High IAT= lean AFR

[SSsuperdave]

Got a problem that I don't know where to look. '00 SS, stock OS, heads,cam screened truck MAF with integrated IAT sensor. After sitting still, engine off, for a few min, upon restart the IAT reads from 120s to 140s. I understand this heatsoak, but the issue is after approx 5sec or so, the AFR goes lean to the degree of 18-20/1. I realize the computer is prolly figuring on less air coming in due to the high temp an fueling accordingly but there has to be a table to compensate for this. BTW, the MAF is disabled during this time due to VE calibrating. Will the MAF correct this when it's enabled, and if so, how do the SD guys get around this? May be a simple fix, but I'm still learning. Thanks for any help.

[Frost]

It's much more tolerant of the heat soak with the MAF enabled. Big cam car SD cars hate life when they get lean from IAT heat soak. For the cars that stay SD and are sensitive to it, I often relocate the IAT to the bumper where it sees outside air temp but does not get hot from the heat of the engine bay.

[S2H]

part of the problem lies within the poor formula that GM has put in the PCM to compensate for IAT/ECT.
theres a complex cylinder charge table...that takes care of that issue, but it doesnt work in the 98-00 years...
if youcan spare the credits... consider upgrading to an 01 or 02 OS and you can use that table to help curb that issue...
its not perfect from the factory.. but its a better start than a non working table in the past years. and you can adjust it a little at a time untill you see much less swings from IAT.

[jimmyblue]

I've been much happier since relocating the IAT to
outside the engine bay. If you do that then error
moves to the low end and rich, rather than big end
and lean, and if you're closed loop the rich error will
easily be driven out by trimming.

[SSsuperdave]

May be a silly question, but how does one upgrade to a later model OS?

[Frost]

part of the problem lies within the poor formula that GM has put in the PCM to compensate for IAT/ECT.
theres a complex cylinder charge table...that takes care of that issue, but it doesnt work in the 98-00 years...
if youcan spare the credits... consider upgrading to an 01 or 02 OS and you can use that table to help curb that issue...
its not perfect from the factory.. but its a better start than a non working table in the past years. and you can adjust it a little at a time untill you see much less swings from IAT.

I literally just got done moving the sensor on a large cammed 2001 SD car here in the shop. At least in OEM form, it's every bit as bad as the previous years. An odd tidbit; it seems the higher the DCR, the less tolerant the setup is for the lean condition of heat soak.

Trying to tune the bias is an endless circle. It's much easier to work around it and get on with life and driving.

[S2H]

I literally just got done moving the sensor on a large cammed 2001 SD car here in the shop. At least in OEM form, it's every bit as bad as the previous years. An odd tidbit; it seems the higher the DCR, the less tolerant the setup is for the lean condition of heat soak.

Trying to tune the bias is an endless circle. It's much easier to work around it and get on with life and driving.

I've found that raising the table typically makes the error much smaller...
getting the IAT to a better place is the other half of the puzzle...

I wouldnt necessarily call tuning bias an endless circle... yes it takes a long time and you have to go back over the ve inbetween bias changes... but you can get that error down to a much smaller window.
its never going to be perfect...just not enough tables to correct for every variable... but it can be closer..

upgrading the OS is as simple as doing a write entire with a newer year OS.. if you dont have a license for it you will have to spend a minimum of 2 credits to write the New OS...

[jimmyblue]

You can't get the right answer by interpolating between
IAT and ECT temperatures, when they're both above
real air temp. Charge bias only works when your real air
temp in the manifold is somewhere between the two,
and predictable in its position-between at a given airflow.

This is one area where the MAF is your buddy. He doesn't
care about temperature.

[S2H]

maybe so jimmy... but the bias has helped greatly in my car... swings less than 5% total from hot to cold IAT's and on varioius days and climates so far.

relocating the IAT to a position in the nose of the intake, and getting an IAT that reacts faster to the actual air temp makes a difference...
when I open the throttle and the air gets moving, my IAT goes right down to ambient temps... when its sitting there and airflow is slow, it goes up because the air coming in is hotter from sucking in engine bay heated air.

its not perfect.. but its better than the stock values...

and even with the MAF.. the temp calculation is still in effect and still screws with the fueling.

