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Running Super Rich? What's up with these Fuel Trim Numbers?


Narms7227

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I'm kind of confused with what these trim numbers mean can someone help me out? I'm getting like 8 MPG. I'm replacing all O2 sensors and my fuel filter this weekend any thing else I should do? I also attached my Error codes which point to bad O2 sensors (Im aware of the misfires too, they where cause by water intrusion into the ignition coil. The issue has since been fixed.) Could the bad O2 sensors be the only issue? Thanks for the help guys.
 

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adsm08

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Fix your misfires.

Fuel trim goes up when the computer adds fuel trying to compensate for oxygen left over after combustion. The problem is that oxygen sensors only see free molecular oxygen in the exhaust, so they can't tell the difference between not enough fuel to use all the oxygen, or a misfire that doesn't burn all the fuel or all the oxygen.

A misfire dumps free oxygen down the exhaust, which looks to the sensor the same as a lean condition, so it tries to add more fuel to compensate. And you have three misfires on the bank with high fuel trims.

Fix them, if you haven't, and clear the memory so it has to relearn the fuel trims.
 

Narms7227

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I replaced the ignition coil and haven't had misfires since. Still getting bad gas mileage and O2 sensor codes though. Any thought on those fuel trim numbers? My understanding is that 0% is the target and that -25% is lean and +25% is rich. Is this correct?
 
adsm08

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You O2 sensor may have been fouled by the misfires. Go ahead and replace them.

Technically 0% is the "target" however the short term trim is a moment by moment correction. It can and will fluctuate quite rapidly as you drive, or idle. I have found that usually the long terms will settle around 10-15%, maybe lower on a fresh engine with all new sensors.

But yes, +/- 25% is the limit that will start setting codes for a rich or lean condition.
 

Narms7227

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adsm08 said:
You O2 sensor may have been fouled by the misfires. Go ahead and replace them.

Technically 0% is the "target" however the short term trim is a moment by moment correction. It can and will fluctuate quite rapidly as you drive, or idle. I have found that usually the long terms will settle around 10-15%, maybe lower on a fresh engine with all new sensors.

But yes, +/- 25% is the limit that will start setting codes for a rich or lean condition.
Click to expand...
Awesome thanks for the insight. I'll replace the O2 sensors and fuel filter this weekend and see how she does. The SOHC 4.0 has 3 O2 Sensors right? 2 up stream and 1 down?
 
adsm08

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Yes. That is most V configured OBD2 engines.'

Make sure to unhook the battery to wipe the memory.
 
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