's probably not just my imagination."
At Bosch, all possible values were read out, and according to them, everything was okay. Well, that doesn't necessarily mean anything.

The Hard Facts:
- Power test bench -> 70kW. According to Bosch, everything should be fine. However, I don't know...
how the exact curve should look across the entire speed range.
- LMM temporarily replaced; no change.
- Replaced the pressure hose leading to the cooling system due to damage from rodent bites, tightened all hose clamps -> problem solved.
- Have the start of the injection process checked at least 2 times -> ok.
- AGR -> The hose was disconnected, but there was no improvement. In fact, there might have been a slight worsening. The sealing plates have already been ordered just to be on the safe side.

Okay, I've been playing around with VAG-Com and logged the following:

Mass Air Flow Barometric Pressure Manifold Pressure Throttle Specified MAP Actual MAP Duty cycle
text
Regulator
valve?
mg/R mbar mbar % /min mbar mbar %
498.8 958.8 1030.2 15.3 1575 1795.2 1173 35.1
821.3 958.8 1723.8 100 2121 1866.6 1948.2 39.8
814.6 958.8 1927.8 100 2646 1836 1866.6 64.5
804.7 958.8 1846.2 100 3087 1825.8 1836 68.5
761.4 958.8 1815.6 100 3465 1825.8 1825.8 68.5
741.5 958.8 1825.8 100 3780 1805.4 1805.4 68.9
714.9 958.8 1795.2 100 3990 1795.2 1795.2 68.9

I hope this is somewhat legible...

I hope I will be helped here .

Does anyone have any ideas?

I once read something in the professional lockpicking community about what's called...
"Exhaust restriction, low mass airflow values in the lower RPM range" - Issue.
From what I've gathered, several people are experiencing this issue.

It seems like the LD (likely referring to a turbocharger or similar device) spool-up is too slow. I'm experiencing overshooting with approximately 2000 RPM (in my case, +0.1 bar!).

I recently, as a test, disconnected the plug from the boost pressure control valve. The boost pressure regulates nicely to 1560 mBar (absolute). I think that's okay. It sounds strange, but it somehow seemed like my car felt more agile; of course, the performance was bad with about 0.6 bar of boost pressure .

Hello Roland,

Your boost pressure is fine, and the control behavior appears normal at first glance.
The air mass is too low. If the mass airflow sensor (MAF) is working correctly, there must be a restriction somewhere that is limiting the amount of air being drawn through the MAF. This restriction limits the maximum fuel injection amount; check the corresponding measurement block. Depending on the quality of the label file from the respective distributor, you may or may not see what I'm referring to.
Examples include: a detached intake snorkel on the loader, a clogged catalytic converter/exhaust system.

Best regards, Rainer.

Hello Rainer,

Thank you for your answer. What I find strange is that the amount of air decreases with increasing speed or velocity. Shouldn't it be the opposite?
What should the LMM values be at RPMs above 2000? (I once heard that for my engine/car, it should be around 850mg/stroke...)

Intake snorkel on the loader -> Are you referring to the black rubber hose connecting the air filter housing to the turbocharger?
There are no visible abnormalities in the engine compartment, and nothing appears to have fallen off.
I have no idea how something like that could be leaky/become leaky.

Catalytic converter/exhaust blocked... that will be difficult to check, right? I don't think there's any rattling noise, as far as I know.

I'm almost certain that the lack of power is related to the subtle suction noise. But I just can't find anything wrong. And the workshops can't find anything either. So, it probably doesn't have anything to do with the timing belt replacement, right?

Next, I'm going to completely seal off the AGR (Abgasrückführung) system with sealing plates to be able to rule it out as the cause. Then, I want to clean the intake manifold and replace the gasket. Maybe that will help. I'm starting to get really frustrated with this car.

Hello,

roland_99 wrote:

What I find strange is that the amount of air decreases with increasing speed or velocity. Shouldn't it be the other way around?

The mass flow naturally increases with engine speed, but relative to each individual intake stroke, it approximately follows the curve of the torque curve. At lower speeds, it initially increases until the turbocharger starts to provide significant boost, then there is a more or less pronounced plateau, followed by a decreasing section. This decrease is caused by throttling losses at higher flow rates and depends on the valve opening times. In TDI engines, this effect is particularly pronounced at higher speeds due to the swirl channel, which is essential for clean combustion. This behavior can only be partially compensated for, even with higher boost pressures.

Quote:

I'm almost certain that the poor performance is related to the subtle suction noise.

But I just can't find anything.

Do you know anyone with a compressor? You need to disconnect both hoses from the turbo and the intake manifold. Use a piece of 50mm round steel, drill a hole through it, insert one pressure hose into the hole, secure it with a clamp, hold the other pressure hose closed with your hand, and have a friend blow into the hole while you watch where the air is escaping. It could also be the intercooler itself; the plastic parts are only held in place by bent aluminum tabs on the frame, and they can sometimes pop off. But how can the electronics not detect anything? It should actually detect an implausibility between the air mass, engine speed, and intake manifold pressure ?


Best regards, Uli.

Uli S. wrote:

But doesn't the electronics notice anything? It should actually detect a plausibility error between the air mass, engine speed, and intake manifold pressure

? } If such an error detection system were present and programmed accordingly, yes.

But then, every aging LMM (Linear Motion Module) would generate a countless number of "enigmatic" (for average workshops) error messages - perhaps the chaos that the developers anticipated was a reason not to implement such a feature?

Was there a solution?
I have the same.
At my mother's, with her 1998 Ahu.
140,000 km

Xander wrote:

Was there a solution? I have the same. At my mother's, with her 1998 Ahu. 140,000 km

What is the same?

It was probably the intake channels that were clogged due to EGR/KGE deposits... I've seen this happen before, and it matches the error pattern of a lot of misfires but relatively low airflow from the top .