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Old 03-06-2006, 11:17 PM   #1
Blennophobic
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Default LTFT goes to 0 in Open Loop? <56K go away>

The attached log shows my long term fuel trim starts out at +15 where it has been consistently sitting lately. Then it quickly goes to 0 as I do the pull. Can someone confirm this is normal because it doesn't make sense to me. I would think whatever fuel adjustment the ECU learns in closed loop should carry through to open loop.... no?

Any other comments on the log are welcome. The knock correction dips down pretty low in the middle and I'm losing a lot of torque between 4000 and 6000 rpm. Above 6000, I will sometimes get audible knock. I'm wondering if this is because of the LTFT going to 0 in OL.


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Old 03-06-2006, 11:29 PM   #2
Hitokiri
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that is the definition of open loop. The car is not using any o2 feedback at all.
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Old 03-06-2006, 11:59 PM   #3
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Quote:
Originally Posted by Hitokiri
that is the definition of open loop. The car is not using any o2 feedback at all.
There can be any number of reasons why the ECU adjusts fuel trims such as changing fuel qualities or to adjust for aging components. Wouldn't we want to maintain those fuel offsets in open loop instead of running 15% leaner than when it was tuned?
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Old 03-07-2006, 09:09 AM   #4
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People with leaks post MAF have reported Fuel Trims in the +12% range at idle or cruise. To be specific... people with fugged up turbo inlet tubes that had holes in them or cuts where the hose clamp holds it on the turbo. Worth a check, especially if it only recently starting doing this. Could also be a wacky/bad O2 sensor.
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Old 03-07-2006, 01:30 PM   #5
nhluhr
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I have noticed that the LTFT will almost always get smaller when you go WOT but it doesn't always go to 0. I think it kinda depends on if the ECU has any experience in Closed Loop at those load sites. . .and frankly, I have no idea how you get it to be closed loop in those loadsites.

I do know without a doubt that the LTFT can be applied to WOT... As a former WRX owner, I installed STi Pink injectors on a stock car. At first it wouldn't do much other than idle but soon I was able to go WOT... it took a long time but eventually it pushed the LTFT down enough that I was able to wind out the gear.
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Old 03-07-2006, 11:37 PM   #6
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Thanks for the replies guys. I changed out my inlet tube to a Perrin in the fall because of a suspected post-maf leak. It seemed to improve quite a bit at that time, but was never quite right. I rechecked all the clamps this weekend and all seems OK. I even pressure tested the section from the inlet to the turbo outlet and it is fine.

That's interesting nhluhr, that's how I'd expect the ECU to act. I'm not totally sure there is anything wrong with mine though. The LTFT going to 0 in the log may not mirror the actual fuel offset the ECU is supplying... I dunno.

Another symptom I've observed is rough idle at start-up that sometimes sets a misfire code. If I watch the STFT at start-up, it goes to negative 15 right away to compensate for my high +ve LTFT, but as the car warms up it gradually settles back to where it was. Does this indicate a bad front O2? Bad MAF? At the price of these sensors, I really don't want to start replacing them unless I've got definitive proof they are bad.

Thanks,

-Steve.
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Old 03-08-2006, 08:23 AM   #7
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you should be tuning your car so that it pulls fuel at idle. ie, a negative ltft. this will result in a much smoother startup/idle.
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Old 03-08-2006, 10:25 PM   #8
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Quote:
Originally Posted by ride5000
you should be tuning your car so that it pulls fuel at idle. ie, a negative ltft. this will result in a much smoother startup/idle.
Thanks Ken, I think my tuner already used 525cc for my injector size with pinks in the ECUTEK tune. Would you think its normal to use a smaller number than that, or is this all indicative of a problem with injectors, pump, or FPR?

Anybody else observed similar behaviour as I described in the post above? ie. car seems to like 0 LTFT offset on cold start, but as it warms up needs a lot of +ve offset? Is this normal or does it certainly indicate a problem of some kind?

Almost seems like a post MAF leak develops as the car warms up... thoughts on what might cause this? All speculation welcome!

Thanks,

-Steve.
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