I've got something going on with 2 trucks that is driving me crazy.
Both of them will slowly richen up at idle.
At first I was chasing my tail on the AFRs. If it was 10% rich I would change VE by 10% and it would over shoot. So I started adjusting ve by 50% of the corrections and it would still do the same thing.
So if it was 10% lean I would only add 3% fuel and it would be close.

But now I have the trucks idling with the hood raised and a 48" shop fan in front to keep them cool so the electric fans won't turn on and put a load on the engine.

They are still richening up. From commanded 15afr to 10afr over several minutes. ECT and IAT are stable.
I'm at a loss. I've tuned thousands of these trucks and never seen this.