Only 0.5 bar of boost pressure at idle

hello,
I've been having a problem with my car (Audi A6 C4 TDI, 1995, 140 hp, AEL) for quite some time: it has too little power across the entire RPM range. I've been to several workshops. At the last one, they measured the power on a dynamometer: 107 hp , 0.5 bar of boost pressure, and 620 (something) of air mass at 4000 RPM. The mechanic told me that the boost pressure should be 1 bar, and the mass airflow sensor should read around 800. So, are the turbo and mass airflow sensor issues in the... (rest of the sentence missing)?
Could it be that the turbo is broken, even though it lasted 50,000 km?
If not, what could be the cause?
What does that cost?
Can you buy something like that used, or would you advise against it?
Best regards.
Martin.

Hello,
Ulf's technical article on troubleshooting TDI engines should help you with your problem!
Low boost pressure is rarely caused by the turbocharger itself.
The insufficient air mass is a consequence of the too-low boost pressure!

Best regards,
Albrecht.

Sure.
Furthermore, 0.5 bar is suspiciously close to the pressure that occurs when, in a turbocharged system with overboost protection, the boost pressure regulator (LDR) is in its pass-through position because, for example, the connector next to it is disconnected.

Hello... well, I can't imagine that the problem is caused by the charger and the mass airflow sensor (MAF). The MAF sensor definitely seems to be defective. You should replace it, and then do a test drive. The boost pressure should then return to normal. By the way, the mass airflow sensor (MAF sensor) costs around €130, including taxes.

Best regards, Mike.

Why should the LMM be discontinued?

If the turbo isn't properly "inhaling" – as I understand it, because it's running on a standard wastegate control – then it's actually not drawing in as much air.

My 1Z engine would settle at around 0.6 bar without the boost pressure control system, but with the boost pressure limiting solenoid valve, it achieves 0.9 bar.

If you also have this solenoid valve, please check if it is being activated and if it is actually "regulating" properly!!

m;

Why should the mass airflow sensor be broken??

If the turbo isn't properly 'inhaling' – as I understand it, because it's running on a standard wastegate control – then it's actually not drawing in as much air.

My 1Z engine would settle at around 0.6 bar without the boost pressure control system, but with the boost pressure limiting solenoid valve, it achieves 0.9 bar.

If you also have this solenoid valve, please check if it is being activated and if it is actually 'regulating' properly!!

m;

'In my case, the boost pressure was also only 0.7 bar, and the air mass was between 660 and 690. I replaced the LLM (likely referring to a Lambda sensor or similar), and now the boost pressure is back to normal.'

Of course, there could be other causes. My car accelerates well up to the third gear, but then the 'acceleration' stops. The maximum speed is about 150 km/h on a flat road.

I'm still looking for a solution...



Best regards, Mike.

Thank you for the answers.
I don't think the LMM is broken anymore (it costs over €500, by the way).
I'll check the solenoid valve, etc., or I'll make an appointment with Bosch. I hope they'll put in some effort and won't immediately try to sell me a new charger.

Thank you for the answers so far.
I don't think the LMM is broken anymore (it costs over €500, by the way).
I'll check the solenoid valve, etc., or I'll make an appointment with Bosch. I hope they put in some effort and don't immediately try to sell me a new charger.

I just checked with VW... you have a mass airflow sensor, not a volume airflow sensor. That's a big difference... but never mind... it's actually unlikely that the mass airflow sensor is defective.

Sorry, I can't think of anything else.

Best regards, Mike.

I finally took my car to Bosch. I was really hopeful this time. Now he called me to say that he couldn't do anything because he couldn't access my software with his PC . Is that even possible?
Okay, I'm going to take some time this weekend and try to check things myself as much as possible. The workshops are starting to get to me.
Does anyone know of a competent repair shop in southern Germany, near Lörrach? I've already been to 4 different workshops, and each time without success. I'm slowly getting tired of it .

I just took it to VW, and they said it's the turbocharger.
What should I do now? Boost pressure is 0.6 bar. I still don't really believe that's the problem, but I also don't feel like going to another workshop.

Hi,
Does it reach 0.5 bar only at 4000 RPM, or does it reach that pressure earlier, somewhere between 2500 and 4000 RPM?
If you are unable or unwilling to follow the troubleshooting steps outlined in the technical article, you will likely have to accept the financial loss. There's always the risk that you'll be charged for a completely new turbocharger for something as minor as a stuck bypass valve.

Hi,

I would do exactly what Christians says - check the boost pressure regulation using relevant technical articles. That way, you at least eliminate trivialities that could become very, very expensive with the workshop's approach.

Best regards,

Jan.

I can only agree with my two previous speakers!

Take a look at my last measurement with and without boost control on the wastegate turbocharger: /viewtopic.php?t=2773&start=48

The only difference between the two measurements is that the connector for the boost pressure control valve (solenoid valve N75) was disconnected in one of them.

You might have a broken wire somewhere between the control unit and the solenoid valve! In the simplest case, the plug might have just come loose or have a poor connection (due to corrosion?).

Have fun debugging!

Regards,
Alex.

I've also looked myself, and at first glance, there doesn't seem to be anything obvious. To search more thoroughly, I also lack the time, tools, etc.
I specifically told the workshop that the problem was definitely related to the turbocharger control, a solenoid valve, or something similar, and they spent 3 hours looking for the issue but found nothing. Surely, they can't all be deliberately incompetent, can they? I also contacted Audi headquarters and expressed my dissatisfaction with the technical competence of the workshops, and asked for their advice. They only responded that they cannot provide a remote diagnosis and that I should contact a specialized workshop . Every time, I pay a lot of money, and in the end, it's for nothing.

Hello Martin119!

You probably have a simple multimeter, or know someone who could lend you one.
This will at least allow you to check if there is voltage at the valve or if the ground connection is okay.
Since this isn't an intermittent issue, you should be able to detect a broken cable using this method. I can provide you with the pinout diagram for the N75 if you need it.

Regards,
Alex.

I'm now, in my desperation, completely at my wit's end. Replaced the turbocharger; the boost pressure is probably okay now.
...but the performance is probably still not as good as it should be .
I was told that the measured airflow is still too low, which is why not enough fuel is being injected. They want to install a new airflow meter as a test to see if it then measures correctly. It sounds really weird .
The workshop before us had come to the same conclusion: it's the turbo and the mass airflow sensor.
Are they all wrong, or am I really in such a bad situation? The repair bill is so high, it's almost the price of a whole other car.