[Bluecat]

A more realistic bias table then what is provide stock goes a long way, but I'm with Frost on this one. I also have been moving the iat to the bumber for certain cars. Specifically ones that don't have any intake ducting like 4 barrel single plane cars and people dead set on running open throttle bodies. In the begining I was trying to mount the iat as close to the tb as possible and tune the bias to match. Much better results with it in the bumper. But you still need to tune the bias accordingly

[S2H]

and its usually only the lower airflows that need to move.. the upper airflows seem to be work well enough for most applications...and its hard to get good multiple temperature data for the upper side anyways

[SSsuperdave]

S I guess the summary is either I need to relocate the IAT sensor away from the engine compartment, upgrade to an 01-02 OS to get temp bias tables to monkey with. And would it make more sense for me to upgrade to a 1 BAR MAF custom OS? I'd get RTT and what else? And lastly, just so I understand correctly, the secondary VE is used when in SD only and the primary is used for a sanity check for the MAF during throttle transitions. Is this correct? And if in SD, using the secondary VE only, how do u log error on the primary VE to tune it? Just trying to understand the operating procedures better. Thanks for any insight.

[dsmlights]

So then I have to change my PCM and go 2bar ? I have the same issue and it sucksss. I have a 99 pcm

[Frost]

All of the cars that I relocate the IAT on become stable enough that no bias tuning is needed for solid operation. With the end of the heat soak comes the end of the issue.

[S2H]

Just curious...where on the bumper do you relocate it to?

have any pics of an f-body?
I'd love to experiment once on my car...might be a useful thing I can do when I tune some other cars around here.

[Frost]

Recently I had a procharged car that was 2bar SD. It made 764rwhp at about 16psi. Stock cubes forged, AFR 205s, 9.8-ish:1 SCR and the cam was something like 235/237 on a 113LSA. He has a small second fan and it blows near the IAT. Hot starts were MISERABLE. I spent literally about 8 hours on things to improve startup and fueling. In stop and go traffic, heat soak was horrible too. On a 98 deg day it would read 155degF after sitting about 7-8 mins. With his IAT AFTER his two methanol nozzles, we really want (need) to see charge temp under boost. I finally gave up fighting it. I used his hobbs switch (1-2psi) that triggers the second Walbro to run another relay. This relay switches the IAT from the unit behind and under his driver's front headlight whenever there is boost to the IAT sensor in the charge pipe near the throttle body and after the meth nozzles. The car runs and drives immeasurably better!! With no wandering IATs, the VE took all of a few minds to get nailed. Sounds crazy to use two IATs and switch, but it worked very very well.

I say front bumper but usually end up behind the headlight, somewhere that it can see outside airflow.

I can get him to take a pic and send it to my phone.

-edit-
It seems that N/A SD cars are more sensitive to this the higher their DCR as well, as I previously mentioned. You will see it in MAF cars to some extent with high DCRs or really large cams in general even if it only manifests in goofy hot starting. IAT relocation works well for those cars also.

[S2H]

in some plastic? in some metal? or just zip-tied to something?

I actually have the foam in the bumper cut out for air flow where the holes in the nose are... and I have a spot in the actual bumper it could go that it would see fresh air and probably still be in a weather resistant spot...

[ls1Mulllet]

I am dealing with this issue right now and my IAT sensor is probably in the worst location ever. I have a volant intake and it sits next to the filter, so even cracking the throttle doesn't get air flowing by the sensor. Scott- in the bias charge, what exactly do you change and by how much? Could you possibly post yours so I could have a reference as to what it looks like? Thanks

[Frost]

relocate the sensor!!

[SSsuperdave]

I finally relocated mine and it works great. No more hot start .