Passat with AHU engine - occasional power drops?

Hello,

I'm currently working on a 1997 Passat 1.9 TDI with an AHU engine, trying to find the cause of a power loss.
The vehicle runs smoothly up to approximately 3000 RPM, but then noticeably loses power and starts to jerk (like when you release the gas and then press it again). The error memory is empty.

I have now checked the entire hose system, and everything is properly connected and leak-proof. I then, somewhat randomly, replaced the magnetic valve (also known as the diverter valve) for the turbocharger, which is located at the front, below the air filter. (It's a brass cube.)
"I took it for a test drive, and the jerking was gone. However, based on my feeling, I think the engine could still pull a little harder, because my brother has an AGR engine (also 90 hp), and his seems to be a bit more responsive. So, I installed a pressure gauge in the turbocharger system and took it for another test drive. During full load, the gauge shows a pressure of 0.8-0.9 bar, with a slight vibration. Question: Is this boost pressure appropriate for the AHU engine, or what should it be? Could it also be a problem with the mass airflow sensor?" According to the owner, it should only be 5 months old, as it was replaced by a VAG part.
Furthermore, when accelerating, a hissing sound starts, similar to a slight whistle from an exhaust manifold or a turbocharger hose. A friend told me that this should be considered normal, as the AHU engines are known to produce this sound. Can anyone here confirm this? Best regards, Luiggi.

Hello,
The boost pressure (supercharge pressure) of 0.8 to 0.9 bar is acceptable for the AHU (1Z), but should be reached at approximately 3000 rpm. The target values in the test are 1800 to 2050 mbar at 3000 rpm (in the 3rd...). Accelerate from 1500 RPM with full throttle.
The manometer's vibration is likely due to the lack of damping in the line.
I don't think that hissing sound is normal. Could the EGR valve be causing you trouble?
Around 1900 rpm, in my car (1Z), with the new mass airflow sensor, the air mass-based fuel injection limiting switches to torque limiting. If that's the case for you as well, then the mass airflow sensor (MAF) should also be fine.
Perhaps a full-throttle log run across the entire RPM range in 4th gear would yield more data (record air mass, boost pressure, and emissions control limitations)?
hg
Herbert.

Hello,

"I couldn't rest until I figured out the issue with the mass airflow sensor (MAF). According to the owner, it was replaced a few months ago by VAG. I took a closer look, and if VAG really replaced it, they only replaced the sensor element. They installed a Pierburg universal MAF element (the original is Bosch). I didn't have a new one on hand, but I found one from a Ford Galaxy (115PS PD AUY) that fits the connector housing. I installed it, and the car is now running completely normally. Since it was a used part, I also drilled it as described in a technical article. This is just a temporary fix until Monday, when a new one will be installed. So, the problem is now solved, except for this hissing noise during hard acceleration."So, it's only building boost under load. That whistling sound, like the Audi 2.5 TDI V6 engines make at lower RPMs? Such a chirping sound...
Perhaps someone among you has installed an AHU and could observe its behavior during acceleration and report back here. I hope it doesn't cause any consequential damage, especially to the turbocharger. According to the owner, he's been driving with it for about a year already. So, with the faulty turbocharger boost control valve and the mentioned jerking/hesitation, he took it to VAG (Volkswagen Audi Group) twice, but the error was never found (although the bills were higher!).
What do you think...?

Best regards,

Okay, I also checked the AGR valve. I actuated the valve with the pressure pump and then released it. It makes a distinct "clunk" sound, so it's clearly audible. It also seems to hold the pressure. And it's not producing any black smoke. That would otherwise be a good sign of a faulty AGR valve...

Hello,

I think I've finally identified the noise today. It's 90% coming from the alternator. It's that hissing sound, which I initially thought was the turbocharger. That's why I also removed the hose from the turbo today and inspected the turbine by hand for any play or scratching noises – the result was all good. Then I focused on the alternator, which starts making these noises or whistling sounds at a certain RPM. What I noticed is that the freewheel mechanism on the alternator moves out about 2mm when I accelerate. Therefore, it probably won't fix the problem by just replacing the freewheel, because something else is wrong with the bearings or inside the alternator itself. What do you think? Should I replace the entire alternator? Best regards.

Hi,

I recently renewed the LIMA (likely referring to a specific component or system), and the problem with the (running noise) has also been resolved.

Okay, here's the solution to the performance problem:

The engine's hesitation at higher RPMs and the loss of power were caused by a faulty solenoid valve and, subsequently, a faulty mass airflow sensor (although there was no error code stored in the system).

Best regards, Luiggi.

Thank you for the feedback!