Golf IV TDI can no longer reach its top speed

Hello, I have a Golf IV TDI with 110 horsepower, likely an AFN engine, which is no longer reaching its top speed. The speedometer shows around 195 km/h (previously about 210 km/h). There's also quite noticeable black smoke coming out at night, which isn't as visible during the day. And when accelerating in 4th and 5th gear, it feels like there's no power; the turbo doesn't kick in as strongly around 1800 rpm as it used to. This problem has developed gradually and only became noticeable after I drove another car with a similar engine!
After studying the forum for a while, I suspect it might be a partially closed EGR valve (due to carbon buildup or something similar). Or what else could it be? I'm grateful for any advice.

Best regards,
Andre

Hello colleague,

I had the same problem after experiencing a VGT failure.

Received a new intercooler, AGR valve, and solenoid valve.

Despite that, my TDI 81kW A3 with the ASV engine was only reaching a top speed of 196 km/h on the speedometer.

After testing the mass airflow sensor (MAF) with a 1551, I found that it was measuring too little air mass, which resulted in an incorrect fuel injection calculation.

You can easily test this yourself. The LMM (lambda sensor) for the 90 hp, 110 hp, and 115 hp diesel engines from the Volkswagen Group are the same. I believe the part number is GTA-....30.

Best regards from Lake Constance.

Hello, the LMM (Lambda Measuring Module) is responsible for controlling soot emissions. If the LMM provides values that are too low, the engine control unit reduces the fuel injection amount accordingly to prevent excessive soot formation.
So, if black smoke is observed coming from the exhaust while the engine is running at full throttle, then either something else is malfunctioning or it's NOT the mass airflow sensor (MAF).
First, I would replace the air filter with a new one and check the intake path for any blockages caused by leaves or other debris. Then, you should inspect the charge air pipes from the turbocharger to the intake manifold for leaks. These often reveal themselves through slight oil traces (the crankcase ventilation system vents before the turbocharger, which means some oil mist can enter the intake system).
The next step would be to modify the AGR valve to check if it's closing properly.
As the final step, and because it's the most time-consuming, I would use a diagnostic tool to measure the air mass intake and boost pressure under full load for several seconds.

Regards, Rainer.

Tobias wrote:

Hello colleague, I had the same problem after experiencing a VGT failure. Received a new intercooler, AGR valve, and solenoid valve. Despite that, my TDI 81kW A3 with the ASV engine was only reaching a top speed of 196 km/h on the speedometer. After testing the LMM (Mass Air Flow sensor) with a 1551 device, I noticed that it measures too little air mass, which results in an incorrect fuel injection calculation. You can easily test this. The LMM (lambda sensor) for the 90 hp, 110 hp, and 115 hp diesel engines from VAG are the same. I believe the part number is GTA-....30. Greetings from Lake Constance

.

Hello,

If you summarize the posts and experiences shared in this forum,
It's probably fair to say that the mass airflow sensor (MAF) cannot be blamed for many problems. While workshops often like to replace it, this doesn't always lead to a solution.

Therefore: It is better (and cheaper) to check the pressure and vacuum systems for leaks and damage.


Bye.


Michael II.