IAT question

[odd boy]

what happens to the car idle when the IAT is disconnected? I know air temp plays a major rule in calculating the air density so it will affect a lot of parameters.

I am asking for that piece of info in order to tune a car that will be used in a region where the temperature varies between (77-85) ºF.

Another thing, how can I force the pcm to ignore the IAT at all???

[hquick]

Have a look at B4901 and B4902

[mr.prick]

Have a look at B4901 and B4902

these 2 tables are used when {B0109} is enabled.

are you the guy that was trying to "trick" the ECU by giving it a set IAT?
high/low IATs have a huge affect on your engine,
there is really no way around it.
i assume this is why we have LTFTs and STFTs.
try relocating the IAT sensor, maybe that will help some.

[odd boy]

thanks to you guys.

[redhardsupra]

i've seen situations where IAT sensor (the hardware) failed, and it just reported constant -40C, which is hugely off for most situations, so your temperature will be misreported, and all temp based modifers will throw off the tune, so spark, airmass, and all other things are gonna go heywire. you dont wanna deal with that.

[eurospec1]

I actually ran into this problem in my C5, that is already run in OLSD.

I was driving home one night, and 35 miles from home when the car started running terrible, i was data logging at the time and saw the IAT reading -40. I just re-did my VE table and everything was fine again, i actually drove the car for quite a while before getting around to fixing it (was a broken wire) with no negative affects.

[rally1]

If you unplug the IAT it will normally run very rich casuing the fuel trims to go a long way into the negitive.

[redhardsupra]

yea, fake cold temps are gonna make airmass go up, thus the computer will dump more fuel than necessary in reality.

[Sid447]

i've seen situations where IAT sensor (the hardware) failed, and it just reported constant -40C, which is hugely off for most situations, so your temperature will be misreported, and all temp based modifers will throw off the tune, so spark, airmass, and all other things are gonna go heywire. you dont wanna deal with that.

I've seen a locally "modified" car (Lumina SS) here which had the IAT sensor located behind the headlight and disconnected from the maf-pipe.
When scanned the IAT reading was steady at -40C and the car was running seriously rich.

[odd boy]

Welcome,

How is EFIlive tuning in UAE?? I am happy to see more people in the Middle East using this great device. "Big welcome from KSA"

Well, let's start with understanding the contribution of IAT in all tables;

1. {B4901} charge temperature blending

The PCM calculates the charge temperature (in degrees Kelvin) using the following formula 273.15+IAT+((ECT-IAT)*factor) where factor is obtained from this calibration, which will be used later in the VE calculations (g/cyl = VE*MAP/charge temperature)

This will be valid if {B0109} is set to yes.

2. {B4902} Charge Temperature Filter

Determines the rate of change of the calculated charge temperature in response to changes in the IAT or the ECT.

3. {B5911} Spark IAT table : Provides intake air temperature correction to spark timing.

4. {B5943} Spark IAT vs RPM Multiplier

This table contains the multiplier values for table {B5911} "Spark IAT Table" based on engine speed. Any value in table {B5911} "Spark IAT Table" will be multiplied by this amount before being summed in the final spark advance calculation.

* EXAMPLES *
0.0 = No Correction applied from IAT spark table.
1.0 = Use the full value defined in IAT spark table.
0.5 = Use 50% of the value defined in IAT spark table.

5. {B5944} Spark IAT vs ETC Multiplier

This table contains the multiplier values for table {B5911} "Spark IAT Table".
Any value in table {B5911} "Spark IAT Table" will be multiplied by this amount before being summed in the final spark advance calculation.
This table can be used to turn off IAT spark correction when the engine is cold (or hot).

So if we assume that the ECT is 80 C and IAT is 30 C, the correction temperature will be 273.15+IAT+((ECT-IAT)*factor )==> 273.15+30+((80-30)*.6) = 347.15 K.

Now will assume that the IAT is -40 C, the new value will be 375.15 K.

