Found out why i'm getting PO174
11-02-2011, 04:07 PM
#1
4th Gear Member
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Join Date: Jul 2008
Location: NC
Posts: 1,956
Found out why i'm getting PO174
Well for the last month i've been getting a P0174 pop up every week or so and my gas mileage has slowly gotten horrible(15 MPGs).
It turns out my MAF is bad for a couple of reasons. One I suspected might be an issue and turns out it is. I'm running a 90MM lightning MAF and months back I went to try to take the MAF out of the housing and stripped both the damn screws they must have had red loctite on them or something. Well needless to say it wouldn't tighten back down all the way so I had to take duct tape to it to keep it tightened down. I havent really bothered replacing it because I drove around afterwards and nothing seemed to be wrong. Its finally leaking though because the duct tape is loosening
The mechanic said bank 2 short term fuel trim was at 12 and bank 1 was at 4 consistently. When he sprayed starter fluid around the MAF seal the short term fuel trim on both banks dropped to zero instantly.
He also mentioned that during the diagnostics test that the MAF was reporting barometric pressure in the 5000 hg range which it should have been reporting in the 150-160ish range. Just to be clear that this is most definitely an issue - there is no funny tuning trick/procedure that would make my tuner adjust how my MAF reports barometric pressure right? This is clearly an issue I assume.
So the MAF wont seal properly to the housing due to stripped threads and its reporting barometric pressure incorrectly. I am a little curious as to why both banks arent reading lean if the vacuum leak is there but this is what i've been told by the shop.
Should I just buy another 90MM Lightning MAF or an SCT 2600? What makes the SCT 2600 support so much more power yet its still 90MM?
It turns out my MAF is bad for a couple of reasons. One I suspected might be an issue and turns out it is. I'm running a 90MM lightning MAF and months back I went to try to take the MAF out of the housing and stripped both the damn screws they must have had red loctite on them or something. Well needless to say it wouldn't tighten back down all the way so I had to take duct tape to it to keep it tightened down. I havent really bothered replacing it because I drove around afterwards and nothing seemed to be wrong. Its finally leaking though because the duct tape is loosening
The mechanic said bank 2 short term fuel trim was at 12 and bank 1 was at 4 consistently. When he sprayed starter fluid around the MAF seal the short term fuel trim on both banks dropped to zero instantly.
He also mentioned that during the diagnostics test that the MAF was reporting barometric pressure in the 5000 hg range which it should have been reporting in the 150-160ish range. Just to be clear that this is most definitely an issue - there is no funny tuning trick/procedure that would make my tuner adjust how my MAF reports barometric pressure right? This is clearly an issue I assume.
So the MAF wont seal properly to the housing due to stripped threads and its reporting barometric pressure incorrectly. I am a little curious as to why both banks arent reading lean if the vacuum leak is there but this is what i've been told by the shop.
Should I just buy another 90MM Lightning MAF or an SCT 2600? What makes the SCT 2600 support so much more power yet its still 90MM?
11-03-2011, 09:16 AM
#3
TECH SAVANT
Join Date: Dec 2006
Location: Saint Augustine, FL
Posts: 10,938
MAFs do not report barometric presrure, but rather the mass (weight here on Earth) of the air passing through them--are you certain he was not referring to kg/h (kilograms per hour)? That is the most common unit however g/s (grams per second) and lb/h (pounds per hour) are also used.
Actually the MAF outputs a signal voltage that is related to the mass of air flowing through it, 0.0V to 5.0V on our cars; this voltage is then converted to mass/time unit flow rate using a transfer function derived from the 30-point MAF Transfer map loaded into the tune. Here is an example of that table (commonly called the MAF Transfer Function, it is really a set of data points from which the PCM calculates the actual transfer function):
The PCM reads the 30 data points and as stated above calculates a regression function (curve) that will let it convert any MAF voltage, up to 4.8V in a stock tune, into the equivalent mass of air being ingested--making that limit 4.99V is one of the first things most tunes/tuners do.
As each MAF design/model produces a different voltage output for a given range of air flows¹ it is essential that the 30-point MAF Transfer map in the tune match the output of the MAF in use. Here are some curves for a few well-known MAFs:
Because the MAF sensor only monitors a small sample of the total amount of air flowing through the tube it could very well be that the physical location of the leaks in your intake just happened to be routing air more or less directly through the sensor making it report crazy amounts of air flow. It is because of the sensor's sampling of just a fraction of the air flowing through the tube that "clocking" a MAF (rotating it to a different "o'clock" position) will often make it perform better, or worse.
