Data on normal readings from sensors?

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TomO - 12 Sep 2006 17:19 GMT

Hi All,

I'm working on a pesky little problem on my 2000 Chevy Astro/4.3L AWD
minivan. The thing keeps lighting the MIL with a code of P0101 -
something about the MAF sensor readings being out of spec.

I connect my OBD-II monitor to the car and I can see the MAF go from .7
lbs/min to about 5.x lbs.minute as I accellerate, so I know that the
sensor is doing something.

My question is about where to find some typical readings information.
I.e.: what *should* the airflow be under normal acceleration, at idle, etc.

Thanks!

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TomO

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aarcuda69062 - 12 Sep 2006 18:31 GMT

> Hi All,
>
[quoted text clipped - 10 lines]
>
> Thanks!

You can download a volumetric efficiency calculator here;

http://www.smp-training.com/Pages/ptsframeset.html

Should calculate grams per second based on RPM, temperature,
engine size, air density (baro) etc. I'd plug in 85% for the
(your) engines VE

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TomO - 12 Sep 2006 20:37 GMT

> You can download a volumetric efficiency calculator here;
>
[quoted text clipped - 3 lines]
> engine size, air density (baro) etc. I'd plug in 85% for the
> (your) engines VE

Thanks.

If I'm using this tool correctly, I see that in order to reach near 100%
Volumetric Efficiency, I would need to see about 12.5 lbs/minute at 2200
RPM and a reading of 10.7 would yield me pretty darned close to 85% VE.

So it sounds like I have an airflow problem to troubleshoot, or perhaps
a vacuum leak beyond the MAF sensor or something along those lines.

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TomO

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TomO - 13 Sep 2006 14:42 GMT

>> You can download a volumetric efficiency calculator here;
>>
[quoted text clipped - 12 lines]
> So it sounds like I have an airflow problem to troubleshoot, or perhaps
> a vacuum leak beyond the MAF sensor or something along those lines.

Well, I did find a vacuum leak. It was a broken plastic line that feeds
the dashboard air controls. Got that patched up, but the readings from the
MAF sensor did not change significantly. I'll have to dig deeper I suppose.

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aarcuda69062 - 13 Sep 2006 19:42 GMT

> > You can download a volumetric efficiency calculator here;
> >
[quoted text clipped - 9 lines]
> Volumetric Efficiency, I would need to see about 12.5 lbs/minute at 2200
> RPM and a reading of 10.7 would yield me pretty darned close to 85% VE.

You're not going to reach 100% volumetric efficiency with that
engine. The idea is to plug in a known reasonable volumetric
efficiency and then fill in the other variables to see if your
air mass measurements agree with what they calculate out to be.

An engine of XXX cubic inches will flow XX grams of air per second
(or lbs. per hour)
at XXXX RPM at XX.X barometer.

> So it sounds like I have an airflow problem to troubleshoot, or perhaps
> a vacuum leak beyond the MAF sensor or something along those lines.

Could be a measurement problem (faulty sensor), could be the
engine isn't breathing as well as it should be (coked valves,
valve timing problem, restricted exhaust, restricted intake).

The engine is an air pump, the mass air flow sensor give a way to
measure the air being pumped thru the engine. Either the sensor
is wrong, the engine can't pump its volume or the problem lies
elsewhere.

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jeffcoslacker - 13 Sep 2006 20:21 GMT

aarcuda69062 Wrote:

> > > You can download a volumetric efficiency calculator here;
> > >
[quoted text clipped - 34 lines]
> is wrong, the engine can't pump its volume or the problem lies
> elsewhere.

That's true...I've seen crimped tailpipes cause MAF codes before...go
back to basics, blow the dust off the vacuum guage and see what kind of
vacuum level you are pulling...

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jeffcoslacker

http://www.automotiveforums.com

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TomO - 14 Sep 2006 15:49 GMT

>> Thanks.
>>
[quoted text clipped - 6 lines]
> efficiency and then fill in the other variables to see if your
> air mass measurements agree with what they calculate out to be.

Yeah, I was just working theoretical numbers to see if I was doing the
calculation correct.

> An engine of XXX cubic inches will flow XX grams of air per second (or
> lbs. per hour)
> at XXXX RPM at XX.X barometer.
>
>> So it sounds like I have an airflow problem to troubleshoot, or perhaps
>> a vacuum leak beyond the MAF sensor or something along those lines.

I found a broken vacuum line, but repairing it did not help the readings
from the sensor. It did get my dashboard air ducts working again though.

> Could be a measurement problem (faulty sensor), could be the engine
> isn't breathing as well as it should be (coked valves, valve timing
[quoted text clipped - 3 lines]
> measure the air being pumped thru the engine. Either the sensor is
> wrong, the engine can't pump its volume or the problem lies elsewhere.

I'll just keep on checking stuff until I come to the true source of the
problem.

Thanks!

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TomO

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