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

Kinda like who came first the chicken or the egg.

As engine gets hotter the PCM reacts by the IAT and water temps feedback sensor values.

As engine gets leaner the 02s feedback reports lean and thus PCM dumps fuel in and thus fuel trims show rich.

Where the IAT sensor is should have no bearing as you can control it's tables as how much timing would be pulled as the lower timing is the richer engine would get

If no tuning then of course the old trick of moving the IAT to a location that is cooler to trick the PCM the temp is lower and not pull that timing.
As seen in one of the IAT tables the PCM can pull a lot of timing out that also then affects the fuel trims that were lean to be rich at the lower timing.

Intersting table, I don't see it using LS1Edit, from what tool it's coming from?

I understand what you mean: hotter = less timing = richer = negative fuel trims = mess between MAF and SD.

I see the condition I described at idle too, where G/Cyl are about 0.12 and timing is not corrected. Are there other tables / influences?

That table screen shot was using Tunercat tuning tool but LS1edit also has multi IAT tables like this one

Ouch! OK, I forgot that table

Let me do a little test: I'll force the PCM to work in SD mode and simulate the IAT signal with a potentiometer, let's see what happens with the fuel trims.

I noticed that the IAT sensor in the Corvette is in a pretty cool place, in the F-Bodies it's pretty close to the engine and can pick up a lot of heat. Have you noticed that? (mine is a 98 Trans Am)

As promised...

I let the car idle for a while, once in SD (MAF disconnected) once with the MAF connected.
Using an extension I put the IAC first in a cold place (AC outlet) than on top of the engine, this to simulate different IAC values.
The real IAT temperature has been the same for all the time of the "experiment".
Here the result:


As you can see (at idle) the PCM fully relays to the MAF, unless it's disconnected. IT's not a mix between MAF and SD.

In SD mode the PCM tries to change the fuel metering according to the gas law (pV = nRT). Because the PCM is working in closed loop the oxygen sensors cause a change in the fuel trims.

The value Fuel Trims = ((LTFT B1 + STFT B1)+(LTFT B2 + STFT B2))/2

pV = nRT simplified is nT = n'T'
n can be considered a value for Fuel trims
nT = initial Fuel Trims * initial IAT
n'T' = end Fuel Trims * end IAT
T has to be in Kelvin

Example (see the graph):
initial T = 70F = 294K, initial FT = 0%
end T = 150F = 339K
339 / 294 = 1.153 = 15.3%
This means that at 150F the PCM "would like" to inject 15.3% less fuel than at 70F, but the oxygen sensors increase the Fuel Trims of that much to bring back the fuel where it's supposed to be (stoichiometric).

OK, on the graph at 150F the Fuel Trims are +14% and not +15.3%.
Maybe there are other correction factors stored in the PCM: if you look at the MAF curve at that same temparature the fuel trims are 1% lower than at 70F. As much as the difference between the calculated and the measured value of the SD curve

Stefano

Re-reading the entire thread: unless the system is in SD the IAT is not responsible for the Fuel Trims changs I see in hot days.

The PCM is pulling timing for sure but not enough to change the fuel trims by -6% / -10% as I described in the 1st post.