Positive trims with higher IAT's
07-24-2006, 12:42 PM
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Positive trims with higher IAT's Why do trims go positive with higher IAT's?
You would think increase in temp air has more volume less density. Molecules farther apart less fuel to maintain correct afr.
You would think increase in temp air has more volume less density. Molecules farther apart less fuel to maintain correct afr.
07-24-2006, 01:46 PM
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I think it's related to the heat-soak compensation built into the PCM. I just run a -3~-5 LTFT at idle to try and battle this. Besides, if it does go a little positive, what's the harm? It's idle...not WOT.
07-24-2006, 07:12 PM
#4
Cylinder charge temperature is the table that deals with incoming charge air temp on fbodies, not sure if the trucks uses the same term for it. The pcm uses a bias between IAT & ECT to determine the temp of the incoming air. It is heavily weighted towards IAT. Since the IAT on fbodies sits on top of the radiator, it is prone to heat soak. This causes the IAT sensor to report a much higher air temp than actual. Since the air temp is cooler than reported, the mixture is actually too lean. The pcm sees this via the O2 feedback and adds fuel by + trims.
Your IAT sensor is in the MAF correct? I would think heatsoak would be reduced to some degree because of this compared to an fbody.
Your IAT sensor is in the MAF correct? I would think heatsoak would be reduced to some degree because of this compared to an fbody.
07-24-2006, 08:04 PM
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Originally Posted by bshell
Cylinder charge temperature is the table that deals with incoming charge air temp on fbodies, not sure if the trucks uses the same term for it. The pcm uses a bias between IAT & ECT to determine the temp of the incoming air. It is heavily weighted towards IAT. Since the IAT on fbodies sits on top of the radiator, it is prone to heat soak. This causes the IAT sensor to report a much higher air temp than actual. Since the air temp is cooler than reported, the mixture is actually too lean. The pcm sees this via the O2 feedback and adds fuel by + trims.
Your IAT sensor is in the MAF correct? I would think heatsoak would be reduced to some degree because of this compared to an fbody.
Your IAT sensor is in the MAF correct? I would think heatsoak would be reduced to some degree because of this compared to an fbody.
07-24-2006, 08:23 PM
#6
Set the bias to 1.0 so it uses IAT and ECT equally. Not sure if you caught the thread on hpt yet: http://www.hptuners.com/forum/showthread.php?t=6345
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07-24-2006, 08:45 PM
#8
I've been SD too long to know if it does / doesn't occur with the MAF. I set my bias to 1.0 and left the IAT in the lid. The fueling is more stable (according to my wb). I've also tuned a car in closed loop SD and noticed a smaller swing in fuel trims.
