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Old 04-01-2010, 02:39 AM   #1
The Critic
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1996 SL2
Default Help Me Decipher Live Data From My Scan Tool

Here is some live data from my Saturn's onboard computer.





The vehicle is a 96 SL2 with 143,000 miles. The Thermostat is a 195F unit. I currently have a mix of 11 gallons Costco 91 octane fuel and 2 gallons 87 octane in the tank, if that matters. There is also a few ounces of Amsoil PI left in the tank as I have just finished a treatment.

I am confused as to why the spark advance is in the negative. More specifically, why I have 14 degrees of negative spark advance at idle. Is this normal? Or is it something bad? Could it be due to the high octane fuel I have in the tank? The car specs 87 octane, but I am running 91 as a test.

Also, in my OnDemand5 reference source it shows that for Oxygen Sensor testing, 0.1 volts equals lean and 1.0 volts shows a rich condition. Mine is showing 0.270 volts, so is that good or bad?

And for the STFT B1 value-- is that the long-term fuel trim?

Thanks.

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Old 04-01-2010, 04:50 AM   #2
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2003 L-Series 3.0L Sedan
Default Re: Help Me Decipher Live Data From My Scan Tool

Spark advance is expressed in negative numbers while retarded timing is expressed as positive numbers. -14 degrees would be interpreted as 14 degrees before top dead center or BTDC which is correct. It does seem confusing at first until becoming comfortable with interpreting live data.

O2 sensor data can be misleading because live data displays will not show the O2 sensor switching frequency adequately; the O2 sensor, when heated up to operating temperature above 600F, will begin sensing exhaust gases and express values between 0.1v-0.9, while displaying instantaneous readings anywhere between 0.4v-0.8v. During cold start running the O2 sensor won't output signals but may be displayed (incorrectly); the engine coolant sensor temperature should be used as a guide to determine when the O2 sensor begins operating correctly; when the coolant temps reach approximately 160F the exhaust temps may be near 600F, the temps needed to allow O2 sensors to start operating. Since there are no exhaust temperature sensors, coolant temps are monitored by the pcm to determine when open loop mode is switched to closed loop mode; when the O2 sensor starts outputting the pcm was waiting when coolant temps were met as part of the parameters switching open loop to closed loop mode of emissions control.
O2 sensor signals should be looked at only when coolant temps are at operating temperatures, at least 180F or higher, to read proper O2 sensor data. The live data should be looked at several times and then the O2 sensor data can be averaged to gauge its capability correctly. It may not display correctly for one scan because the switching speed and signal output of the O2 sensor is faster than the readers capability and may confuse anyone reading live data. LCD screens do not switch fast enough when seeing live data streams and adds to the confusion. A proper live data stream would be seen better on a computer display with the O2 numbers continually changing; the displayed numbers will change quickly as the pcm switches quickly from lean to rich mixtures while the O2 sensor reads the mixtures. The frequency change isn't reflected accurately on LCD's but can be interpreted when more numbers are shown, a trend showing more lean volages than rich voltages (lean meaning less than 0.5v, rich as more than 0.5v).

The 0.270v can be considered as too lean but may be misleading unless you see many of the same low voltages when the engine is fully warmed up. Ideal O2 sensor readings fluctuate between 0.4v and 0.8, perfect air fuel ratios near 0.5v. Read about O2 sensor operation and what ideal a/f ratios are as expressed in O2 sensor voltages while getting background information on O2 switch speeds compared to LCD speeds that can interfere with correct readings.

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Old 04-10-2010, 01:03 AM   #3
greenman
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Default Re: Help Me Decipher Live Data From My Scan Tool

Quote:
Originally Posted by The Critic View Post


And for the STFT B1 value-- is that the long-term fuel trim?
STFT B1=short term fuel trim on bank 1. A range of plus 10 to minus 10 is ideal.

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