This is also why it is important to have a straight 6" or better run of piping before the MAF, why the OEM MAF screen is there, and why slot style MAFs are less sensitive to "clock" position than older small sample port units.
------------------------------------------------
¹ - Each MAF will have what I will call a "natural" (it isn't really) air mass vs. output voltage relationship, however this relationship can be adjusted by the designer/maker to be whatever they would like it--some can even be adjust in the field, mechanically or electronically.
So-called "calibrated" MAFs are adjusted to trick a particular stock tune to work with higher capacity injectors--this is why they are a tuner's nightmare--read more about that here.
Actually the MAF outputs a signal voltage that is related to the mass of air flowing through it, 0.0V to 5.0V on our cars; this voltage is then converted to mass/time unit flow rate using a transfer function derived from the 30-point MAF Transfer map loaded into the tune. Here is an example of that table (commonly called the MAF Transfer Function, it is really a set of data points from which the PCM calculates the actual transfer function):
The PCM reads the 30 data points and as stated above calculates a regression function (curve) that will let it convert any MAF voltage, up to 4.8V in a stock tune, into the equivalent mass of air being ingested--making that limit 4.99V is one of the first things most tunes/tuners do.
As each MAF design/model produces a different voltage output for a given range of air flows¹ it is essential that the 30-point MAF Transfer map in the tune match the output of the MAF in use. Here are some curves for a few well-known MAFs:
Because the MAF sensor only monitors a small sample of the total amount of air flowing through the tube it could very well be that the physical location of the leaks in your intake just happened to be routing air more or less directly through the sensor making it report crazy amounts of air flow. It is because of the sensor's sampling of just a fraction of the air flowing through the tube that "clocking" a MAF (rotating it to a different "o'clock" position) will often make it perform better, or worse.
This is also why it is important to have a straight 6" or better run of piping before the MAF, why the OEM MAF screen is there, and why slot style MAFs are less sensitive to "clock" position than older small sample port units.
------------------------------------------------
¹ - Each MAF will have what I will call a "natural" (it isn't really) air mass vs. output voltage relationship, however this relationship can be adjusted by the designer/maker to be whatever they would like it--some can even be adjust in the field, mechanically or electronically.
So-called "calibrated" MAFs are adjusted to trick a particular stock tune to work with higher capacity injectors--this is why they are a tuner's nightmare--read more about that here.
11-04-2011, 08:56 AM
#4
4th Gear Member
Thread Starter
Join Date: Jul 2008
Location: NC
Posts: 1,956
Yeah I think this issue is a little out of their hands. I really don't think my MAF is reporting anything incorrect electronically, I just know it can't properly seal itself and its being held on by duct tape which worked for a while. Its still pretty tight but it doesnt take much at all for air to leak in. Its something thats been bothering me knowing its just duct taped on so now is a good time to fix that to eliminate the possibility of it being the culprit.
I'm lucky enough to have a dyno shop nearby and I called and the guy told me he could have a 90MM LMAF for me today so i'm gonna leave work an hour early and drive a county over and pick it up. I'll swap it in, reset my KAM and then drive around for a day or two and i'll know immediately based on STFT, LTFT and gas mileage if its fixed. I do not believe this will fix it. If the MAF was leaking and causing the alleged A/F issue then then both banks should be lean. I have money on an exhaust leak as according to this shop the bank 2 front o2 sensor checked out ok so it cant be that. If it isnt an exhaust leak or the MAF i'm gonna be looking at....clogged injector/bad injector, vacuum leak elsewhere, hell I dunno what else.
I'm lucky enough to have a dyno shop nearby and I called and the guy told me he could have a 90MM LMAF for me today so i'm gonna leave work an hour early and drive a county over and pick it up. I'll swap it in, reset my KAM and then drive around for a day or two and i'll know immediately based on STFT, LTFT and gas mileage if its fixed. I do not believe this will fix it. If the MAF was leaking and causing the alleged A/F issue then then both banks should be lean. I have money on an exhaust leak as according to this shop the bank 2 front o2 sensor checked out ok so it cant be that. If it isnt an exhaust leak or the MAF i'm gonna be looking at....clogged injector/bad injector, vacuum leak elsewhere, hell I dunno what else.
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