TDI 90PS: Strange Mass Air Flow Sensor Issue

Hello everyone,

Recently, my Passat TDI 90PS was running like a snail, with no acceleration. I unplugged the mass airflow sensor (MAF sensor) and, lo and behold, the engine runs relatively well. Currently, I'm driving with the MAF sensor unplugged. Normally, the car should run worse and the limp-home mode should activate. However, neither of those things is happening. Can I be sure that the MAF sensor is defective? I'm hesitant to buy a new one based on this alone, as they are quite expensive!

Who has advice?

Best regards, Franz.

Hello Franz,

A forum participant once posted about a "makeshift solution" to allow the car to still move somewhat, even with a defective mass airflow sensor.
If I remember correctly, he connected the +5V power supply of the mass flow sensor directly to the pin for the measurement output of the mass flow sensor, and he didn't connect the mass flow sensor itself, for example, via a 100-ohm resistor.
This would constantly trick the engine control unit into believing that a very high air mass was being drawn in. In the case of TDI engines with distributor injection pumps, this should prevent any reduction in the injection quantity, allowing the engine to maintain its normal power output.
Of course, in that case, soot reduction will no longer occur, and the mass airflow sensor (MAF) is installed for that purpose, as well as for the EGR system.
However, I can't tell you how your engine control unit will react to such a modification; you'll have to try it out yourself. I've never done it myself.
If your car runs well under load (full power) and you don't have any leaks between the turbocharger and the mass airflow sensor (MAF) where unmetered air could be drawn in, then you can likely invest in a new MAF sensor.
You can measure the pin assignments of the mass airflow sensor (+5V, measurement ground, measurement output of the MAF sensor) or use the search function, as the pin assignments have been posted before.

Best regards, Rainer.

rl2664 wrote:

With the connector disconnected, the engine runs with a fixed mass airflow sensor (MAF) value of 555mg/h. Purchase a new one and have the dealer verify and document its correct function. (1551 characters) And keep the receipt in a safe place!

Best regards,
Ronny.

(I've copied this here to create a thread.)

Hello Rainer,

Thank you for your input. I guess I'll probably have to get a language model (LLM) after all. Maybe I can borrow one to test it out. I'll ask the dealer.
'What I still don't quite understand is that my mass airflow sensor (MAF) outputs a voltage of 1.5 - 4V at idle, yet the car runs very slowly with the MAF connected. What kind of error could the MAF be producing? It seems the fixed value I'm using works well because I'm actually quite satisfied with the performance now.'

Thank you again.

Best regards, Franz.

Franz wrote:

What I still don't quite understand is that my mass airflow sensor (MAF) outputs a voltage of 1.5 - approximately 4V at its output, and yet the car runs very slowly with the MAF sensor. What kind of error could the MAF sensor be producing?

Hi Franz,

Under what conditions did you measure these voltage(s)?

Franz wrote:

Only what is still not entirely clear to me is that my mass airflow sensor (MAF) outputs a voltage of 1.5 - approximately 4V at its output, and yet the car runs very slowly with the MAF sensor. What kind of error could the MAF sensor be producing

?

Hello Franz,

"I've never encountered anything like this before. Assuming you've measured correctly, it would be interesting to log the values using a diagnostic tool to see what values the engine control unit is receiving."
A partial rebuild of the mass airflow sensor takes less than 5 minutes and is an alternative if the possibility exists.

Best regards, Rainer.

Hello Ulf,

I measured the specified voltages both when the vehicle was stationary and while it was moving (I installed cables inside). The voltage readings were taken at engine speeds between 900 and 4000 RPM. As my title suggests, this is a strange phenomenon.

Best regards, Franz.

Hi Franz,

Okay, "approximately 4 volts" seems a bit low for a mass airflow sensor (MAF) when operating at maximum airflow = full load at 4000 rpm, and with correct boost pressure. It should probably be closer to 4.5 volts.

However, even with the mass airflow sensor disconnected, the car should still have reduced power.
The opposite scenario, as in your case, seems so distorted that I would spontaneously believe there's a defect in the engine control unit – although, based on general experience, this is extremely rare.

Could you please provide us with some more details? For example, give us the performance "before," the performance "now" with and without the mass airflow meter (MAF), perhaps using the 2000-4000 rpm test as an example, and any other noticeable issues.

Do you already have a reliable error code readout or measured values during a test drive?
And most importantly: is the turbocharger pressure correct?

Hello Ulf,

Regarding the performance, here's some background information. I took over the vehicle from a relative about a year and a half ago, with 220,000 km on the odometer. It was a company car and always serviced at a VAG (Volkswagen Group) workshop. I believe that's also the main problem. Besides wear and tear parts, very little else was ever replaced, although there was already a noticeable drop in performance at higher RPMs (limp mode was activated). About two years ago, the mass airflow sensor (MAF) was replaced once, but the problem wasn't resolved.
Now, thanks to the help I received on the forum, I've replaced the turbocharger boost control valve – the problem is solved. This just confirms once again that you can't rely on the 'parts exchange' program offered by VAG. That's why I have a lot of trust in workshops. When I look at the invoices my relative has shown me (he gave them all to me), it's mind-boggling. The last one was for over 4000 DM for maintenance at 200,000 km. That's insane. And the actual problems weren't even fixed.
Okay, let me briefly describe my experience since I've owned the car. From the beginning, I had the impression that the car was underperforming (I always attributed it to the weight - I had previously driven a Golf TDI with 90 horsepower). I then tried a simple tuning modification (1.2kOhm resistor). After that, the car ran quite well. However, even then, I noticed that there was no change whether the mass airflow sensor (MAF) was connected or disconnected (I discussed this with you about half a year ago). I didn't pursue it further because the performance was acceptable at that point, although there were occasional limp-home modes. Those issues have now been resolved. For some time now, the performance suddenly dropped to a minimum, until I realized that disconnecting the MAF sensor restored the power, as reported. However, I can't really say whether it could perform even better.
So, that was quite a long story, but I wanted to give you a detailed account of what happened. By the way, my engine now has 247,000 kilometers on it.

Thank you again for your cooperation.

Best regards, Franz.