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Impact of Aftermarket Injectors on STFT & LTFT's

TheSloMoShow

NAXJA Forum User
Location
Columbus,OH
Good evening everyone,

I was curious how one would determine the appropriate impact to your STFT & LTFT based on a new set of injectors you install.

For example, does an injector set that is either 10% lower or 10% higher in fuel flow rating (at same rail pressure of 49psi, 2001 XJ here) mean the LTFT's should indicate 10% in either direction pending whether or not the injectors you bought are smaller or larger?

I have installed Bosch's 4-pin hole 280155784's.
From what I've read, some say they are nearly identical (~22.6lbs/hr) in fuel flow rating as stock spec and some say as heavy as 24.9lbs/hr.

So I'm curious what LTFT's I should really expect from this set, assuming that the LTFT's should vary a bit from the LTFT's I was used to seeing with the stock set in the Jeep (~ +/- 3%).

My main purpose for asking this question is that LTFT's currently are about -7% or -8%. I've been battling what I thought was erroneous O2 sensors, but if these LTFT readings seem OK for this injector set, then potentially the O2's are A-Ok.
 
Mine reads a final LTFT at -11% each time I reset and let it run through a learning cycle. That is with the same 784's on a 2000 XJ.
Have the intention to try another set of Ford injectors before going back to some rebuilt stockers.

Sent from my SGH-I337M using Tapatalk
 
Mine reads a final LTFT at -11% each time I reset and let it run through a learning cycle. That is with the same 784's on a 2000 XJ.
Have the intention to try another set of Ford injectors before going back to some rebuilt stockers.

Sent from my SGH-I337M using Tapatalk
Appreciate the reply.
Out of curiosity, how has your cooling system been running? Any issues at idle?

Anyone else who runs these injectors and who has access to OBDII, please post up your fuel trim data.

Sent from my SM-G900V using Tapatalk
 
Might get more responses if we knew WTF STFT and LTFT stood for. I build Jeeps and fly airplanes so I know literally hundreds of acronyms, but those are new ones for me. I'm betting the lack of response is because I'm not alone.
 
Might get more responses if we knew WTF STFT and LTFT stood for. I build Jeeps and fly airplanes so I know literally hundreds of acronyms, but those are new ones for me. I'm betting the lack of response is because I'm not alone.

Here goes...

They stand for Short term fuel trims, and Long term fuel trims.

The ECU will compile an average of short term fuel trims based off of O2 sensor readings, which produces the long term fuel trim. The long term is essentially what the ECU has learned about it's running environment in order to maintain a 14.7:1 fuel ratio.

For most of the time (low load conditions), the ECU is using the O2 sensor and these trim values to maintain the proper fuel ratio. However, once you go past a certain load point, the ECU reverts to using the injector maps for direct injector output. This is because you'll need an A/F ratio that is richer than 14.7. Most OEM O2 sensors are narrow band, meaning they are extremely accurate at zeroing in on that ideal 14.7 ratio, but outside of that value they are essentially useless.

Once in open loop the larger injectors will be flowing more fuel regardless of fuel trims as the ECU is controlling the injector directly from these maps. In other words, a larger injector is pointless unless the engine build calls for it.

As for the 784's, they are close enough to stock that I wouldn't worry about a 10% LTFT.

Hope that helps. :)
 
Yep, good explanation too. I'm familiar with the basic operation of a fuel injection system, and have built many a standalone EFI engine harness, but had never seen anyone use those particular acronyms, nor ask questions about them.
 
Thank you Boostwerks.

I looked at the post then moved to the next saying, That is too scientific for a simple cellular RF (Radio Frequency ) technician.
 
As for the 784's, they are close enough to stock that I wouldn't worry about a 10% LTFT.
Sorry, but I don't understand your comment.

If they were close to stock, my LTFT would ideally be at or around 0%.

10% would indicate to me they are far off from spec, being it that the CEL pops on at 20% I believe.

Am I wrong?

You don't happen to have the 784's installed do you?

Sent from my SM-G900V using Tapatalk
 
I wouldn't worrie about it mate,
The PCM was pulling fuel @3% with the oem. it's now pulling fuel @6-7%, considering they are a bigger injector than stock, and whole argument for the swap is that they atomise fuel better which, means that they have to inject less fuel per cycle to get the same burn. Not a massive amount but that is the direction of the mod.
Their will be bigger differences in the type of fuel you put in that heap. Fuel being a witches brew of different compounds varies Between Different distributors in your area, there are differences in fuel composition in different regions across the country, let alone the differences between winter and summer fuel, esp in colder climes.
If yr that worried about it get access to a wide band oxygen sensor see what it's doing around town, then see what happens at wide open throtel.
What sort of mileage you getting ?
I was taught that as a rule of thumb, single digit long term fuel trim was ok in a stock system ( cause there are differences in fuel, and idiosycronies that each individual machine can have) It's when you get to double digits it *might* be indicating a problem, if you chuck in aftermarket injectors, well then you should expect the envelope to open up a bit.
Good luck with it.
 
