MAF and STFT values...
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Joined: May 2012
Posts: 33
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MAF and STFT values...
The diagnostic printout for the '98 F150, 4.2L V6, shows that both the MAF and STFT values fluctuate quite a bit when the truck is on the road:
f150_98_v6.doc
The MAF values are between less than four and 150 gm/sec, depending on the position of the gas pedal. This seems to be normal, albeit I do not know what the MAF values should be; does anyone know?
The STFT values fluctuate between +14 and -16 percentage, that does not seem to be correlated to the position of the gas pedal as well as the MAF does. The majority of the reading is negative, indicating that the engine mixture is too rich most of the time and ECU tries to lean the mixture. The too rich mixture is also evident in the tail pipe's inside color, which is dark black.
Running rich could be an indication of failing sensors, or the ECU is not capable to adjust after a certain point in its operation reached. The truck has never had the "Check Engine" light on and there is no code found by the reader to indicate any issues.
I still don't like running the engine too rich and looking for options.
Is there a way to make adjustment for the "base" air/fuel mixture? If not, is there a way to further test the sensors and ECU for the correct operation?
f150_98_v6.doc
The MAF values are between less than four and 150 gm/sec, depending on the position of the gas pedal. This seems to be normal, albeit I do not know what the MAF values should be; does anyone know?
The STFT values fluctuate between +14 and -16 percentage, that does not seem to be correlated to the position of the gas pedal as well as the MAF does. The majority of the reading is negative, indicating that the engine mixture is too rich most of the time and ECU tries to lean the mixture. The too rich mixture is also evident in the tail pipe's inside color, which is dark black.
Running rich could be an indication of failing sensors, or the ECU is not capable to adjust after a certain point in its operation reached. The truck has never had the "Check Engine" light on and there is no code found by the reader to indicate any issues.
I still don't like running the engine too rich and looking for options.
Is there a way to make adjustment for the "base" air/fuel mixture? If not, is there a way to further test the sensors and ECU for the correct operation?
Senior Member
Joined: Sep 2011
Posts: 419
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From: Minnesota
I took a look at your data you received. The fluctuation in short fuel trims is normal. It does negative after a decel and positive when accelerating! The short trim is for making the oxygen sensor switch lean to rich. Long fuel trims are for what the PCM has learned from the short trims. Ideal situation is the long trim=0 and the shorts go plus to minus to make the o2 switch lean to rich. If you see short trims too high at idle will soon drop the short to 0 and the long will increase. +or-13% will turn the light on for long trims. If the the long and short trims are withing limits the truck should be running at stoichiometric 14.7 to 1. I do not use the grams per second for MAF reading, so I don't know if that would be a good reading or not. MAF and BARO readings somewhat go together. A poor MAF reading or vacuum leak can make the BARO reading go way low. Anything under a 150hz reading is showing something. Also calculated off elevation. I am about 900ft above sea level! I usually like to see 150-160hz, not constant since pressures change slightly all the time. A dyno and 3 gas analyzer may also help. Programming can be changed but usually are set more rich for power gains. My race bike is set about 12.3 to 1 and my toyota supra is set even richer for boost.
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Joined: May 2012
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Thanks TRD...
I certainly don't want to gain more power, so replacing the ECU programming is not an option; the engine runs too rich as it is.
The barometric sensor is together with the ECU, at least that what I recall from my searches. Its value at your elevation is just about right, based on what I found:
Referenced link:
http://www.fordf150.net/forums/viewtopic.php?t=69459
I am pretty much at sea level, take or give couple hundred feet most of the times. I don't know how to read the values from this sensor, but I'll look it up.
It seems that following your suggestions is in order, check the vacuum hoses, clean the MAF sensor, and test the barometric sensor. After 14 years, it is probably due for one. Especially, after reading this from the same link above:
It seems that the way to reset the "base" air fuel mixture is, quote:
I guess the same would apply for rich symptoms and it could go away as well, if the MAF is disconnected after cleaning.
I certainly don't want to gain more power, so replacing the ECU programming is not an option; the engine runs too rich as it is.
The barometric sensor is together with the ECU, at least that what I recall from my searches. Its value at your elevation is just about right, based on what I found:
BAROMETRIC PRESSURE REFERENCE
Barometric Pressure (in. Hg.) Barometric Pressure (kPa) BARO/MAP PID (Hz) Altitude above sea level (ft)
3.5 11.8 89.3
5 16.9 92.8
10 33.8 104.6
15 50.7 117.0 14,000
20 67.5 129.6 10,000
21 70.9 132.5 9,000
22 74.3 135.4 8,000
23 77.7 138.3 7,000
24 81.1 141.1 6,000
25 84.4 144.0 5,000
26 87.8 146.9 4,000
27 91.2 149.8 3,000
28 94.6 152.8 2,000
29 97.9 155.8 1,000
30 101.3 158.9 0 (sea level)
31 104.7 162.0
31.875 107.7 164.7
Barometric Pressure (in. Hg.) Barometric Pressure (kPa) BARO/MAP PID (Hz) Altitude above sea level (ft)
3.5 11.8 89.3
5 16.9 92.8
10 33.8 104.6
15 50.7 117.0 14,000
20 67.5 129.6 10,000
21 70.9 132.5 9,000
22 74.3 135.4 8,000
23 77.7 138.3 7,000
24 81.1 141.1 6,000
25 84.4 144.0 5,000
26 87.8 146.9 4,000
27 91.2 149.8 3,000
28 94.6 152.8 2,000
29 97.9 155.8 1,000
30 101.3 158.9 0 (sea level)
31 104.7 162.0
31.875 107.7 164.7
http://www.fordf150.net/forums/viewtopic.php?t=69459
I am pretty much at sea level, take or give couple hundred feet most of the times. I don't know how to read the values from this sensor, but I'll look it up.
It seems that following your suggestions is in order, check the vacuum hoses, clean the MAF sensor, and test the barometric sensor. After 14 years, it is probably due for one. Especially, after reading this from the same link above:
MAF sensors can get contaminated from a variety of sources: dirt, oil, silicon, spider webs, potting compound from the sensor itself, etc. When a MAF sensor gets contaminated, it skews the transfer function such that the sensor over-estimates air flow at idle (causes the fuel system to go rich) and under-estimates air flow at high air flows (causes fuel system to go lean). This means Long Term Fuel Trims will learn lean (negative) corrections at idle and learn rich (positive) corrections at higher air flows.
If at least two of the previous three steps are true, proceed to disconnect the MAF sensor connector. This puts the vehicle into Failure Mode and Effects Management (FMEM). In FMEM mode, air flow is inferred by using rpm and throttle position instead of reading the MAF sensor. (In addition, the BARO value is reset to a base/unlearned value.) If the lean driveability symptoms go away, the MAF sensor is probably contaminated and should be replaced.