What is your long term fuel trim running?

Brian83

Brian83

TJ Enthusiast
Joined
Mar 25, 2019
Messages
390
Location
Oklahoma City, OK
For those of you with the capability of monitoring your fuel trims what are you running for Long Term Trims? Ive been working to get my jeeps fuel mileage back to an acceptable level from 8mpg. The latest was finding my Cat had been gutted. Its a 99 Federal Emissions so only one Cat. With the gutted cat i was running -13% LTFT after replacing the cat and driving half a tank I'm down to -9% LTFT. Everythind Ive read says 0, plus or minus ten, is "Good". Im just curious what you guys are running to see if I need to try anything else. Id love to get this thing to 15mpg but the way I drive Im not sure that's realistic, lol.
 
I'm at 6% LTFT, just checked the other day when I downloaded a new program for all my vehicles. I get about 13mpg, 4.0 5sp on 33s.
 
qslim said:
I'm at 6% LTFT, just checked the other day when I downloaded a new program for all my vehicles. I get about 13mpg, 4.0 5sp on 33s.
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Are you using standard scanner to read that? If so where do find the fuel settings? I’ve got one I’ve used for me kids car. I didn’t think about using it.
 
Ericict said:
Are you using standard scanner to read that? If so where do find the fuel settings? I’ve got one I’ve used for me kids car. I didn’t think about using it.
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I downloaded OBD fusion for iPad/Iphone because it has a lot of "dealer only" data that I can read for my wife's Lexus and my Silverado. It's 10 bucks for the IOS download & another 10 each for late-model stuff if you want to read body ECU codes, traction/VSC etc. The basic version reads all the generic CARB PIDs for the TJ though (fuel trims, O2 waveforms, MAP pressure, etc.). You have to get a bluetooth OBD dongle for it as well, the one I ordered for the app was about $40.

It gets me about the same amount of data that I used to read with my Snap-On Modis that I paid $500 for nearly 20 years ago when I fixed cars for a living.
 
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qslim said:
I downloaded OBD fusion for iPad/Iphone because it has a lot of "dealer only" data that I can read for my wife's Lexus and my Silverado. It's 10 bucks for the IOS download & another 10 each for late-model stuff if you want to read body ECU codes, traction/VSC etc. The basic version reads all the generic CARB PIDs for the TJ though (fuel trims, O2 waveforms, MAP pressure, etc.). You have to get a bluetooth OBD dongle for it as well, the one I ordered for the app was about $40.

It gets me about the same amount of data that I used to read with my Snap-On Modis that I paid $500 for nearly 20 years ago when I fixed cars for a living.
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I just plug my scanner and found live data. I need to figure out what I need to look at for that info. I can see 24 items. Have figure out which code.
 
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I have never used the scanner in this way, always looked up a code, checked the component and replaced. Learned a lot. Idle they are at 10.9, at 1500, 10.9/10.2, at 2500 they are 4.7 and 3.9. @qslim, I read that the actual are the longterm minus the short term. In which case these numbers would change. This is all new to me so not sure if that makes a difference. Does your setup allow you to reprogram? I’m getting close to 16 in the city, but we don’t hot rod it. We are taking a road trip in a couple of weeks so I will see what my hw mileage is at that point.
 
Ericict said:
I have never used the scanner in this way, always looked up a code, checked the component and replaced. Learned a lot. Idle they are at 10.9, at 1500, 10.9/10.2, at 2500 they are 4.7 and 3.9. @qslim, I read that the actual are the longterm minus the short term.
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No, there's no subtraction involved when it comes to interpreting ST/LT fuel trim. Your ST number reflects how much fuel is being added/taken away this very second to meet the pre-defined ideal ratio (14.7 parts of air to 1 part of fuel), your LT fuel trim reflects how much fuel has been added or taken away over a given period of time to meet the ideal ratio. Positive numbers indicate that the computer is adding fuel (lean condition), negative numbers indicate that fuel is being taken away (rich condition).

Your fuel trim #s could be indicative of a slight vacuum leak; while at idle the 02 sensors are reporting more air in the exhaust than the PCM wants, so the PCM compensates by adding fuel. When the engine is revved to 2500 the extra air moving through the engine kind of masks the vacuum leak, and the fuel trim #s come back closer to zero/ideal. +10% isn't super high, but in my experience single digits in either direction are what you want to see.

Here's some good reading on fuel trims if you want to get more into it from one of my favorite automotive sites:

https://www.aa1car.com/library/what_is_fuel_trim.htm
Ericict said:
Does your setup allow you to reprogram?
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No, I use OBD fusion because it has more parameter IDs than the standard CARB stuff, so I can look at manufacturer-specific parameters if I ever need to.
 
qslim said:
No, there's no subtraction involved when it comes to interpreting ST/LT fuel trim. Your ST number reflects how much fuel is being added/taken away this very second to meet the pre-defined ideal ratio (14.7 parts of air to 1 part of fuel), your LT fuel trim reflects how much fuel has been added or taken away over a given period of time to meet the ideal ratio. Positive numbers indicate that the computer is adding fuel (lean condition), negative numbers indicate that fuel is being taken away (rich condition).

Your fuel trim #s could be indicative of a slight vacuum leak; while at idle the 02 sensors are reporting more air in the exhaust than the PCM wants, so the PCM compensates by adding fuel. When the engine is revved to 2500 the extra air moving through the engine kind of masks the vacuum leak, and the fuel trim #s come back closer to zero/ideal. +10% isn't super high, but in my experience single digits in either direction are what you want to see.

Here's some good reading on fuel trims if you want to get more into it from one of my favorite automotive sites:

https://www.aa1car.com/library/what_is_fuel_trim.htm

No, I use OBD fusion because it has more parameter IDs than the standard CARB stuff, so I can look at manufacturer-specific parameters if I ever need to.
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Thanks for the link. Good stuff. Looks like I’ve got some work this weekend. I’ve just changed the plugs. Good it still be off because of the old plugs? I do need to go run it again. I didn’t let it get up to operating temperature either. More proof that I need to pull the reader out and just look at the system to better understand what’s happening. Based on MPG and idle, I would have never thought there was an issue.
This Jeep doesn’t have the Oem intake (CAI) and a larger TB. Could that be causing it?
 
Ericict said:
I didn’t let it get up to operating temperature either.
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Oh, ok... If that's the case you can completely ignore those #s you were looking at because it was running in 'open loop operation'. When the engine is cold the PCM doesn't take 02 sensor as feedback, it's just trying to get the engine up to operating temperature as quickly as possible based on the coolant temp sensor. Once you're up to operating temp, the PCM switches to 'closed loop' and takes feedback from the 02 sensors to maintain a good air/fuel ratio. Your fuel trim #s are only valuable when you're in closed loop.
 
Did I miss something? The OP seems to have a rich condition though, the LTFT's are negative.
 
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dbbd1 said:
Did I miss something? The OP seems to have a rich condition though, the LTFT's are negative.
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Yes, I'm not sure what is causing the rich condition just yet. I have a rear 02 sensor that is all over the place after a cat replacement. I also changed injectors prior to the cat replacement so I'm about to spend $75 on OEM injectors from the salvage yard as I think I tossed mine, just to see if that pulls everything back in order.
 
Is your coolant temp sensor showing about 210 after warmup?
Is the precat O2 sensor in known good condition (and the right brand/model)?
 
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