Suddenly lean???

[hquick]

What would cause my tune to suddenly go from almost spot on to about 3-4 points leaner (AFR) across the board?
Can't think of anything I did to cause this. I logged all of my O2's just to make sure my WBO2 hadn't gone bad....but the results seem to verify what the WBO2 is telling me.
I didn't notice any particular issues with the truck's driving.
I was thinking exhaust leak before th O2's but it would have to be on both sides which is just too much of a coincidence.
I cleaned and re-dialled in the MAF for the moment...so it seems to be OK'ish now, but it was damned near perfect before and the MAF shouldn't have needed to be touched.
Any thoughts?

[ChipsByAl]

Were by any chance your fuel trims reading positive @ WOT in the past? If the weather changes and the trims go negative, the PCM would not still be adding the excess fuel to keep the mix where you want it @ WOT.
Al

[hquick]

Well....I usually don't log the trims because I'm running OLMAF. MAF is set at 1500rpm so as to try and catch the lean tip in issues.
We haven't had any major weather changes between the tune being very good to all of a sudden garbage.

[swingtan]

Hi Howard,

Tuesday was a scorcher and the change may have upset something. I'd firstly try a PCM reset to ensure no trims are being stored. Then check what the IAT's were when the lean-out occurred as you may have reached a point you had not seen before.

Simon.

[hquick]

Thanks for the reply Simon.
This started last week before the stinking hot weather.
I cleared all learned trims...that I know of.
So far I've re-fitted the MAP sensor to make sure of no leaks and changed the MAF for a brand new one (I had in the shed).

[hquick]

OK...a bit of a follow up.
I solved the problem.
Seems during the last update....which I downloaded just before this issue started, I somehow ended up with an issue in one of the .cax files relating to transient fuelling.
Well...yesterday I was looking into the 0411 4x4 COS issues a few of us seem to be having and I downloaded the .cax file which allows switching of 4x4 and platforms. When I opened my tune....alot...actually most of the transient tables/parrameters were 'out of range'.
I reset them to how I had them prior to the lean issues I experienced. I also reset the MAF table and everything was back to normal.
Thanks for your help guys.
Let this be a lesson to everybody to ALWAYS check/take notice of the 'OUT OF RANGE' tab.

Just for interests sake....if anyone wants to see the cause of this issue.
Compare these two files.

[Stealth97]

Glad to hear you figured it out!

[hquick]

Thanks Eric.
Actually....yesterday it went south again!
No idea what happened. It was spot on after I reflashed with the fixed out of range parameters.
I drove around all day....no issues.
Next time I started it and drove to work.....up the creek again!
I'm wondering if I'm getting dodgy fuel. Last night it was so lean it was pinging badly.
re-adjusted the MAF table and it's fine again????
No idea at present...haven't had much time to look into it.

[Stealth97]

Sounds very interesting. Let us know what comes about! Let us know if it goes haywire again. I'm real interested.

[hquick]

OK....didn't realize a small change in B4307 "desired airflow" would make such a large difference to the AFR. Seems it is the case.
For the moment I've gone back to OLSD to double check my VE tables.