What causes a -20% fuel trim?

Yo~

Just finished replacing the 3 injectors on the driver side this past weekend and my fuel trims are -20%. replaced the 3 injectors on the passenger side 2 weekends ago but the trims were +10% at idle only because I had the throttle body gasket on backwards...... im sure it was fine if i had done it right to begin with...... anywho. what all can cause the fuel trims to be -20%?

not a leaky fuel injector, i already inspected them connected and disconnected. consistent on all injectors.

other thoughts are mass airflow is reading more air than what's passing through.

oxy sensor might be bad but I didn't touch it and it was working just fine before

it's possible the fuel pressure regulator somehow got a little more clogged after doing the drivers side causing too high of pressure to the injectors. or maybe i shouldn't have used brake cleaner to clean the inside of the fuel rail.......

the computer/EFI system doesn't go through a learning process after being disconnected for so long, does it?

any other thoughts?

I replaced the fuel filter too. The trim gets closer to 0 as the RPMs go up.

 

Rich usually isnt a MAF, its possible but usually misses the air resulting in lean.

The fuel regulator is a possibility, the rails being disturbed may have pushed sediment into the back of the rail. I have seen 1 bad FPR on a 5vz, its hard to check because fuel gauges don't connect easily.

Next is temps, is the coolant temp correct once at operating?

 

That's right. the temp sensor. Yes, the readings are perfect just like before. reading 199-205ish when warmed up.

 

a vacuum leak causes more are to come in than is being monitored, forcing to system to add fuel to compensate. causing a positive fuel trim. my situation is the opposite.

 

New injectors are squirting more than they should be? Air leak is going to cause + not -.

There's no additional learning required. The learning IS the short and long term fuel trims. What's your short? If it's +20 and long is -20, then you're balanced.

 

that's what's odd. the spray pattern seems fine across all injectors and the first 3 i replaced didn't cause a - trim. i also replaced the 1 that looked like it was spraying too much. if there was an injector that was off, it should cause a misfire but the engine is running fine.

 

See my updated message, sorry for the edit

 

10-4. short term is hovering around 0% so it's good. just my long term is -. I'm starting to really suspect the FPR right now...

 

I'm inclined to have you keep driving and see if it works itself out before you go tearing into things again. You should hear and feel the engine change if it's running too rich.. gets more groggly.
Any time you disconnect the battery, the ECU will re-learn, btw.

 

right, another odd thing. I've disconnected the battery many times before and with multiple vehicles. it's never done this before.

 

I'm going to replace the FPR. I bought a Herko brand like 5 years ago for ~$35 and it's done very well until this. Going with them again.

 

@CodeSeven

If you would, come back and update with your results. I have been reading TW for about 4 years and I have not read a thread where someone had to replace their FPR. Lots of leaking injectors, fake injectors, bad sensors,.... but I don't remember a thread where the FPR turned out to be the root cause.

The last post in this thread has a tip for looking for signs that your FPR is bad:

https://www.tacomaworld.com/threads/p0172-code.744847/#post-26519556

 

No check engine light? If not, pay no attention to fuel trims. Take it through a drive cycle as described in the workshop manual to set the readiness monitors. If no check engine light is on. Start looking for trouble only after the check engine light comes on. Then you would pinpoint test the DTC.

 

10-4 if changing my FPR fixes my - fuel trim it just means that the brake cleaner i used to clean the fuel rail damaged the FPR diaphragm or gunk broke free and clogged the diaphragm. it wouldn't have gone bad on it's own. I didn't remove the FPR when i cleaned the rail out. and I know brake cleaner can melt some rubbers and plastics.

I know it doesn't matter. I'm just super OCD about making sure my truck is running properly. I've always had my truck running around 0% fuel trim. this just bothers me more than it should

 

I had a MAF sensor harness that caused fuel trim issues in my Mazdaspeed3, I could wiggle the black ground wire leading into the harness and hear the engine idle change and see on the monitor the trims go way negative.

The one thing I paid most attention to in my modified Mazdaspeed was LTFT's. The first sign of something going wrong would be seen there, boost leak, vacuum leak, fuel system etc.

 

that was my next step after swapping the FPR. Thank you!

 

check for vacuum at the FPR vacuum line at idle. next your observation regarding the MAF might be correct. You can verify this by reading the g/sec of air flow at idle. It should be close to engine displacement in liters so your 3.4 should read 3.4 ish g/sec of air… if it shows much more, say 8 g/sec you need to investigate. If the FPR diaphragm leaks now, it is just a coincidence.

 

Looks good to me. it may be a bit high but 797 rpm X 3.4 L / 60 seconds = 45.16g/s. what Idle are you assuming? 600rpm?

P.S. Side note
The gauge works just fine so the 2nd temp sensor is in working condition.
The EFI temperature reading matches so the 1st temp sensor works fine as well.
Not the temp sensor.

 

the MAF's g/sec look fine, what I do see is a 40 degree drop in coolant temp just by raising the RPM's? I am assuming that these readings are one right after the other.

 

Correct. this is just the warm up process