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Old June 12th, 2017, 19:27
Ecomike's Avatar
Ecomike Ecomike is offline
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Re: Impact of Aftermarket Injectors on STFT & LTFT's

Since my 87 Renix Jeep is showing STFT to be about 30-50, way under 128, I looked this up in the Jeep-Snap-On MT-2500 guide book.

STFT: ST fuel trim numbers lower than 128 indicates that the ECU is commanding a short term lean mixture.

But the Renix Jeep manual says that low O2 sensor voltage is a rich mix, and that is what I am seeing in open loop at idle when I get it to hang up in open loop at idle after holding it in the higher rpm miss fire state for a while according to the book (the scanner says it is lean, LOL)

So if I assume the low O2 sensor voltage I am seeing of about .6-.9 volts out of 0-5 volt range, is a rich signal, as open loop should be, then it makes sense that the ECU STFT would trying to move it back to 2.45 Volts by commanding a leaner condition in order the get it back to closed loop status, as it does on mine (it is slow sometimes 2 minutes to get back to closed loop, but it does get there), then I can make sense of the data I am getting and then suspect that one or more injectors is sticking open and leaking excess fuel making it rich, while the STFT and ECU are trying to correct it?

At this point this is just an unproven theory as there is nearly equally conflicting data out there on what low O2 sensor data is saying.

The part that bothered me was that I was always under the impression that ECUs switched to a rich mode in open loop (failure of closed loop mode), but that may not be true ( I know this is heresy LOL) as if the ECU is in rich mode it may be commanding a lean direction switch to try and get the rig into closed loop!!!!! Perhaps there is a time domain delay where if the ECU can reach closed loop mode it stops trying after a few minutes and then switches to a data table rich mode and ignores the O2 sensor????

But how does one know when or if that is happening???

EDIT: Then the manual says that in open loop the ST Fuel trim goes to a fixed value, usually 128. WTF, now they tell me,

But mine is switching from 30-50 in open loop, not parking at 128 after a rev up and operation with the miss going on for 10-30 seconds.

So is Open Loop, truly open loop, or is their a permanent open loop, and part time open loop?

The manual suggests comparing Injector on time to the STFT value. May need to do that later.

It is dawning on me that the O2 sensor data is live sensor data, while the STFT is a what the ECU is telling the injectors to do, a command!!!! One is sensor output data the other computer control instructions. I was confusing the STFT with data with sensor data, it is not.
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34 MPG , '85 2WD Cherokee Pioneer with custom installed, 64 hp, 2.2 L Nissan SD22 Diesel 5 spd Manual; & 4 Renix XJs, '87 Wagoneer 4.0, 4WD, 89-Cherokee, 4WD, '87 Cherokee 2WD, & '89 Cherokee Pioneer 2WD, all 4dr. #2091

Last edited by Ecomike; June 12th, 2017 at 19:46.
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