Hi all,

I recently changed the O-ring on my oil filler tube as it had obviously been leaking for some time. I then read about the impact of air leaks on short-term and long-term fuel trims and thought I'd take a look at them. I had never checked them previously.

I went for a drive with my UltraGauge and could see the LTFT on both banks is erratic - changing quite regularly rather than being static. I then used Car Scanner to log the data and can see it seems to change between several specific values during the journey:

Bank 1
-0.78125
0
0.78125
1.5625

Bank 2
-1.5625
-0.78125
0
0.78125
1.5625
2.34375

The STFT seem as expected, on the whole between -10 and +10 (with some exceptions which I think were under larger throttle openings).

No driveability concerns, no warnings/errors on the dash, ~38 MPG according to the trip computer between 50 and 70 MPH on the motorway this morning.

Anything to worry about/further checks needed/possible explanations?

Thanks!

 

I did a bit more research following a comment on the UK forum. My research to date had suggested that the LTFT didn't really move (unless you induced or rectified an air leak, say). It seems this may differ between manufacturers. shows a Jaguar 4.2 V8 running normally (no fake lean/rich scenarios) and the LTFT changes quite regularly (like mine).

On a BMW forum I also found this very helpful data plot over a 48 mile trip and you can see the LTFT on both banks are also very erratic.

In conclusion, I suspect this erratic LTFT may be entirely normal and my numbers are actually pretty good.

 

The PCM has a sort of table of values corresponding to engine load. It learns LTFTs and much more for each cell.

It's much easier for a DIYer / ordinary car enthusiast to ignore the above and instead just check for sane fuel trims as so often posted (hot engine, parked, at idle & again at about 2500rpm).

(By all means read up on load cells.)

 

Thanks - that's helpful - I think I came across this concept in one of the more tuning-focused videos I watched.

Perhaps unhelpfully, I also recently changed my battery during which the old one was removed from the car for a few days whilst I tried to revive it, so that may be impacting things. I've probably driven 150+ miles since over a period of several hours on different days but maybe not across all RPMs etc..

 

Right from the workshop manual.
Reset the battery. (That is, disconnect the battery. Touch the disconnected leads together to flatten all capacitance in the car system)
Then: " 5. Restart the engine and allow to idle for approximately two minutes (this will allow the ECM to learn the idle values). "

Then: go for an italian tune up drive. all the gears, all the revs.

 

Which reset the learned values (fuel trims etc) and OBD monitors.

Normally should relearn quite quickly - can be sped up by following the drive cycles.

Note that some monitors have specific requirements (e.g. EVAP needs not too much or little fuel).

If some won't set, first check all drive cycle info and then post which ones won't set and hope for ideas! Oh, and see if any pending codes - but each unset monitor means a bunch of codes can't flag.