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MAF sensor and fuel trim values


ab_slack

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2002 Explorer 4.6L V8

Chasing a problem where vehicle seems to run fine but when keying off then trying to restart after a 10 minute drive it dies. I am trying to understand the fuel trim values as relates to being lean or rich. I did get a P0171 code at one point suggesting lean, likely a vacuum leak.

In the process I got another MAF sensor. Original sensor typically shows some positive long term trim values. 5% to 10%. Short term trim values are about eh same but sometimes go negative.

If I swap for a new sensor, the short term trim values go up to 30% even as high as 40% and the long term values typically drop to 0% and hold so steady that it seems it isn't trusting the readings it is getting.

Vehicle seems to run fine with either sensor once RPMs are up over 1200. At idle vehicle doesn't want to run with either sensor.

trying to get some insight to understand the values.

Thanks,

Andrea
 


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Dirtman

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Unplug the IAC valve. Does the engine rpm drop and almost stall or stall? If not you have a vacuum leak. Find it and fix it and stop fiddling with fuel trim numbers.
 

acsnowrider

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Chasing a problem where vehicle seems to run fine but when keying off then trying to restart after a 10 minute drive it dies. I am trying to understand the fuel trim values as relates to being lean or rich. I did get a P0171 code at one point suggesting lean, likely a vacuum leak.

In the process I got another MAF sensor. Original sensor typically shows some positive long term trim values. 5% to 10%. Short term trim values are about eh same but sometimes go negative.

If I swap for a new sensor, the short term trim values go up to 30% even as high as 40% and the long term values typically drop to 0% and hold so steady that it seems it isn't trusting the readings it is getting.

Vehicle seems to run fine with either sensor once RPMs are up over 1200. At idle vehicle doesn't want to run with either sensor.

trying to get some insight to understand the values.

Thanks,

Andrea
Long term fuel trims at 10%, on engines with high mileage, (100,000) are pretty typical-- My own 04, ran these trim percentages, actually up to 12-13%. The mil light comes on, usually, at 25%. Short fuel, will widely vary, due to eng temp, altitude, ect.
I prefer to use "smoke", to test for vacume leaks. Its very conclusive, and you can save yourself a lot of time, and aggravation by using this method

If you have a vac leak, your short fuel trims, will stay high after engine warm up, and the Long trims, will "average" out to the high side as well--Using this thought process, allows you to evaluate a engine VERY effectively!!:cool: Also, watch your "BARO" hertz reading--sea level, should have a reading around 155, to 157 hertz. Vacume leaks, will also translate into low Idle Air Control valve percentages--around 20%, and sometimes lower! Typical MAF sensor voltages should be at .6 to .9 volts, at idle. Hope this helps.
 

ab_slack

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Unplug the IAC valve. Does the engine rpm drop and almost stall or stall? If not you have a vacuum leak. Find it and fix it and stop fiddling with fuel trim numbers.
Great idea. I have checked for leaks, found a couple minor ones, there doesn't seem to be anything big, but it is a nice simple check. I suppose the only problem with the test is that tthe engine stalls anyway at idle although when it stalls it seems to flood the engine.
 

ab_slack

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Long term fuel trims at 10%, on engines with high mileage, (100,000) are pretty typical-- My own 04, ran these trim percentages, actually up to 12-13%. The mil light comes on, usually, at 25%. Short fuel, will widely vary, due to eng temp, altitude, ect.
I prefer to use "smoke", to test for vacume leaks. Its very conclusive, and you can save yourself a lot of time, and aggravation by using this method

If you have a vac leak, your short fuel trims, will stay high after engine warm up, and the Long trims, will "average" out to the high side as well--Using this thought process, allows you to evaluate a engine VERY effectively!!:cool: Also, watch your "BARO" hertz reading--sea level, should have a reading around 155, to 157 hertz. Vacume leaks, will also translate into low Idle Air Control valve percentages--around 20%, and sometimes lower! Typical MAF sensor voltages should be at .6 to .9 volts, at idle. Hope this helps.
Certainly high miles at 312K though only 160K on this engine. The components around are mostly original factory.
 

Dirtman

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Did you fix the vac leaks or just find them?
 

acsnowrider

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Certainly high miles at 312K though only 160K on this engine. The components around are mostly original factory.
It sounds like you have a decent diagnostic tool, as you asking about "live PID" values--Check the ECT, or temp voltage--Had a few that intermittently varied, causing the computer to make inappropriate fuel adjustments! wiggle the wires on the top of the sensor, if you can, and lightly tap the sides of the sensor (key on, engine off)--Voltage should remain stable. Then watch it engine running--You can also watch your PCM voltages, to evaluate that relay you swapped!
 

07nhbpsi

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2002 Explorer 4.6L V8

Chasing a problem where vehicle seems to run fine but when keying off then trying to restart after a 10 minute drive it dies. I am trying to understand the fuel trim values as relates to being lean or rich. I did get a P0171 code at one point suggesting lean, likely a vacuum leak.

In the process I got another MAF sensor. Original sensor typically shows some positive long term trim values. 5% to 10%. Short term trim values are about eh same but sometimes go negative.

If I swap for a new sensor, the short term trim values go up to 30% even as high as 40% and the long term values typically drop to 0% and hold so steady that it seems it isn't trusting the readings it is getting.

Vehicle seems to run fine with either sensor once RPMs are up over 1200. At idle vehicle doesn't want to run with either sensor.

trying to get some insight to understand the values.

Thanks,

Andrea
5-10% is good for short term fuel trim @ idle, now when are u looking @ long term fuel trims? I was taught long term u would rev motor up to 2k rpm then check them....hold it steady around 2k to 2.5 k and observe . Short term fuel trims are gonna be all over the place with a vacuum leak, and of course if your 02 sensors are worn out..... long term won’t read right either with a vacuum leak and worn 02 sensors.... best to replace 02s’ at 100k for optimum performance and most accurate fuel trim readings!
 

ab_slack

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Final update. I don't know if it appeared in the troubleshooting, but ultimatly found two issues that were interacting.

First is the connector for one of the O2 sensors had come in contact with the Y pipe and melted shorting some connections. After connector replacement that kept some codes from re-appearing but trim values were still messed up. I eventually tracked this to a fuel problem, possibly just the filter, or a pump that was getting in trouble when warm. I replaced both.
 


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