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ramairbrc 11-11-2011 07:23 PM

MAF reading question
I have the typical P0174 and P0171 codes. I follow the typical DIY, and get to this point:

How Do I Know if the Problem Is the Mass Air Flow Sensor?
There is a very effective "truth test" for any Mass Air Flow Sensor. Start the engine, let it idle, and then check the Barometric Pressure reading on the scan tool data. If the reading is about 26.5 Hg and you are close to sea level, you know that you have a defective Air Flow Meter because it is telling you that you are at about 4500 feet above sea level. (These conversion tables will help.) When the Mass Air Flow Sensor sees this Barometric reading, it adjusts its Air Density table and then "under reports" the actual amount of air entering the engine. It does this because the Barometric Pressure Sensor is actually part of the Mass Air Flow Sensor.

So, I went to Autozone and used the scan tool, with the following results:

MAF (LB/M) 0.43

MAF (gr/S) 3.22

Since the previous post states: 26.5 Hg, I don't know whether my MAF readings indicate that it is working correctly or needs to be replace. I live near sea level.

Thanks in advance.


lejeunebimmer 11-12-2011 12:52 AM

Before jumping to the MAF did you first check for vaccum leaks? More likely than not your culprit right there, MAF's don't generally fail all that often and generally fail from over-oiled air filters, or damage while being handled, or improper "cleaning."

Your above mentioned method also seems to pertain to MAP sensors not MAF sensors

As the names imply (Manifold Absolute Pressure sensor) and a Mass Air Flow sensor. Two entirely different things my friend, and our cars do not have a MAP.

Case in point your units as measured from the LD capture were 0.43 pounds a minute, or 3.22 grams a second which is the "mass" of air entering the engine and the time interval it is entering at.

Where as the reading of 26.5Hg is actually a reading of pressure or in this case roughly 83kPa (which I would like to point out your source was wrong again)

Lets think about this at 26.5Hg you're talking about almost (close enough) to two atmospheres of air, do you using common sense think that the air is thicker at sea level or in the mountains? Clue: its thicker (more dense) at sea level so you're reading will be higher at sea level than at altitude. So your source is backwords for starters, and secondly his numbers are pretty decently off, since you're not going to have essentially twice the atmospheric pressure entering the engine at idle. This number would be close to WOT at sea level....

End of story. Check for vaccum leaks in the intake boots report back.

ramairbrc 11-12-2011 09:04 AM

Thank you for the response. I did take the intake area apart yesterday and looked for potential vacuum leaks. Though the boot itself looked intack (I read that many have cracked), I noticed that the small vacuum line on the "t" was in bad shape, so I replaced it.

The car does have almost 130K miles and an AFE Magnum Force intake with an oiled filter, which is why I wanted to know how to check if the MAF sensor was bad. The filter was horribly dirty, so I followed the instructions and cleaned it, and then slightly oiled it (not my first such filter). I bought some MAF sensor cleaner, and did that while I had it all apart.

I did notice that the MAF sensor readings appeared to be "over time" with the M for minute and the S for second.

So, we'll see if that helped prevent the P0171 and P0174 codes.

lejeunebimmer 11-12-2011 12:26 PM

Well since you cleaned it the MAF, and replaced the t then I suspect you're probably good to go.

As for the MAF its self going bad, when it goes it will manifest its self as much more than just lean codes. The car will run absolutely horribly (if it will even run at all). So you'll know when it goes. Like I said they do sometimes fail, but it is by no means common on any car.

ramairbrc 11-27-2011 08:42 AM

OK, long update, but need help, please.

I have since replaced the oxygen sensors to rule that out. I also removed and recleaned the MAF. And, I checked for vacuum leaks using the starter fluid. Since I found none, and on a whim, I shot some starter fluid into the vacuum port in the intake hose... and nothing!! No change in RPM. So, I put my vaccum gauge into the vacuum port: ZERO. I checked the gauge to ensure it was working, etc.

How is it possible that the brakes work if the vacuum port reads zero? I tried to "feel" any vacuum by blocking this with my finger, nothing.

I did remove the oil filler cap, and could hear and feel vacuum there, so I "believe" the CCV is still working.

Do I need to change just the intake tubes? Would the intake manifold gaskets have gone bad?

Any advice, please.

ramairbrc 11-27-2011 08:45 AM


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