Odd, maxed out fuel trims on 07 Q7 3.6
03-18-2019, 01:38 AM
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Odd, maxed out fuel trims on 07 Q7 3.6
Months ago, with my B5 Passat ageing, I bought a 2007 3.6l Q7 for super cheap off a friend who didn't want to deal with the issues it was developing. So far I have worked through the sun roof leaks, sagging headliner, bad wire to the shifter and trans that was causing a no start issue and many other annoyances.
What I am having difficulty with now is how rough it is running. Hooking up my new Ross-Tech I get a few codes but none related to the Maf or O2 sensors but when observing blocks 032 and 033 I see the fuel trims seemingly maxed out at 4.5% long term and 25% short term.
Simply unhooking the Maf gets the short term trims to immediately start cycling above and below 0 which seems condemning of the Maf but within 5 minutes of run time the short term trims are right back to 25% and no change with long term holding at 4.5%.
If I clear the codes while it it running with the Maf unplugged then again the short term trims cycle around 0 at first and with 5 minutes of run time they again creep to 25% with the long term trim oddly holding at 0%.
Banging my head here as it seems the O2 sensors are falsely reporting, but both of them? I must not be interpreting the data correctly. Any help is greatly appreciated.
Zach
What I am having difficulty with now is how rough it is running. Hooking up my new Ross-Tech I get a few codes but none related to the Maf or O2 sensors but when observing blocks 032 and 033 I see the fuel trims seemingly maxed out at 4.5% long term and 25% short term.
Simply unhooking the Maf gets the short term trims to immediately start cycling above and below 0 which seems condemning of the Maf but within 5 minutes of run time the short term trims are right back to 25% and no change with long term holding at 4.5%.
If I clear the codes while it it running with the Maf unplugged then again the short term trims cycle around 0 at first and with 5 minutes of run time they again creep to 25% with the long term trim oddly holding at 0%.
Banging my head here as it seems the O2 sensors are falsely reporting, but both of them? I must not be interpreting the data correctly. Any help is greatly appreciated.
Zach
03-23-2019, 03:44 PM
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You're correct to doubt both O2 sensors reading incorrectly and failing exactly the same at the same time. I made an account just to answer you - I've been in vehicle diagnostics for over 25 years and I felt I needed to chime in here. I've diagnosed this exact issue at least 50 times.
You likely have a vacuum leak between the MAF sensor and your engine. Air is getting into the engine that is not being read by the MAF, this extra oxygen is read by the O2 sensors and they increase fueling to compensate. Check your intake tubing and also all hoses connected to the valve covers. It doesn't take much of a leak to increase fuel trims to the level you're seeing at idle. If you hold the RPM to 2000 you'll likely see the fuel trims start to come down because the leak is very little compared to the volume then engine is pulling in.
Another possibility is your MAF sensor has been saturated with oil by an aftermarket K&N style air filter (if you have one). If that's the case brake clean or contact cleaner on the MAF wires will fix it. It must be a no residue type cleaner.
Good luck!
You likely have a vacuum leak between the MAF sensor and your engine. Air is getting into the engine that is not being read by the MAF, this extra oxygen is read by the O2 sensors and they increase fueling to compensate. Check your intake tubing and also all hoses connected to the valve covers. It doesn't take much of a leak to increase fuel trims to the level you're seeing at idle. If you hold the RPM to 2000 you'll likely see the fuel trims start to come down because the leak is very little compared to the volume then engine is pulling in.
Another possibility is your MAF sensor has been saturated with oil by an aftermarket K&N style air filter (if you have one). If that's the case brake clean or contact cleaner on the MAF wires will fix it. It must be a no residue type cleaner.
Good luck!
03-29-2019, 04:59 AM
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Thank you for that info. I did try to clean the original MAF with brake clean. I change. I bought another MAF at a junk yard. No change. I am able to stall the engine with my hand over the MAF opening and do not hear a vacuum leak while doing so. I know that is not definitive.
What I have noticed is lambda is around 1.48 no matter what the fuel trims are doing. Something seems wrong in the O2 sensor feedback loop. What would cause this in both O2 sensors other than a defective ECU?
What I have noticed is lambda is around 1.48 no matter what the fuel trims are doing. Something seems wrong in the O2 sensor feedback loop. What would cause this in both O2 sensors other than a defective ECU?
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