Sorry, but I don't understand your comment.

If they were close to stock, my LTFT would ideally be at or around 0%.

10% would indicate to me they are far off from spec, being it that the CEL pops on at 20% I believe.

Am I wrong?

You don't happen to have the 784's installed do you?

Sent from my SM-G900V using Tapatalk

Your ECU is making the needed adjustments for the slightly larger injector. Sure a LTFT of 0% is ideal, but that's in a perfect world.

I do have the 784s installed actually, and have roughly the same LTFT. I also have a wideband and am showing roughly a 12:1 AFR when WOT, which i'll admit is a little rich, but nothing that I'm overly worried about.

Are you getting a CEL? Unless your driving everywhere WOT, you shouldn't notice a difference in MPG either.
 
I get about 15-16mpg.
No CEL's.

Who makes the wideband O2 that you installed, and what are your AFR values at cold and hot idle?

Sent from my SM-G900V using Tapatalk
 
I get about 15-16mpg.
No CEL's.

Who makes the wideband O2 that you installed, and what are your AFR values at cold and hot idle?

Sent from my SM-G900V using Tapatalk

It's an AEM. It runs a stable 14.7 hot and cold. The only time it idles rich is right at initial start up, which is typical as the narrow band sensor needs to heat up to get a correct reading.
 
Been working a rough engine during acceleration and at higher rpms. It is a Renix, and I am using the MT-2500 scanner. Its zero for fuel trim is 128, and it goes from 0-256 IIRC. a LTFT and STFT according to the snap on manual should be 128. Mine is reading about 30-50 on the STFT, and 128 on the LTFT. One of my few clues so far is going closed loop on start up with no heater power to the O2 sensor heater, and hanging up open loop at idle for 2 minutes after a miss fire/back fire at higher rpm. All the usual tests have been good. Down to a possible valve or fuel injector sticking or fouled up issue.

So on a stock system that was working, nothing changed, a low almost 100% of the time STFT means what to you folks?

Good evening everyone,

I was curious how one would determine the appropriate impact to your STFT & LTFT based on a new set of injectors you install.

For example, does an injector set that is either 10% lower or 10% higher in fuel flow rating (at same rail pressure of 49psi, 2001 XJ here) mean the LTFT's should indicate 10% in either direction pending whether or not the injectors you bought are smaller or larger?

I have installed Bosch's 4-pin hole 280155784's.
From what I've read, some say they are nearly identical (~22.6lbs/hr) in fuel flow rating as stock spec and some say as heavy as 24.9lbs/hr.

So I'm curious what LTFT's I should really expect from this set, assuming that the LTFT's should vary a bit from the LTFT's I was used to seeing with the stock set in the Jeep (~ +/- 3%).

My main purpose for asking this question is that LTFT's currently are about -7% or -8%. I've been battling what I thought was erroneous O2 sensors, but if these LTFT readings seem OK for this injector set, then potentially the O2's are A-Ok.
 
Interesting, awesome info on page 3

http://www.autoserviceprofessional....-works-and-how-to-make-it-work-for-you?Page=3

"
If LTFT or total fuel trim is more than 10% negative, the PCM thinks the air/fuel ratio is too rich, so it is leaning out the mixture calculation to return STFT control to the correct range. This presents three possibilities:

  • Not enough air is reaching the combustion chambers.
  • More than the commanded amount of fuel is reaching the combustion chambers.
  • One or more sensors is reporting incorrectly.
One thing that restricts airflow into the cylinders is a damaged catalytic converter that causes high exhaust back pressure. At idle this can drive fuel trims in opposite directions, producing a positive STFT and negative LTFT. You can see signs of high exhaust back pressure on a scan tool; calculated load will be low at wide-open throttle (WOT) and fuel trims will trend negative as engine speed increases.
The most obvious source of excess fuel is a leaking injector, especially at idle. In this case, STFT will be low but increase with engine speed as the extra fuel represents less of the total fuel requirement. Excessive crankcase vapors or a flooded EVAP canister can also mimic excess fuel, especially at idle. If the oil hasn’t been changed in a long time, especially in an older engine with a bit of blow-by, fuel in the oil can drive total fuel trim negative. Often simply changing the oil will demonstrate this condition by returning the fuel trim numbers to normal."
 
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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