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Old 05-17-2019, 06:59 PM   #1
RayLivingston
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How To Tell If MAF is Bad...

My short-term idle fuel trims have been sky-rocketing lately. Oddly, using OBDFusion the long-term trims ALWAYS show zero.

There are NO intake leaks - I've smoke tested the hell out of it, and found absolutely nothing. But, my idle fuel trims, which were around 10-12% perhaps a month ago, are now up around 15-20%. So, I am suspecting either a tired MAF or pre-cat O2 sensors are the most-likely candidates. They are all the original, factory parts, with 230K miles on them. So, they have served their time, but have been fine until the last few months. I do notice the Bank1 O2 signals look quite different from the Bank2 O2 signals. Both Bank1 signals switch much more often than the Bank2 signals do.

Above idle (anything over about ~1000-1200 RPM), the trims immediately come down under 5%, which still seems higher than they used to be.

So, how do I determine whether the MAF or O2 sensors are at fault? I am seeing 3.15 g/sec air flow and 0.28 g/sec fuel flow at idle.

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Ray L.
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Old 05-17-2019, 08:15 PM   #2
adsterb
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I'm no expert but here are my two cents..

When you take a look at fuel trims and they're high at idle but lower when you accelerate this is indicative of a vacuum leak - that's what I've learned from the 50skid in his video about how to read fuel trims His videos are definitely worth watching. Some vacuum leaks can be pretty hard to locate - not always being visible from looking into the engine bay. There's a line that goes from the intake manifold to the fuel filter that's all the way in the rear of the car for example, and I remember hearing or reading somewhere that if the intake manifold gasket is leaking - unlikely, it'd be very hard to see.

He also makes a video on oxygen sensors and how to read their voltages . Both at idle and at speed your Bank 1 Sensor 1 and Bank 2 Sensor 1 sensors should be fluctuating up and down at a fairly constant rate.

Your Bank 1 Sensor 2 and Bank 2 Sensor 2 voltages should be steady most of the time, if they're fluctuating this indicates a failing catalytic converter.
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Old 05-17-2019, 08:23 PM   #3
Archbid
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I know you are not an interval maintenance guy, but your o2 sensors are at double their lifetime. Yeah, they are probably bad. And the MAF too. If you live near SF, ping me and I will loan you mine to test!

Out of curiosity is it the cost or the principle? Because the car has gotten a huge amount of mileage on those sensors and you shouldn't feel bad about replacing them.

The 0% LTFT is really strange - I have very low LTFT, but they still range from -1.8 to +1.8 during driving - very infrequent that both are at 0. I wonder if you have an ECM issue.

Last edited by Archbid; 05-17-2019 at 08:24 PM.
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Old 05-17-2019, 08:27 PM   #4
swordsman11868
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If bank 2 sensors fluctuate it could be failing O2 sensors, not failing cats. Unless proven otherwise.


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Old 05-19-2019, 11:28 AM   #5
RayLivingston
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Quote:
Originally Posted by Archbid View Post
If you live near SF, ping me and I will loan you mine to test!
Thanks for the kind offer. I've gone ahead and ordered all new O2 sensors, and a new MAF. Should be here in a few days.

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Ray L.
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Old 05-19-2019, 11:44 AM   #6
Hot_tub
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Post before and after butt-dyno update results
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Old 05-19-2019, 12:29 PM   #7
RayLivingston
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Post before and after butt-dyno update results
It runs fine as it is. I don't expect any performance change, just need to be able to pass a smog check, which has never been a problem until now.

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Ray L.
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Old 05-19-2019, 02:57 PM   #8
Archbid
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Originally Posted by RayLivingston View Post
It runs fine as it is. I don't expect any performance change, just need to be able to pass a smog check, which has never been a problem until now.

Regards,
Ray L.
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