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Mass Air Flow

posted May 27, 2016 18:09:17 by CharlesAcosta
I have a 99 Mercury Montaineer with fuel trim on both banks going a little high with RPM up. Not high enough to set a MIL. I noticed on my scan tool that the Baro PID reads 147.9 hz when it should read at about 155 hz at less than 1000 feet above sea level and my two resistors were dirty. Since the sensor is part of a housing which is riveted to it, I can only clean the hot wire but can't reach the cold wire behind it. Didn't help. I drove at WOT several times to update the Baro but remained the same at 147 range. Also the fuel trims were the same. I remember watching Paul's videos where he mentions that the computer can store other sensors position or readings in memory, disconnecting the battery can resolve the issue. In the repair manual for my truck it says to disconnect the battery with no specific reason as to why when replacing the sensor. Is the MAF set in memory? Thanks.
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15 replies
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Dylan said May 29, 2016 12:54:17
Did you perform a WOT test while checking your MAF value, not only the Baro PID? And also O2 sensors?

I remember watching Paul's videos where he mentions that the computer can store other sensors position or readings in memory,


Could you post a link to this video you're referring to?

What are your fuel trims?
[Last edited May 29, 2016 12:58:56]
Belgium, Europe
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CharlesAcosta said May 29, 2016 14:55:22
O2 sensors were lean during WOT. MAF voltage around 1.3, good power and grounds (IAT included) in idle only. I can't remember the videos specifically. One has to do with the TPS and the other I believe was the EGR position valve.
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Dylan said May 29, 2016 15:23:21
Could you post this in 'the repair section'? A lot more people will notice your post and be able to contribute.
Belgium, Europe
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Dylan said May 29, 2016 18:46:07
O2 sensors were lean during WOT. MAF voltage around 1.3


Don't like the lean O2 sensors at WOT and MAF voltage is pretty low also. The O2's are switching at idle, just asking?

Can you give me some feedback on the fuel trims.
Belgium, Europe
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Dylan said May 29, 2016 19:01:05
Also do you have SD Premium? Could link you to some videos.
Belgium, Europe
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CharlesAcosta said May 30, 2016 04:55:03
The O2 are switching during idle. Fuel trims are at B! 12% B2 17% at WOT
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Dylan said May 30, 2016 10:46:21
What are your trims during steady rpm let's say 3000? Not at WOT
Belgium, Europe
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CharlesAcosta said Jun 07, 2016 22:43:16
Sorry for late reply. Come to think of it I only reached 3000 rpm but had to slow down due to traffic (NY no surprise there lol). The fuel trim hovered 17 to 20% then stabilize when I ease the throttle. I decided to change the sensor since cleaning it didn't help. I disconnected the battery. Don't know wether I erased the KAM but I got the P1000 code. Now my Baro reads 159 hz and now at WOT my fuel trims are negative 20's when before it was positive 20's. The O2 are rich during WOT
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Dylan said Jun 08, 2016 17:28:42
A P1000 is not uncommon when disconnecting battery. Did you check your monitors? Are they all complete? If no other DTC's it should go away after drive cycle.
Are your trims balancing out? Short term <-> Long term?
O2's indicate rich at WOT so that's ok.

Belgium, Europe
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CharlesAcosta said Jun 09, 2016 22:29:46

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Dylan said Jun 11, 2016 08:59:10
Thank you for posting data capture. I see positive short term trims and negative long terms that are balancing out. STFT's +20% <-> LTFT's -20%. From frame 560 both are around 0%. When was this capture taken? Right after replacing your MAF? If possible could you post one with vehicle stationary at steady 3000rpm?
Belgium, Europe
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Tyler said Jun 11, 2016 20:22:21
Wow, I really like the graphing capture, Charles. Sometimes I wish the Snappy ShopStream software would allow overlaying scan data graphs like this.

Also, I think you're good to go after the MAF replacement.
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Dylan said Jun 12, 2016 09:20:59
Wow, I really like the graphing capture, Charles. Sometimes I wish the Snappy ShopStream software would allow overlaying scan data graphs like this.


Yeah pretty cool indeed 😉
Belgium, Europe
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CharlesAcosta said Jun 14, 2016 00:13:49
Thanks guys. Dylan here is the capture you asked for. I started to notice my fuel trims getting a little high when idling. I suspect a vacuum leak since you'll see the numbers improve with higher RPM. The first capture I posted was taken a week after I replace the MAF. The two previous to this on was identical, I just didn't record it at the time.
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Dylan said Jun 14, 2016 17:23:26
Hey Charles. Yes indeed fuel trims do get better at higher rpm. At idle on Bank 1 you're still in the +/- 10% zone. Bank 2 LTFT is a bit higher 13/14% ......Not that high of course.
If you're worried about a vacuum leak I suggest you test it. Water bottle, smoke test,...
Belgium, Europe
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