What causes a -20% fuel trim?

Driving your truck through a drive cycle is it’s basic learning process. Short term and long term fuel trims are only telling you what the ecu is doing to maintain a 14.7 AFR. It’s an adaptive strategy, and I’m sure you’re aware of this. You said you’ve replaced injectors, did you clear codes and all adaptive memory afterwards? It doesn’t even matter if you did or not, my point is the same either way. Let’s say in theory that you did have clogged injectors, or maybe your new injectors are flowing a little more than the old ones. In that case, there’s nothing wrong with seeing a negative LTFT because the ecu is pulling back fuel to maintain the 14.7 AFR. Two things that have remained a constant is the MAF and the O2 sensor. Like it or not, the O2 sensor is more powerful as the ecu commands the injector pulse width based on the feedback from the 02 sensor.
All I’m trying to say is that your engine is running correctly in both scenarios as far as I’m concerned, as long as the CEL stays off….keep driving! Your engine size has not changed, but the amount of fuel it is receiving most likely did change, therefore the ecu is acting accordingly to keep your truck emission compliant, and your fuel mileage the same.
I completely understand you being OCD, as I am as well. You maybe doing exactly what I do all the time and “overthink” the situation.

 

Negative trims of -18 or worst should throw a code. No experience with the Tacoma under such conditions but that kinda of reading indicates something wrong. If the trims were just re-learning it should move back toward 0 pretty fast at first then settle in.

 

Swap the injectors from bank 1 to 2 and see if the trims switch, if not then try switching the O2 sensors and see if the problem follows. That could help narrow down what it is or isn't in a hurry

 

I can’t argue with that either. I don’t know what the magic number is that the LTFT has to pass to set a DTC. I do agree that a -18 seems excessive, but if the ecu wasn’t reset after installing the new injectors, then it might not be that uncommon. I was just trying to get OP to do a drive cycle and then recheck the fuel trims.

 

Our trucks only have one upstream oxygen sensor. So switching banks with the injectors wouldn’t do anything to my knowledge. Going back to the old ones may.
This kind of diagnostics is easier on the newer vehicles with 2 upstream and 2 downstream oxygen sensors. Then you can decide what is common to one bank or the other or both.

 

Nice feedback/brainstorming. Thanks guys. Though I have to say it's too difficult to swap injectors on the fly to try that method. Not as easy as swapping coils. I'm going to drive it for a bit and wait for a new FPR to come in and see what happens. I'm still kinda set on me accidentally damaging/clogging the FPR when I cleaned the rail with brake clean. We'll see what happens next weekend!

 

I can't contribute anything, but very interesting thread.

 

No worries. I couldnt really find anything i liked regarding negative fuel trims so i planted a seed and it's growing nicely.

 

Short term trims of 20% are fine, its the long term fuel trims you need to watch, so if the 20% is long term, then its running lean. A lean condition will create a rich condition though because the ECU is compensating for the unmetered air its detecting by the o2 sensors and dumping more fuel into the mixture until it gets a 14:5.3 fuel to air ratio. If this happened after replacing injectors, double check the wire harnesses for the injectors or anything else that may have come disconnected. If nothing else, reinstall the factory injectors. My hunch is something is disconnected though. One more thing; even if no CEL is on, you may have a pending code that has not triggered the light yet; connect a code reader to the OBDII port and see if any codes are pending.

 

@CodeSeven,

What's the STFT at 2500 RPM versus at idle? If they are always less than 10 percent, you may be good, and the computer is just taking time to relearn. Normal = +/- 10 percent TOTAL fuel trim.

I know that you said it's NOT a leaking injector, but unplugging them isn't going to tell you if a seal is leaking. It only tells you if that injector is dead or clogged.

Mark

 

I agree with ^^^. Total fuel trims are what we are looking for, LTFT +/- STFT = total fuel trim, with 0 being desirable but 10% “acceptable”.
This is a situation where you have to decide just how much time, effort and money you want to put into perfection which might not be achievable.

 

My Short term is consistently around 0 throughout all RPMS.

long term goes from -20 around idle to ~-3% above 2krpm. it's all consistent. (as fuel pressure is used up by higher RPMs, pressure in the rails goes down, this is a sign that there's too much pressure overall in the fuel rails.)

my readiness monitors are all completed except for the evap monitor, which always takes a long time to self diagnose. it should complete today if i drive around enough. we'll see what happens after that. i doubt it'll change the trims.

another side note i wanted to mention. when i installed the injectors, i put dielectric grease on the orings to help them enter the fuel rail and intake manifold. maybe a bit of grease broke off and clogged the FPR.

disconnecting the injectors will show a leaky injector that is stuck open. if an injector is spewing fuel while disconnected, it's stuck open, and spewing more fuel out while it's connected.

@
Glami

 

I've had this truck for 9 years and this is the first major problem it's ever had. It's worth putting another 5k in to it IMO

 

Dielectric grease is a bad idea, and can contaminate the O2 sensor. Likely not the issue here though. Dodge, Ford and others say to use engine oil for the seals. I doubt it could clog the regulator.

You said that your fuel pressure is high? Have you used a gauge? Several things can mimic what you are describing, including leaking injector seals (since you say internal leaks are likely not an issue). Cheap and easy to do, and could certainly mimic your issue, because the O2 sensor would add fuel that the MAF/ECT/IAT says it shouldn't need, and a battle between sensors and the Computer is the result.

Mark

 

the only indicator i have that the fuel pressure is high is the possibility that the fuel trims are negative because too high of pressure would be spraying too much fuel when the injectors open. the injectors are external from the chamber so any leaky seals would noticeably spray on the intake manifold. yeah, timmy used actual gasoline as a lubricant for the seals. i didn't think about oil. at this point im just going to wait and see what happens when i swap the regulator. another option would be to give the FPR vacuum and see if that causes the trims to go positive.

 

Makes sense. Would be better to see the actual fuel pressure though.

As far as the seals...I wasn't very clear (Sunday afternoon beers because of snow today) I meant the LOWER injector seals, not the upper. Lower seals would leak air IN, because they are under vacuum when charging the cylinder.

Mark

 

True, but leaking lower seals would cause a positive fuel trim, not a negative.

 

I would go back to square one.....right were it started...

why did you replace injectors....miles?.....brand?
what about other 3...brand and what not...

if you rule everything else out....it has to be these...

and how many miles on new set.....I've heard of 70~80 miles for the ecu to reset sometimes...

what about fuel pump....is it shot and ecu is trying to compensate....
replacing the press reg is a no brainer.....probably not it...but now have the old as backup.....

 

cylinder misfire 5. went away after doing 1-3-5 injectors. ran perfect and better than before. didn't need to do 2-4-6 but figured i might as well. now that it's all done, even though i have the - fuel trim, my truck is running stronger than it's ever ran.

 

Where did you get the injectors from?