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High LTFT at idle- normal?
I had some error codes just after a gas refill, lean bank 1/bank 2.
Do not know if it was cause of coincidence. I monitored the LTFT fuel trims and they were +15 to +20. I have since cleaned the MAF sensor, and actually replaced the two upstream O2 sensors as the car has done 120k Miles. Now, LTFT is around +3-5 at cruise, STFT hovers around zero +/- 2 ro so. I observe that on closed throttle during coasting, as the rpm approaches idle, the LTFT increase back to +15%, and during the coast the STFT drops to -10% and reduces to zero as the rpm returns to idle So, at idle 600rpm, the LTFT sits at +15%, and STFT is zero. Raising the RPM to even just 700rpm, while in drive, the LTFT drops again to +3%. Is it normal that the LTFT is high at idle, I was thinking not. I cannot find or hear any air leaks. Full load PCV valve seems open, plenty of suction through hose at idle. Oil dip stick seals Fuel pressure is 55lbs MAF and O2 sensors are clean or new. Reset ECU by disconnecting the battery. Could injectors be weak and causing lean at low pulse widths? What else can I check, seems tough to pull the injectors? Appreciate any other things to check, bit baffled at this time and do not have any data to know what the normal values of trim is at idle and how quickly it changes off idle. |
If the LTFT are high at idle and come down off idle that would indicate vacuum leak. if the problem was MAF or a sensor the the LTFT would not get better off idle. Is it 05 xjr?
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It will take awhile for the LTFT to fall normally, speed it up by clearing the trims from the ECM by doing a hard reset. As long as idle n cruise speed STFT trims are small, the long terms will remain low.
When in closed loop, short term trims are used. When open, during coasting/deceleration, or cold start, LTFT numbers are used. |
ok, will do a new reset. Last time I did, ltft was at zero, and stft went way positive. If it were a vacuum leak, why do the values go back so positive with the rpm so close to idle, but not idle. I also note that just off idle, while ltft goes close to zero, the stft goes way negative.
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if the STFT goes positive at startup, you've got a leak. ST LT trims really dive to low values at highway speed, but if you think about it, a small leak won't have very much affect when the MAFS is trying to pull 15-50 g/sec. of air compared to running at idle where it sits around 4.4-4.5 g/sec. The same sized leak impacts trims at idle or 'decel' more than anything else.
Have you read these documents I attached that was put together by one of our jag techs? Read the 'Primer' document first, then Part Two afterward. I've been hunting down and diagnosing my lean condition for a couple months, and I'm confident now I've fixed them all. At idle, my car's ST fuel trims bounce between -0.8 and +0.8, with an occasional 1.8 here and there. LT at idle is at 5.5/6.2, and falling the more I drive the car. Yes, at decel, when I go into 'closed loop', my ST sometimes shows large negative values for a split second, then returns to normal. My LT never reaches close to 0.0 as you stated, however, not even at a quick acceleration (2.3/2.3 is what my scanner usually shows). Pay more attention to consistent ST trims when using a constant RPM, like at idle (650 RPM, 1,000, 1,500, and 2,000. The ST values should not fluctuate much at these consistent RPMs, they should still bounce just above and below 0.0 if all is sealed up well and sensors are working; if the ST values spike, and remain elevated, keep hunting for air leaks. |
Originally Posted by H20boy
(Post 669785)
if the STFT goes positive at startup, you've got a leak. ST LT trims really dive to low values at highway speed, but if you think about it, a small leak won't have very much affect when the MAFS is trying to pull 15-50 g/sec. of air compared to running at idle where it sits around 4.4-4.5 g/sec. The same sized leak impacts trims at idle or 'decel' more than anything else.
Have you read these documents I attached that was put together by one of our jag techs? Read the 'Primer' document first, then Part Two afterward. I've been hunting down and diagnosing my lean condition for a couple months, and I'm confident now I've fixed them all. At idle, my car's ST fuel trims bounce between -0.8 and +0.8, with an occasional 1.8 here and there. LT at idle is at 5.5/6.2, and falling the more I drive the car. Yes, at decel, when I go into 'closed loop', my ST sometimes shows large negative values for a split second, then returns to normal. My LT never reaches close to 0.0 as you stated, however, not even at a quick acceleration (2.3/2.3 is what my scanner usually shows). Pay more attention to consistent ST trims when using a constant RPM, like at idle (650 RPM, 1,000, 1,500, and 2,000. The ST values should not fluctuate much at these consistent RPMs, they should still bounce just above and below 0.0 if all is sealed up well and sensors are working; if the ST values spike, and remain elevated, keep hunting for air leaks. Thanks for confirming that at idle even, the levels are at zero, I was worried I was chasing a ghost |
Hmm, i'm a little confused, maybe i missed something.
If your STFT only bounce around +2, the LTFT should quickly fall to these levels. My LTFTs over the last two days have fallen from 5.5/6.2 to 4.7/4.7 yesterday, and today they have dropped even further to 3.9/3.1. You should be getting the same result if those STFTs do not get high, or only spike on decel or accel. Any chance a 'cold' start results in a blast of elevated STFTs , but after a warmup, the STFTs level off? That can happen, and it will affect LTFTs just as soon as the car goes into closed loop and sees those ST numbers...reaching closed loop status isn't very long after startup...just needs time to heat up and such. |
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