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MAF reading high

Help, Advice, Owners' Discussion and DIY Tutorials on Volvo XC90s. The XC90 proved to be very popular, and very good for Volvo's sales numbers, since its introduction in model year 2003 (North America).
richardst
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Year and Model: 2006 XC90 2.5t
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richardst

MAF reading high

Post by richardst » Fri Jul 26, 2019 9:14 pm

I am chasing a problem with a high MAF reading and would appreciate any ideas on how to diagnose.

2006 XC90 2.5t 210k miles

Previous owner had new Bosch MAF installed last year. I purchased a new Bosch MAF and get the same results, leading me to believe it is not a problem with the sensor.

Codes are either 1210 MAF flow too high or 120C MAF signal too high.

Watching the live MAF values with a computer I see 28-30 kg/hr at idle which seems high from the numbers I've seen others report, like maybe double what others have seen. At full load near redline I see about 630 kg/hr which seems about right for the rated 208 bhp. After a reset, long term fuel trims eventually go high and throw codes after 10-20 miles of driving. After a reset air mass correction factor climbs from 1 to 1.15 after just a few minutes of idling.

I have checked for vacuum and boost leaks and believe everything is tight, confirmed by multiple smoke tests. I also overhauled the PCV system with new parts, cleaned throttle body, replaced intake, exhaust, and turbo gaskets. All good things at this mileage but none seem related to the MAF high value issue.

I have removed the ECM, cleaned the contacts, reseated. Same for the MAF connector. Wiring looks OK at the MAF connector. I cannot find a pinout for the ECM, that would be helpful as I could then test the wring between the MAF connector and the ECM socket. Anyone have even a partial pinout that shows where the MAF wires terminate in the ECM socket? I hate to tear apart the whole harness looking for a wiring issue without testing continuity/resistance first.

Anyone have ideas other than a wiring issue that would cause a high MAF value at idle and/or low loads?

Can anyone confirm that I am correct in thinking the MAF value in the OBDII data is derived solely from the MAF sensor? If I'm wrong and the OBDII MAF value is calculated using inputs from O2, temperature, or other sensors then I would need to open up the scope of diagnostics to other potentially bad sensors or wiring.



richardst
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richardst

Problem found - bad ground

Post by richardst » Sat Jul 27, 2019 11:13 am

To answer some of my own questions and hopefully help someone out in the future:

The pinout for the MAF connector is:
1: not used
2: battery voltage ~+12-14v
3: ground
4: +5v
5: signal

I checked resistance from pin 3 to one of the nearby chassis ground points and it was about 8 ohms which seemed high. Cut that wire and spliced in a wire to connect pin 3 directly to the chassis ground, resistance less than 1 ohm. Reported MAF at idle is now in the 12-15 kg/hr range depending on whether or not AC compressor is active and transmission is in P or D. So far have driven about 10 miles with no codes, air mass correction factor and all fuel trims stay within a percent or two of 1 which seems healthy. I am pretty sure the bad ground was the culprit.



richardst
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richardst

General thanks

Post by richardst » Sat Jul 27, 2019 11:16 am

As you may notice I am new to the forum with this thread. I would like to generally thank everyone who participates here, I have found answers to a lot of questions by searching the forums. I imagine there are are many others who find valuable information here yet do not sign up just to post a "thank you."



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MoVolvos
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Re: MAF reading high

Post by MoVolvos » Sat Jul 27, 2019 12:39 pm

-
Always nice to see a new member with DIY research and diagnostic skill. Hope you'll continue contribute.


Blessings,

BKM

1998 S70 T5 - Donated
2003 S80 T6
2003 GS300
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Re: MAF reading high

Post by jimmy57 » Sat Jul 27, 2019 4:23 pm

When looking for bad grounds use pin-out box (everyone has those lying around) or back probe connectors of components. With engine running and the MAF loading that bad ground you will see voltage when there should very nearly zero V. 0.3 V is usually considered normal limit but for sensors it should be no more than 0.15V. T pins slide through the rubber seals on the back of connectors readily and do no harm to make the connection. Ohm meter use can give you false reading even with battery disconnected due to capacitors in circuits.



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