Mass Air Flow Sensor Failure?

Hello everyone,

Due to the issue I described in my post yesterday, I tested my A3 81 kW TDI (engine code AHF) on the highway today, and with a lot of effort, I only reached 175 km/h instead of the expected 200 km/h. Overtaking maneuvers on country roads are also quite difficult. However, the engine control unit has not entered a limp-home mode. There is still some torque at low RPM. There is no visible soot buildup.
The test at full load, at approximately 3000 RPM, yielded the following results:

Quantity: Group 10, Field 1:
Air mass is 750, target is 800-1100 mg/H.

Quantity: Group 0, Field 4:
Injection amount is 190, target is 199...214.

Could the estimated power loss of approximately 25-30 horsepower be caused by the mass airflow sensor (MAF), and would there be a correlation with the measured values?

To what extent could there be a connection with sporadic error 17656: Injection start control, regulation difference?

Thank you for your feedback.

Best regards,
Jörg

Hi.

et wrote:

... The test at full load, at approximately 3000 RPM, yielded the following results: Quantity: Group 10, Field 1: Air mass is 750, target is 800...1100 mg/H

.
Clearly, there's not enough airflow. Either there's a significant air leak bypassing the mass airflow sensor, or the sensor is faulty, or the boost pressure is too low.
Please also check this.

Quote:

Could the estimated power loss of approximately 25...30 horsepower be caused by the mass airflow sensor (MAF) and correlate with the measured values?

Yes.

Quote:

To what extent can there be a connection with sporadic error 17656: Injection start control regulation difference?

Here's a possible translation:

"Spontaneously, I'm thinking of this possibility: by preventing the injection process from starting, the control unit reduces the injection volume and the turbocharger pressure."
If the error is occurring sporadically, I would test the cable from the needle lift sensor for any damage, as well as the sensor itself (by tapping it).
Have you ever observed the target and actual values for the start of injection during normal driving conditions?

Best regards, Rainer.

Hello Rainer,

Thank you for your suggestions!

With the new LMM (mass airflow sensor), it's running again. The air mass is approximately 900 mg/H, the boost pressure is 2.0 bar, and reaching 200 km/h is no longer a problem. The intermittent error "sporadic fuel injection start timing control difference" is no longer occurring.
Perhaps a brief history is in order:
- sudden loss of power during overtaking maneuvers.
- After each start, the engine control unit switches to an emergency mode after a short period of operation, the engine runs noticeably louder, the driving dynamics are impaired, and there is no soot production.
- Washed the LMM with alcohol and dried it with a hairdryer.
- The motor no longer engages the emergency program, but there is no power output from mid-range RPMs onwards.
- unfortunately, the measurement data blocks were only read out after that, with the result as posted.
- Injection start values during operation at full load (3000 rpm) are approximately 12° BTDC (Before Top Dead Center), which is within the target range of 8-14° BTDC, so it is normal.

"Quite expensive, these mass flow meters!!! It looks very simply constructed. I took out the insert (with a Torx key access...) and had a look. Does anyone know what's hidden under that potting compound?"

Perhaps a note should also be added to error 17656 in the error database, indicating the possibility of an LMM issue. If error 17656 reappears for me (which I hope it won't), I will contact you again regarding it.

Best regards,
Jörg


P.S. Great website, my compliments! I've been browsing here for a while, but I've just now signed up.