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Discussion Starter · #1 ·
I got a 99 jeep wrangler. I got a ob2 scanner and has live readings. Does anyone know where i can find what the reading should be at? Thanks
 

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Does your scanner have a settings tab where you can enter the VIN number.
 

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What make and model scanner do you have
 

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Discussion Starter · #6 ·
Its an autozone actron scanner. 99 tj 2.5l and looking to see what map readings and the o2 should read. A few other that i cant remember. I have a idle issue. Runs good but shakes alittle like a idle sensor would do, but not sure if its another one.
 

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As Tangofox007 said you will see a quickly changing voltage number on your O2 sensor readings. These are the fast changing level of oxygen that is being measured in the exhaust. The ECU is using the readings to constantly adjust the fuel injector pulse widths to keep the A/F ratio as close to 14.7:1 as it can. You won't get a "reading" on this but rather an indication that it is varying, and hopefully, within a range that the ECU can control it well.
Probably a more useful reading will be that of the short term and long term fuel trims. The LTFT will show how well the engine is running and if the fuel mapping in the ECU is well matched to the engine.
Example: People that put huge injectors on their vehicle without changing the base fuel map will see a large negative LTFT number as the ECU trims back by shortening the injector pulse widths. Of course there is a maximum that it can compensate for conditions, but most will try to tune for less than a 10% LTFT. 5% is better if obtainable. This number can clue you in if something is way off in your engine, such as a bad vacuum leak, cracked manifold, dead cylinder, failing sensor, or mis-matched components. The computer is building a new "Base map" which is the LTFT.
Short term fuel trim is what is being read right now, In the moment. This can vary quite a bit as the barometric pressure changes, or the temperature of the air or temperature of the engine. Maybe you are wheeling around at 12000 ft altitude and the STFT is really trying to lean things down from yesterday when you were at 4,000 feet. The STFT numbers are being used to rewrite your LTFT. With some time, it will change the LTFT.
It may sound a bit complicated but there are several good articles on the internet you can look up to learn more. Look for low LTFT numbers for an indication that all if functioning well and the computer is not having to make big changes to get things running right. Most ECU's start throwing trouble codes when fuel trims get to 35% or more. My experience with my 2003 shows it can go to 40% w/o a trouble light being thrown.
 

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Discussion Starter · #11 ·
Thank you. My short term is around 0 it moves alittle neg to pos but on 5%. Long is at 14%. Just trying to figure out my new rebuilt has rough idle
 

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Your STFT is low because it has made a good base map for injector timing on the LTFT. If your LTFT is positive, it is adding 14% MORE time to the average injector pulse because the oxygen sensors are indicating a lean reading. The ECU is richening it up just a bit. Probably in a reasonable range. You might have a small vacuum leak somewhere.
 
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