TDI 130 PS (AVF): Noises When Releasing Gas

Hello,

"When I release the accelerator pedal while accelerating (e.g., before each gear change or when releasing the clutch), I hear a distinct, noticeable squeaking noise. This noise is also audible even when not shifting or releasing the clutch; simply abruptly releasing the accelerator pedal is enough to cause it."

It's fine if I don't hear anything, as long as I only release the accelerator when there's no more acceleration happening, essentially when I'm just cruising.

"The people in the workshop think it's normal. A valve would release excess pressure. But that can't be right. TDI engines don't have what's called a wastegate valve; only gasoline turbo engines have those, and they have a throttle valve. Diesel engines, as is well known, don't have a throttle valve. Therefore, this valve is also missing."

It's not the exhaust gas recirculation system; I temporarily disabled it for testing purposes, but it didn't make any difference.

What the heck is wrong with me? That noise is driving me crazy!

PS: The turbocharger was replaced at 2,700 km due to complete turbocharger failure. It now has approximately 10,000 km on it. The noise has been present since the beginning (even with the old turbo).

Hello,

There were some issues with the turbocharger hose attachments on certain Passat AVF models. However, that problem should have been resolved by the model year of your car (if you want, you can send me your vehicle identification number via private message).
How did you disable the AGR (Abgasrückführung)? Did you just unplug the electrical connector, or did you block the vacuum hose?
Are those more like croaking or whistling sounds?

Hello Michael,

My Passat has the chassis number (please see the private message). It was manufactured around the end of 2006, beginning of 2007.

It's not a whistle, but clearly a croaking sound.

I removed the fabric-covered hose from the vacuum pump and sealed it with a suitable screw, as described in a troubleshooting guide for an older TDI engine. Unfortunately, it didn't solve the problem.

The noise is always present in the situations described above (i.e., whenever the acceleration process is interrupted, but there is no noise if acceleration is not occurring). It occurs regardless of whether the engine is cold or warm, and the outside temperature is also irrelevant.

I would be very grateful for any further assistance!

Greetings.

Rudi.

Hello Rudi,

Please check the motor mounts. Pay particular attention to any damaged screws.
According to the vehicle identification number (VIN), the turbocharger boost hoses on your car are in good condition.

I had also suspected the engine mounts. I'll take a closer look at them.

I had completely forgotten about that idea. Back then, I thought that with a new car, the bearings couldn't possibly be defective.

If a bearing is defective, the symptoms described could occur. Of course, when the gas is suddenly released, the dampers have to work very hard.
Is the motor hydraulically dampened, or what?

Thank you for the tip!

Hello Rudi,

The bearings on the left and right sides are hydrostatic bearings. The bearing at the front and bottom is a rubber buffer.

Hello.

I drive an A4 Avant 1.9 TDI PD with 96 kW. I also experience a similar squeaking noise in the situations you mentioned. However, I'm more inclined to think it's more of a rattling sound. The dealer told me that it's the sound of excess exhaust gas being released and that I shouldn't worry about it.

- Colour-Lightning

Colour-Lightning wrote:

I was also told by the friendly service advisor that it's the sound of the excess boost pressure being released, and that I shouldn't worry about it.

If "excessive boost pressure" were to be lost, you should definitely start thinking about things – for example, whether you have enough money to buy a new turbocharger.

Your friendly mechanic, of course, is taking it all in stride (assuming he believes what he told you), and he might even be sensing a big repair job coming up.

I definitely advise you to consider switching workshops. Because if someone tells such a load of nonsense, I wouldn't trust them anymore...

Okay, I'm ready. Please provide the German text you want me to translate.

Hello.

Thanks for the link. But when did a TDI or a diesel engine start having a throttle valve? The boost pressure is released with every gear change. That's actually a good thing, because when you press the accelerator again, new boost pressure is built up with fresh, cold air. If the boost pressure remained constant, the load on the crankshaft would be too high. Just like you can hear the rattling at low RPM during acceleration; otherwise, the force on the crankshaft would also be too great. Only above a certain RPM is the full boost pressure used. At least, that's what I was told by a friend who is a master technician at a friendly customer service department.

Colour-Lightning wrote:

The boost pressure is released with each gear change. That's actually a good thing, because when you accelerate again, new boost pressure is built up with fresh, cold air. If the boost pressure remained, the stress on the crankshaft would be too high.

Even if your colleague is an expert in friendly customer service, that doesn't mean it's a free pass for a series of hits in the blunder department 1.

@ Colour-Lightning

Okay, so a gasoline engine only releases the pressure before the throttle valve in order to prevent the turbocharger from working against pressure. Some gasoline turbo tuners like to install what's called a "pop-off valve" to abruptly release the pressure and keep the turbocharger spinning, so that it can build boost again immediately when the accelerator is pressed.
You should listen to the legendary Walter Röhrl from his Turbo Quattro days. Initially, they had something like a blow-off valve, but that wasn't enough, so they always drove "against the brakes," meaning they kept the accelerator pedal pressed even while braking to keep the turbocharger spooled up. This was also known from early Formula 1 turbos (Renault).