Folks, I'm curious about my LTFT's sitting at -4 to -5% when at idle.
Note: 4.2 naturally aspirated. Denso sensors replaced last year.

See below.

Idle at around 600 rpm. Varies maybe 10 rpm in either direction.
LTFT's are indicating the ECU is adjusting 4-5% for rich on both banks. I know it's not much, but why isn't it 0%?
Because STFT's on both banks are operating correctly, adjusting around 0% more or less. Letting STFT's oscillate around 0% for awhile does not change LTFT.
(Note the graph captures instant data, so Bank 1 says 3% when that's lowest reading at that instant as the STFT cycle oscillates near 0%)
O2 sensors on both banks the same, correct at .6 - .7 volts.
MAF is correct at idle at around 7%....

So this is more curiosity than something being significantly wrong.

Question is, why doesn't LTFT drift to normal (0%), if STFT seems correct, and O2 sensors read correctly, and MAF reads correctly.
Air filter changed last year, so not clogged. Both banks doing the same, making a leaking injector unlikely.

So, if you have experience at how all these work together, what do you think is going on that makes the ECU adjust out a mildly rich condition on both banks?

Panthera

 

Imagine you cleared the trims (don't), it's likely your STFTs would then be -ve until the LTFTs gradually adjusted to where they are now.

STFTs near or at 0 (for a given engine LOAD/RPM/etc) means the LTFTs have learned (to whatever stable values).

Slightly -ve LTFTs and about the same both banks could be some common item slightly misreading e.g. MAF. If it's the MAF (and it may not be), a clean might help - but those are good LTFTs and I like to leave well alone!

 

Thanks -- greatly appreciate your thoughts. I agree the current trims are good. It took some diagnostic work, since when I bought the car they were greater than +15 due to various vacuum leaks, etc. Got pretty good at smoke testing and patience.

Agreed, the STFT's tell the core tale, and they're fine. I'm mostly curious how the system works.

In particular, I'm just puzzled at why the LTFT's don't seem to shift. I always thought that LTFT was solely a calculated mathematical reflection of what the STFT's displayed -- LTFT as a smoothed curve of the STFT over time.

I think I'll try constant data capture at 1500 and 3000 for a minute each and see what STFT does. Maybe there's a very different STFT trim at higher RPM's. Again, thanks.

 

Dont get worked up too much about fuel trims - you need to understand that perfection is not possible in life, what is perfection? the magical air fuel ratio of 14.7 : 1 ... its not possible for any vehicle to maintain that which is why there is a large tolerance, I think the EML comes on if they go above 15 percent or somthing at which stage the ECU says sorry I cant do any more work here guys compensating for air fuel ratios lets call it a day.

+/- 4% is nothing, I think I seen the same in my 4.2 also a few months ago. If they were 10 percent yeah that is concerning, if the fuel trims are very positive or negative whilst driving that is obviously not desirable also.
The best way to assess the engine's health is capture the LTFT over a drive and average them out, I would guess that on a healthy engine the trims would be near pretty close to 0%

 

I'd like to chime in that, if you changed your injectors at any point, the newer injectors (at least on the 5.0) flow more fuel
than the old injectors which will result in a negative 4 (-4) or so LTFT. This is normal unless you updated your ECU to reflect newer injectors.

 

Also a clogged catalytic converter can also cause issues with fuel trim

 

Good point -- but these are old-style 4.2 injectors, operate differently than the DI types in the 5.0.

Interesting. Does one clogged affect both banks?

 

I agree, it's certainly in tolerance and not a problem. This is a "how does it work" inquiry. I've learned a lot with this car, and I'm still curious.

Thanks, everyone. I'm going to collect more data. In the 4.2 NA, commonalities across both banks are: Air filter (new), MAF, throttle body, fuel pressure sensor (new), injector pulse timing, manifold pressure sensor, fuel temperature sensor. Time to get out SDD.

 

They shouldn't "shift" (change). They should (and yours have) settle. Then the STFTs can adjust over their full range should they need to - such as a momentary thing that interferes with fuel/ignition/whatever.

The settled LTFTs are also what determine whether things are OK / codes flag.

They are. But the idea is to move the value (if steady, or the steady component of the value) into the LTFTs and zero the STFTs (well, zero is an ideal and engines tend not to be ideal).

A modern PCM tends to have a whole lot of LTFTs & STFTs, worked out for lots of combinations of engine load/RPM/etc. So you'll be looking at different ones (hopefully close to zero if you can do a steady speed - well, it's probably engine LOAD for your car but speed on a consistent road surface is a decent proxy for that).

You can think of a table of sets of LTFTs (& corresponding STFTs), where the cell in the table is chosen based on some combination of engine operating conditions (speed etc). Every time a particular cell is chosen its LTFTs & STFTs are used / updated. Then kept when another cell is chosen (due to slightly different operating parameters - you're going faster or whatever). Ideally each cell's LTFTs will settle (and be near zero).

The above data (about a cell, effectively) can be put into the Freeze Frame stuff if a DTC flags, and can help diagnose what went wrong - so you may get a Lean/Rich code for example plus RPM, LOAD and so on.

By way of example, lean codes often flag at idle but misfires might be uphill at 50 because that stresses the coils, and that data is in the freeze frame.

 

It may help to think of LTFTs as a reflection of how close the engine is (over time) to an ideal newly made engine. The STFTs ideally fluctuate a bit around zero to cope with momentary disturbances (car jolts, fuel pump doesn't love that, or whatever).

But with (say) an air leak, the STFTs have to cope quite a bit and if they keep at similar non-zero values for a while then that value is in effect a way the engine is now behaving over time and goes into the LTFTs. The STFTs go back to zero (or near it), back to coping with momentary stuff.

 

Edit

 

Be more concerned about big differences between Bank 1 and Bank 2, they should both be the same, differences indicate on issue on the respective bank, (air or fuel - pumps, rails, injectors, intakes, vacuum, etc).