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  #1  
Old 04-22-2019, 08:00 AM
Rotorblade Rotorblade is online now
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A few days ago I've replaced my Siemens MAF sensor (I suppose it was OEM; it is very well built) with a new one presumably from Hella, however the fuel trims go now all over the place, up to 20-25% although the Stft+ltft is within 15%. Also if I push the throttle then I let it go the engine sometimes almost dies for a second then comes back or when I stop to a traffic light.
This new sensor looks poorly made, out of a single type, black plastic, rough finish and few printed details. The original one is different. It wasn't cheap, at almost 150$ so it was supposed to be decent quality.
The old one reads at idle 3.57g/s but the Hella shows 4-4.2g/s, fr time to time it drops briefly to 3.5g/s(16C outside temperature, 40C intake).
Now the Siemens sensor is back on my car. I will see how it runs to make again a comparison.
The reason I've replaced it is I got some idle fluctuations (500-1100rpm) without rpm going too low for the first 60s after each start up. Also after 10 years I thought it might be slightly decalibrated.

Any thoughts ?
The engine is 2008 3.0si n52

Thanks.

Last edited by Rotorblade; 04-22-2019 at 08:02 AM.
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  #2  
Old 04-22-2019, 06:41 PM
wifesauto wifesauto is offline
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i would give the whole intake system a good look at.......vacuum leaks and look to see any coking in the throttle body
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Old 04-22-2019, 09:01 PM
Andyz4 Andyz4 is offline
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Mein Auto: 2006 Z4 3.0si
The Siemens maf should be oem, agree with wifesauto on possible vacuum leak since you didnt report any codes. Check the pcv hose on the valve cover, they get brittle and crack after repeated heat cycles.
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  #4  
Old 04-23-2019, 11:55 AM
Rotorblade Rotorblade is online now
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The fuel trims stay at -5 to -7% and when reving it up to say 3000rpm it goes to -10 sometimes to -15%. So it doesn't seem to have any vacuum leak but I'll check again. Anyway with the siemens maf it seems to run smoother than with the hella which is possibly counterfeit.
Other sensors readings
o2 0.75V at idle.
From INPA
The vanos move to the target values
Filtered rough running: ideal per cylinder:
no 1 goes sometimes down to -2.5
The others vary no lower than -1.5
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Old 05-18-2019, 01:46 PM
Rotorblade Rotorblade is online now
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I've fitted today an oem continental sensor and I get better readings now, within 0 and - 5% stft+ltft bank1! And the bank 1 is close to 0. So there are some little problems on the bank 1. That means the Hella sensor is either counterfeit or poorly designed.
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