Then will put each value in the VE equation with assuming that VE=80, MAP= 35

1. When IAT = 30 C, corrected temp = 347.15 K :

g/cyl = VE*MAP/charge temperature ==> g/cyl = 80*35/347.15 = 8.06

2. When IAT = -40 C, corrected temp = 375.15 K :

g/cyl = VE*MAP/charge temperature ==> g/cyl = 80*35/375.15 = 7.46

From that, do you think the car will run richer or leaner if the IAT is -40 C???

[joecar]

Odd Boy,

It didn't make sense until I realized your calculator failed you... :doh2:


So if we assume that the ECT is 80 C and IAT is 30 C, the correction temperature will be 273.15+IAT+((ECT-IAT)*factor )==> 273.15+30+((80-30)*.6) = 347.15 K.

Now will assume that the IAT is -40 C, the new value will be 375.15 K.
I get:
273.15+30+((80-30)*.6) = 333.15K
273.15-40+((80+40)*.6) = 305.15K


1. When IAT = 30 C, corrected temp = 347.15 K :

g/cyl = VE*MAP/charge temperature ==> g/cyl = 80*35/347.15 = 8.06

2. When IAT = -40 C, corrected temp = 375.15 K :

g/cyl = VE*MAP/charge temperature ==> g/cyl = 80*35/375.15 = 7.4680*35/333.15 = 8.40 g/cyl
80*35/305.15 = 9.17 g/cyl

(I know you are just using VE = 80 for example sake... the VE would be closer to something like 2-3 g*K/kPa.)

Regards
Joe
:angel_innocent:

[odd boy]

It is sweet when u get confused and then clear. Correct me if I'm wrong, the elaboration I made is right (apart form the calculation part)

[joecar]

Your elaboration seems to be correct (you correctly used the concepts of temperature weighting and VE "actualization").

Just remember that the VE table is not a percentage, but it is a normalized value having units g*K/kPa, and by applying charge pressure and charge temperature (under the conditions in which the charge entered the cylinder) you obtain the actual VE in g.

[joecar]

The result of your elaboration using correct calculation would be that at -40C the VE is almost 10% greater (richer) than at +30C.

[odd boy]

Thanks, and think about posting my+ur write up with the required corrections and result

[Sid447]

As I understand it,

Assuming the same atmospheric pressure and elevation; an IAT/OAT of 30c gives a higher Density Altitude than -40c does.
Therefore if the D.A. is higher which means the air is thinner or has less oxygen, it doesn't require as much fuel (to maintain a decent AFR) and the power drops as a consequence of this.
(Though there is some payback with less drag and thus less power required for a given speed depending on temp lapse rate).

If the IAT is not signalling the PCM to decrease fuel in higher temps and increasing it in lower temps then maybe GM have got it wrong and need to compare notes with with an aero engine manufacturer like Pratt & Whitney.

http://www.obd2crazy.com/techdiat.html

[odd boy]

See what we came with, it is in line with ur understanding


The result of your elaboration using correct calculation would be that at -40C the VE is almost 10% greater (richer) than at +30C.

Use these two graphs as a future reference

3856 3857

Should u have any question u can call me on +966504699641.

[joecar]

The second graph shows it... cold air is denser than hot air... we don't even have to mention DA (to keep it simple).

I don't follow why you say GM got it wrong...?

[Sid447]

The second graph shows it... cold air is denser than hot air... we don't even have to mention DA (to keep it simple).
I don't follow why you say GM got it wrong...?

Understanding DA is the whole foundation of air fuel mixture and performance!

Anyway if post #10 is a fine example then I'll say no more. :grin:

[joecar]

I didn't see in that link anything that says differently that what was discussed... I am considering what you said and I didn't want to miss the point... :doh2:... I want to understand everyone's insight. :rockon:

[joecar]

Understanding DA is the whole foundation of air fuel mixture and performance!Maybe... but I think understanding what the units g*K/kPa means is even more fundamental to tuning. :):):)