It's a while now (years!) that I notice something strange: my fuel trims are pretty constant between -2% and +2% depending on the season, fuel and so on.
In hot days, when the car doesn't move too much (like city traffic) the fuel trims suddenly go negative to maybe -6% / -10%.

What I find out is that as long as the signal from the MAF is similar to the calculated VE the PCM uses the MAF.
When the difference becomes too big the PCM uses the calculated VE.

In the city traffic, when the PCM uses the MAF the fuel trims are OK, when it switches to VE the fuel trims go negative.

If I force the PCM to use the VE and the car is moving the IAT is pretty low and the fuel trims are similar as when the PCM uses the MAF. When the car is stuck in traffic the IAT rises ant the fuel trims go low. the higher the IAT the lower the fuel trims.

It's clear that the IAT plays a fundamental rule in this issue!

Is it possible that the position of the IAT sensor is wrong?
Is it possible that the PCM isn't using hte IAT signal correctly?

The IAT sensor is in the stock position (stock filterbox).
I haven't messed with the VE table or any other temperature related table.

So well... as in the title: how does the IAT influence fuel trims?

Stefano