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Old 01-17-2013, 09:53 AM   #41 (permalink)
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Fuel trim is the deviation from the normally expected amount of fuel the engine is injecting in response to oxygen sensor feedback. For example, if the oxygen sensors detect the engine is running lean (not enough fuel and too much air), the computer will raise the fuel trims (positive) to increase the amount of fuel being injected until the oxygen sensors detect the correct air/fuel ratio has been achieved (14.7:1). If the engine is running rich, then the amount of fuel is decreased (negative fuel trims). Fuel trims are split into two types, short term and long term. Short term is an instantaneous adjustment, while long term is a running average used as a "baseline" adjustment specific to a subset of engine operation (e.g. idle, cruise, etc). The two fuel trims added together will always equal the total fuel trim adjustment at any time. In a perfectly running engine, both short and long term fuel trims will be very close to 0. If you introduce a vacuum leak (which will cause the engine to run lean), you will first start to notice the short term trim rising. Let's say it goes up to +15 and stops. As time goes on, the long term fuel trim will start to increase. Lets say it jumps to +7. At that instant the short term trim will decrease to +8 (notice they both total +15). In another minute or two the long term trim might jump to +15, at which time the short term trim will be close to 0. The fuel control has now fully adapted to the lean condition. If you fix the vacuum leak, you will see the opposite occur (short term will ramp down to -15, and it will start to become more positive as the long term trim comes down from +15 until they both meet at 0).

You will not get a lean/rich code (e.g P0171/P0172) until the long term fuel trim reaches 20-25%. However, by the time the fuel trim reaches 10% you usually will have a problem, so that's why we use 10% as a threshold.

Long term fuel trims are "remembered" for each phase of engine operation, which is why you see the long term trim change when you increase engine speed or load. If you reset the codes, the long term trims will start at zero and adjust the next time the engine operates in that range.

Give the computer a few days of driving to re-learn all the correct fuel trim adjustments, and if you still see excessive fuel trim numbers then you should go after fixing them.

Here's a video that talks about fuel trims in more detail:

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Old 01-17-2013, 09:58 AM   #42 (permalink)
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Thank you for that, some very good information. I knew I was supposed to watch it, just wanted to know why!
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Old 01-17-2013, 11:43 AM   #43 (permalink)
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Ok, drove it 60 miles to try and get the monitors ready, 40 miles hwy 20 city and they aren't ready. My gas mileage has went up dramaticly after all this work, was getting about 15mpg, now Im getting 23mpg. However now my fuel trim is higher than it was before 10.7 bank1 and 11.7 bank 2 at idle. While driving down the road temp is 177, at idle it is 199. I do not have a condensor as the wife hit something and I had to take it out because it was bent up and broken, so I figure that may be why it is so much cooler running down the road. p0420 was usually the first code to pop, now p0430 is pending but not a hard code, and I haven't seen p0420 yet. This is getting frustrating! Drives a whole lot better though.
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Old 01-17-2013, 11:47 AM   #44 (permalink)
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What monitors aren't ready? I had to pull the battery cable the other day and just driving 20 miles had everything but evap ready for me. Evap to be ready needs to be over a certain temperature. I'm wondering if it actually readied up today.
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Old 01-17-2013, 11:51 AM   #45 (permalink)
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What monitors aren't ready? I had to pull the battery cable the other day and just driving 20 miles had everything but evap ready for me. Evap to be ready needs to be over a certain temperature. I'm wondering if it actually readied up today.
It's about 55 outside today. Cat monitor and EVAP not ready.
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Old 01-17-2013, 11:53 AM   #46 (permalink)
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Another weird thing is my temp on the dash shows about half way at about 195, but my scanner reads it at 177.
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Old 01-17-2013, 12:03 PM   #47 (permalink)
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If you have P0430 pending, obviously the catalyst monitor is not going to pass. EVAP is difficult to pass so that might take a few days or weeks.

If you have to pass emissions testing, then you're going to need a new Y-pipe/catalytic convertors. That or else try one of those products I mentioned earlier.

Temp gauge for the dash and the computer use separate sensors, so they won't always read identical.
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Old 01-17-2013, 12:10 PM   #48 (permalink)
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^ I don't want to admit that I need a cat
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Old 01-18-2013, 10:52 AM   #49 (permalink)
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I was checking the vacuum on the engine, it is now 19 at idle, 2 in higher than before, but I heard an exhaust leak from bank one, I will be checking that out later today, since bank 1 is the only code that came back, bank 2 seems to be ok for now, and my fuel trims are higher than they were before I changed the o2 sensors, could an exhaust leak raise the fuel trims?
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Old 01-18-2013, 11:02 AM   #50 (permalink)
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Quote:
Originally Posted by Jmxtwiztid View Post
could an exhaust leak raise the fuel trims?
Yes, if air is getting sucked into the exhaust before the oxygen sensor it will register as a lean condition and add fuel.
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