Need help understanding a STFT problem!
08-09-2012, 01:03 PM
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Need help understanding a STFT problem! I am going to try to explain this the best I can. I have an intake on the car that was cracked and repaired. The repair looked good and the car fired up and runs fine. When the car was being tuned the guy stated that the intake must be leaking because the STFT’s keep climbing up (+) at idle. They will get up into the high 20’s pretty quick. As soon as I open the throttle at all they come right back in line. He turned off the LTFT’s so the car would be drivable until I get the leak sorted out. This all seems to make sense to me until I started watching the STFT’s while driving. If the car is just idling they climb well into the high 20’s but if I turn the a/c on the drop back to +/- 5 or so and stay there. If the leak is causing the trims to be out of whack why isn’t it happening with the a/c on? Any insight would be greatly appreciated
08-09-2012, 01:58 PM
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Wouldn't the increased opening of the T/B with a/c commanded "on" have the same effect as adding throttle?
Edit: Just guessing that the leak is enough to cause issues in normal idle load/cell area. Add a little load (a/c pump), and the air requirement for the increased load changes enough (and the leak is small enough) that the leak becomes masked.
Just a guess though...
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Edit: Just guessing that the leak is enough to cause issues in normal idle load/cell area. Add a little load (a/c pump), and the air requirement for the increased load changes enough (and the leak is small enough) that the leak becomes masked.
Just a guess though...
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Last edited by salemetro; 08-09-2012 at 02:21 PM.
08-09-2012, 02:14 PM
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i Know absolutely nothing about tuning cars. I just like to understand things when possible and use common sense when trouble shooting things. Are you saying that when i turn the a/c on that the throttle blade or iac open up further? if so i guess that would be the answer. i know there is no difference in the idle rpm with the a/c on or off. if it's just the iac doing the work when the a/c is on i'm guessing it must be a small leak in the repair because with the a/c running the trims look good. thanks for any help
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08-09-2012, 04:05 PM
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This is probably O2 sensors too old or cold. Trimming is
dumb and only reacts to that the sensors are (or aren't)
doing. If heater and switching codes are killed you can follow
a bad sensor(s) right into the weeds. The added accessory
load makes more exhaust heat and brings them back around.
Of course more actual air (load) will also bury a leak.
dumb and only reacts to that the sensors are (or aren't)
doing. If heater and switching codes are killed you can follow
a bad sensor(s) right into the weeds. The added accessory
load makes more exhaust heat and brings them back around.
Of course more actual air (load) will also bury a leak.
08-09-2012, 04:11 PM
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This is probably O2 sensors too old or cold. Trimming is
dumb and only reacts to that the sensors are (or aren't)
doing. If heater and switching codes are killed you can follow
a bad sensor(s) right into the weeds. The added accessory
load makes more exhaust heat and brings them back around.
Of course more actual air (load) will also bury a leak.
dumb and only reacts to that the sensors are (or aren't)
doing. If heater and switching codes are killed you can follow
a bad sensor(s) right into the weeds. The added accessory
load makes more exhaust heat and brings them back around.
Of course more actual air (load) will also bury a leak.
08-09-2012, 05:50 PM
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well on the drive home i was watching the iac counts. at hot idle with the a/c off it was between 15-30. a/c on went to 75-95. i'm guessing the a/c is allowing the iac to "hide" the leak
08-09-2012, 06:33 PM
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You're right on track Matt - the fact that the idle doesn't change when the a/c is on explains it - because you are already getting the extra air from the leak... Fixing cracked intakes never seems to work - everyone I've heard of trying it always ends up replacing it...
08-09-2012, 06:34 PM
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If you want to know if it's an intake leak, let it sit at idle with the positive fuel trims then spray carb cleaner or propane around the intake manifold. If their is a leak your o2's will peg rich and you will watch the stft's drop negative.
08-09-2012, 06:51 PM
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That works most of the time, but not always when the intake is cracked on the underside - had it happen once - bought a used FAST and it was cracked on the bottom and that didn't show it. I took it off and pressure tested it to find the crack...
08-10-2012, 12:46 PM
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You're right on track Matt - the fact that the idle doesn't change when the a/c is on explains it - because you are already getting the extra air from the leak... Fixing cracked intakes never seems to work - everyone I've heard of trying it always ends up replacing it...
08-10-2012, 06:47 PM
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08-11-2012, 12:31 PM
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i think this is the road i'm gonna take. i just wish they still sold v1. the entry price of v2 has kept me from pulling the trigger for fear that i wont be able to pick it up and end up with a $900 code reader. LMFAO