Does a motor become richer, or leaner overtime

[Jack Daniels 383]

I have 250,000 km on my stock 6.0 L motor. I know there is a leak in the exhaust manifolds/ head mating surface. I suspect it is running falsely rich because of an influx of fresh air. I was toying with temporarily disabling the O2 sensors, and just letting the computer run from it's baseline maps for a bit. However I was curious, with a motor which is very worn in, is it going to run richer then it did when it was new. Or put another way - if you did a trial, and took a motor with 20,000 km on it (broken in - but still new), and let it run on perfectly tuned maps (0 short or long term trims) perfectly stoich, and then you put 10 years and another 250,000 kms on the motor - compressions down in theory, sparks maybe a little weaker, exhaust doesn't flow as well as new, etc. - would you expect it to run richer or leaner?

I'm just curious if anyone has an opinion/ facts on the matter.

My thoughts were it is less efficient in general, so more unburnt fuel heading out the tailpipe -> ergo richer.

Or an opinion, other then fix the air leak, as far as disabling the o2 for now and letting it run straight off the tables. When I reset fuel trims, and was in closed loop I was running .072- .74 volts, foot to the floor, or cruising - however I'm guessing air leaks had a lot to do with this.

If anyone wants to see what a vehicle like this runs like I have attached a log. I reset the fuel trims multiple times. At 2.5 mins, then at 3.5 ish, I reset and turned off trims. Before this scan the fuel trims had been reset a short but forgotten time ago. I wish there were a way to go backwards in scans, rather then restart all the time.
black chevy trip around the block w speedo changed trip 2.hpl

[JamesLinder]

Assuming MAF still works 100% correctly, it will measure the air mass flow correctly and the VE correction factors
will be updated during periods that meet the qualifications of dynamic air steady state to yield correct fueling. You
will just have less air mass flow for the worn motor than the new one at the same RPM levels.

[Jack Daniels 383]

I hadn't actually thought of it that way - however it makes a lot of sense.

[Gerrot]

Whether it be a Map(Speed Density) or MAF system both should compensate with fuel trims either way in any instance. Hence why we have and use fuel trims for correction on almost anything that is fuel injected today!

If you choose not to use these...and not sure why you wouldn't, it will still be running on the VE or MAF table to make it run for any factor it need be. Cold or hot, high altitude or low altitude, Should be no difference in fuel mixture.

Say an engine pulls less vacuum at idle because its old...Just saying for purpose. It would just simply read a different part of the MAF table or VE table and choose a different pulse width for the injector and spark output for the coil/ignition.

[JamesLinder]

Actually, if you have headers which of course changes position of the OEM narrow band sensors, the fuel trims no longer
work correctly. There is a transport delay that is impossible to compensate for due to the lack of parameters in the
tune file that would be needed to adjust for it. In fact, after being moved further away from the exhaust valve of the
stock exhaust manifold when installing headers, the OEM oxygen sensors no longer stay hot enough to work accurately.
For this reason, many tuners leave the tune open loop permanently with both STFT and LTFT disabled. Once you dial
in the MAF and VE tables, there is really no reason to use fuel trims unless you have stock exhaust, and even then I'm
not sure they work like they are supposed to. One thing about the VE table, the values you put in there are always
updated with corrections from the MAF during states that qualify as steady-state, however the limit factor is set by
GM at 1.03, or 3% so once you get a ton of mileage on your motor the VE tables would not be able to correct enough
to compensate for the loss in VE from when the motor was new. And, if your motor is speed density, which is not a good
choice for most naturally aspirated street vehicles because the MAF is the heart of the control system, then your VE tables
do not have a MAF to provide correction factors PERIOD. Another problem with a lot of S/D tunes, no one recalibrated the
IAT to make certain that it is reading correctly. If your motor sets long enough without being started, the IAT should
read the same temperature as the ECT. You will find that some of the IATs are off as much as 23 degrees Fahrenheit; that
is, they read too high leading to lean conditions since the VE computation in the PCM relies on a temperature compensation
factor that is more biased to IAT than ECT.

[JamesLinder]

and yes the OEM O2 sensors have heaters, but they are not sufficient to make them work as intended when you move them that much
further down the exhaust pipe where the temperatures are so much less at low flow exhaust conditions (idle and coasting).

[jimmyblue]

I'd be worried more about things like injector mismatch that
develops over time (silt, etc. - been there more than once)
than a change to the pumping VE. Bankwise fuel delivery
control depends on matching for accuracy in every hole.
Mismatched fueling leads to a "soup" of rich here, lean there,
the wideband just sees the oxygen left over from the lean
ones (narrowband sensors too).

The talk about VE correction factors is interesting. To me
it seems like that wants zeroed before you try to do any
VE table tuning, or your work will drift. No idea how to do
that though.

[jimmyblue]

If you choose not to use these...and not sure why you wouldn't

Because false trimming is worse than no trimming, and
some setups just don't let the sensors work well enough
to trim reliably right.

[Gerrot]

If everything outside the engine was is good OEM shape, there should be no reason to not use GM's well spent engineering on fuel trims.

[jimmyblue]

Perhaps. Show of hands - anyone here still stock?

[5FDP]

I have 256k miles on the stock block. Stock intake, stock maf, valve train and cam.

I only have exhaust stuff done, but still have the factory manifolds.

I've done a VE and MAF tune to it already.

[Jack Daniels 383]

I'm obviously stock too. Not that I don't have plans, but currently stock as stock can be.