Wideband values
More on understanding your car and fuel trims.
When your car is idling or cruising it is always making little changes to your fuel trim calculations, this (if you have way to datalog or monitor, like an AccessPort) will show up under the STFT (Short Term Fuel Trim). That value is averaged over time and committed to the LTFT (Long Term Fuel Trim). If you look at the long term fuel trim, what you can look for is if your LTFT goes over +/- 15%. If that happens you'll likely see a CEL. If it goes +15% it means it's finding that you're leaner than expected from the MAF calculations. This means you likely have a vacuum leak somewhere where air is getting in without the MAF seeing it. Or, it could be an indication your MAF is malfunctioning and could need replacement. Same with -15%. Means it's trying to over fuel you, so you'll probably need to look at your fuel pressure or test your injectors for proper operation.
The whole reason I'm relating these fuel-trim issues to a WB AFR gauge, is when this stuff starts to happen, you'll start seeing erratic readings on your WB. When my MAF and primary O2 failed on my Subie, I would see at idle it would hover around 15.4-15.7, then would fall on it's face and read 17.8-20.0 and would stick. but still idled okay and well it was subaru so it always sounded like it was missing
When your car is idling or cruising it is always making little changes to your fuel trim calculations, this (if you have way to datalog or monitor, like an AccessPort) will show up under the STFT (Short Term Fuel Trim). That value is averaged over time and committed to the LTFT (Long Term Fuel Trim). If you look at the long term fuel trim, what you can look for is if your LTFT goes over +/- 15%. If that happens you'll likely see a CEL. If it goes +15% it means it's finding that you're leaner than expected from the MAF calculations. This means you likely have a vacuum leak somewhere where air is getting in without the MAF seeing it. Or, it could be an indication your MAF is malfunctioning and could need replacement. Same with -15%. Means it's trying to over fuel you, so you'll probably need to look at your fuel pressure or test your injectors for proper operation.
The whole reason I'm relating these fuel-trim issues to a WB AFR gauge, is when this stuff starts to happen, you'll start seeing erratic readings on your WB. When my MAF and primary O2 failed on my Subie, I would see at idle it would hover around 15.4-15.7, then would fall on it's face and read 17.8-20.0 and would stick. but still idled okay and well it was subaru so it always sounded like it was missing
If you're not under wide open throttle, only partial, you should see it oscillate a little. That's because a leaner burn will yield more efficient power, however you're ECU is smart enough to see a lean mixture and add more fuel.
The 1st O2 sensor in your exhaust, is actually a wideband. (the quality may/may not be what the aftermarket one you bought). But it's what the ECU uses to adjust your fuel trims.
Also, you may see your mixture look a little lean under just a few pounds of boost or rpms lower than like 4k revs. That's normal, because you're car isn't into the heavy load tables of it's fuel calculations yet.
The most important part of a WB AFR gauge is to watch your AFR when you have the pedal stomped and your in some heavy boost. That's where a lean burn could cause detonation that will destroy your engine.
However, it's good that you notice what it's doing under partial throttle. If under partial throttle you suddenly see it going to like 17.0-20.0 You'll know something isn't working right and can start doing some investigating.
The 1st O2 sensor in your exhaust, is actually a wideband. (the quality may/may not be what the aftermarket one you bought). But it's what the ECU uses to adjust your fuel trims.
Also, you may see your mixture look a little lean under just a few pounds of boost or rpms lower than like 4k revs. That's normal, because you're car isn't into the heavy load tables of it's fuel calculations yet.
The most important part of a WB AFR gauge is to watch your AFR when you have the pedal stomped and your in some heavy boost. That's where a lean burn could cause detonation that will destroy your engine.
However, it's good that you notice what it's doing under partial throttle. If under partial throttle you suddenly see it going to like 17.0-20.0 You'll know something isn't working right and can start doing some investigating.
When you cut the throttle and you coast, the gauge goes full lean with "---" as a reading. Is that because you've cut the throttle and there's no more fuel being used, and the sensor is only "sensing" oxygen?
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richardjh
09+ Ralliart Engine/Turbo/Drivetrain
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Jun 18, 2011 10:16 AM