My LTFT are normally +/- 2%, STFT jump a little up and down, but not more than +/-5%.

From time to time, especially in hot days or after a spirited driving the LTFT make a jump to the negative side, especially the low load cells (0..11 and idle). They may go -7% or even lower. At the same time I see a richer AFR at WOT than usual.

After I turn off the engine, wait 5 min, the fuel trims go back to the old values.

How is this possible?


A couple of thoughts:

- No codes, no misfire, no overheating

- The Baro value goes up to 104 kPa after a WOT (clear why, does it matter?)

- After disconnecting/reconnecting the MAF or the IAT I notice something similar: does the PCM go in some kind of safe modus and injects more fuel?
If I had a bad IAT or MAF contact that would explain the negative trims. However: no IAT nor MAF codes.

- The original position of the IAT for my Vortech kit was after the air filter. Now it's just before the TB (I wanted to know the charge air temperature). The IAT can go pretty high (up to 170F, especially at idle in traffic, this because the bypass valve is recirculating the air).
Is that a reason for the PCM to mess with the fuel?

- the MAF is before the blower.


...any idea?

Thanks - Stefano

Without hard data I can only make a guess that during WOT its too lean and when going back to EPA mode the O2s are reporting lean so PCM is adding fuel, or maybe COT is kicking in or the tune at those RPMs when leaving PE mode has a fuel flow or VE values incorrect.
Also could be a temp issue and PCM temp tables are forcing PCM to go rich but if it clears up within 10 minutes of drive time its not a big deal.

I suspect the high IAT is driving this issue and why I suggest our water/meth injection kit since it reduces IAT about 40 degrees. That nozzle should be of course right before IAT so it can see that colder airmass

Maybe the popoff valve where it is in the path is effecting airmass or is leaking and sucking in hotter air ?
One other thing is w/meth when its comes on/off cause 02s to report rich and lean and fools PCM for a short time.
I see this all the time after w/meth goes off since when its on the PCM is told fuel trims have gone rich so PCM leans out, then w/meth being off its now too lean and PCM goes to richen up again

Yes, but that would cause a short fluctuation of the fuel trims, right? In my case they stay negative during several minutes.

COT is disabled (maybe that's not a good idea...) VE are stock.

You mean the coolant temperature? the fans are set to keep it 195 - 210F.

The nozzle is installed after the blower, about 10" before the IAT. During WOT the recorded IAT remains around 80F.
There is no intercooler and it's true: the PCM sees hight IAT values in certain situations (slow traffic, engine at idle during long periods).
I just don't see how to change the relationship IAT/fuel in open loop using LS1Edit.
I only have a fuel adder for WOT (which in my case is set to 0 for IAT>50°C).

The Vortech kit recirculates the air: the MAF is installed before the blower. Because of the recirculation the air becomes hot when the car isn't moving (it picks up the heat of the engine bay). Is that a good idea to insulate the air ducts?
The air from the bypass valve goes back right before the blower, about 15" after the MAF. ok, maybe it produces some turbolence... but 15" is pretty far away (?)

I know this situation and the WI ON/OFF points are set to avoid it.

All in all I think too it's a temperature issue (IAT). But so far I haven't discovered how to influence that.
The original Vortech setup had the IAT right after the air filter, so that the PCM always sees fresh air and not the reality.

Here how it looks like:

Another hint:

When the fuel trims are not acting weird they are at idle (cell 19) somewhere between 0 and + 2%. STFT are pretty close to zero.
If I disconnect the MAF (engine still running) the fuel trims aren't really changing.

When the engine is hot and the fuel trims are negative (as described in the beginning of this post) and I disconnect the MAF the fuel trims go back to 0-2%. I reconnect the MAF and they go negative again.

It looks like the VE is commanding a pretty correct quantity of fuel.
But I can't imagine what causes the sudden negative change of the fuel trims.
I can't imagine that the MAF suddenly is measuring 7-8% more air.
I was thinling the MAF was getting too much heat because of the new location (Vortech setup) and I made an insulation. It didn't help.

It's the MAF? What else can change with temperature?

It's a sudden change, not something that increases constantly with temperature or some other signal... when the fuel trims make a jump it takes few seconds!

Again without PCM OBD data its hard to guess.
It might be a IAC issue or simply your looking at this with too much depth since PCM only reacts to LTFTs once every 10 minutes and at the time these changes are seen is the PCM is in "fuel learn OFF" and if that is the case no matter what the values are they are being ignored and not added to the fuel trims averages.

As too MAF being too hot that should not effect those exposed sensors since it is measuring the mass.
The new MAF used in the C6 Z06 does not even have a MAF case, it is just the electroncs stuck into the airbridge.

Check airflow tables to decel, park and idle for anything weird or even timing values at the moments before and after this issue is seen and what the grams/cyl are.
Also still suspect is any bleedoff/popoff valve and also where its located

I finally got it!
It all depends on the IAT sensor location.
Depending if the sensor is locatet before / after the blower, in or outside the air duct, the fuel trims change.

To discover it I disconnected the MAF forcing the PCM to go in SD: in this particular situation the temperature has a HUGE rule in the fuel calculation (p*V = n*R*T) A few Fahrenheit more or less and the fuel trims make a jump.

I think that with wrong IAT values the calculated air mass was a lot different than the air mass reported by the MAF, causing the PCM to jump between SD and MAF. Is that possible?

JR: where do you install the IAT in FI applications?
Theoretically it's better after the blower, in my case it't better to put it close to the air filter: it needs to feel the outside temperature This results in very similar fuel trims between SD ans MAF.

I recommend the IAT sensor being before the boost and close to outside weather because it stays fairly constant.

Reason GM startng with 85 mm MAF included the IAT in it rather then older versions IAT was in the airbridge

With boost heating the air up and PCM seeing that causing surging as boost flows ramps up and down and then several tables in the PCM adjust and take into account what IAT is since its assumed the hotter the air is the worse the quality of cylinder charge and pinging so that had to monitor and adjust to limit this by pulling timing and increasing fuel flow so you do not want PCM to be getting hotter IAT all the time.

The maggie supercharger tended to fool the PCM to go rich as they install IAT right into the intake manifold and you see that with more surging with that design

I guess that using a WI system (and I have one!) the situation gets even worst. With the IAT in the boost side, after the WI nozzle I was able too see temperature drops from 180 to 80 F in seconds!

What an hard job for the PCM to manage the engine...