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Old 10-09-12, 10:12 AM   #1
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Default AEM CAI:LTFT and IAT Mystery Solved

Two things about the AEM CAI on my car has had me searching for answers for years. One is why my IAT increases every time I begin a logged run. The other is how my LTFT, thus MAF CAL, often simply "makes no sense." Trims will be fine then change... so I reset and drive and it's OK again. Or input to the MAF CAL won't do what they should have... at least for long.

These are MINOR things. The increase in IAT only amounts to two or three degrees. The LTFT trims have been within acceptable levels for a long time. It's just that the IAT increase escapes logic, and the LTFT should be both closer and more stable.

I've wrapped the CAI tube/sensor with insulation to see if it was that. Nope. I added a shield at the turbo thinking it may be IR. Nope. In fact, nothing I've tried has influenced these nagging "WTF"s. Somewhere I have a thread about my CAI mods, in which I ensure ambient to the filter... mentioned because I have a temp probe in the fenderwell at the air filter. I know what the temp is at the filter. There should be a correspondence with the IAT sensor that hasn't been there.

So I decided to see if the heat was being conducted by the AEM intake tube to the MAF sensor/IAT sensor. I insulated the MAF sensor from the tube's heat with mica wafers used with transistors. In doing so I coincidentally electrically insulated the MAF sensor's metal mounting plate from the AEM's intake pipe. This is important to later.

Logging after this showed a change in IAT behavior. It was logical now.

Logging also showed a startling shift in LTFT numbers... but still not as stable at finding a trim and settling into it as I'd like. They changed, but.

Then I found that the MAF sensor's mounting plate was a Ground in that ciruitry. (later found this is not so on my 11 STi) That meant the AEM's big pipe was a big floating ground that I'd removed which began to explain some things. With the MAF Sensor's ground now disconnected from the pipe, and seeing the changes this caused,

Final changes:

1 - I added a ground from the AEM intake pipe to the chassis. In the process I also grounded the AEM heat shield. Its paint kept it insulated electrically from the body.

2 - In addition, later, I cut the B+ wire (#3, center) and inserted a simple L/C line filter and ferrite "beads" before and after to remove some of the incredible garbage my O-scope revealed there (the injectors share this supply line). There is a voltage conditioning circuit integral to the MAF/IAT sensor, but I figured less garbage in, less garbage out.

3 - I left the sensor electrically insulated from the CAI's metal tube.

Logging with the MAF Sensor electrically independent from the intake tube, and with the tube grounded, has solved my issues. The IATs are now predictable and logical. My LTFT has stabilized at levels that finally make sense.

The MAF sensor in the OEM intake is mounted in plastic and doesn't have the AEM's issues to deal with. No wonder the AEM CAI has a bad tuning reputation. However, the B+ is the same...

This is just a FYI that may cause others to explore the idea. It is only for tweaks like me that spend too much time tuning minutia, and nothing that really affects the car in any substantive way. To me, however, it means a lot.

EDIT


While my line filter for sensor B+ and the grounding of the AEM CAI's metal pipe have solved some long-standing issues for me, there has been a more profound result that is noticeable every drive. The engine's operation is incredibly smooth in all RPMs and loads.

This was immediately noticeable, but it seemed so normal that I questioned whether it had always been this way. However, I definitely remember it being different... there would always be a place somewhere in a gear I knew it was time to shift. Eiither it would become uncomfortably jerky, or stumble, taking away my throttle's liinearity and inserting instead a region of instability. So, I shifted, and got so used to it I never noticed it... until I was in slow traffic and the car's behavior became an embarassing display of undicipliined behavior and poor driving.

Now, however, I can leave it in any gear, under any load whatsoever, and use as many RPMs as I want... and it is always a song wiith the texture of silk. This car's SPT exhaust with it's innate STi voice is now an unblemished pleasure I enjoy sharing with the world. It is never anything less now.

I haven't updated this thread, because there has obviously been no interest. But a couple of threads I recently visited reminded me not everyone enjoys what I do. Maybe most do, I don't know. But definitely some do not.

For example:
massive negative af learning A, B and C only Post #4

https://forums.cobbtuning.com/forums...Hesitation-Fix!

Maybe my car is an exception. I'm updating this thread for those who identify with "stumble" and "hesitation" issues.

EDIT

FINAL RESOLUTION!!

All vestiges of "stumble and hesitation" gone.

See here: 08 STI Stumble/Hesitation/Lean Spot starting with post #48

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Aracheon, vermont and utahphunk like this.

Last edited by SeeeeeYa; 12-19-12 at 08:56 AM.
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Old 10-10-12, 06:58 AM   #2
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Default Re: AEM CAI:LTFT and IAT Mystery Solved

interesting point though I don't understand why the extension of ground on pipe for MAF can be a problem since it's floating.
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Old 10-10-12, 10:27 AM   #3
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Default Re: AEM CAI:LTFT and IAT Mystery Solved

That make alot of sense.

If the stock mass is mounted on an insulator, and then mounted on something conductive, but insulated you could have a voltage on the conductor.

You would have thought that if the MAF is grounded to its metal base that this would have bled off any charge. Of course if a radio can get messed up from emf from the alt. then a big antenna like the CAI could soak up alot.

Good find.
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Old 10-10-12, 11:34 AM   #4
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Default Re: AEM CAI:LTFT and IAT Mystery Solved

well yeh if the floating ground area is huge then it can act like an antenna picking up noise around..
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Old 10-10-12, 02:40 PM   #5
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Default Re: AEM CAI:LTFT and IAT Mystery Solved

Wow good article. Makes a lot of sense when it comes to nailing down the scaling on my AEM. I have the had the thing for almost three years now and I have been running into a very similar problem. Scaling it should be the same each time no matter what from one map to another, I am finding that ti changes very minutely each time.
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Old 10-10-12, 06:24 PM   #6
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Default Re: AEM CAI:LTFT and IAT Mystery Solved

Someone over at NASIOC also found out that grounding matters, just recently:

Stumbling/Hesitation Problem - MAF Grounding Error (read everything first) - NASIOC

Different symptoms, but same root cause.

I had to kick myself because I ran into a related problem about a real ago, but never figured it out until I read his thread... My car started acting like I had a vacuum leak, and I'm sure now that it was because the paint that was insulating my homebrew CAI's MAF housing from my chassis finally wore through and started conducting.
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Old 10-10-12, 06:33 PM   #7
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Default Re: AEM CAI:LTFT and IAT Mystery Solved

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Old 10-10-12, 07:26 PM   #8
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Default Re: AEM CAI:LTFT and IAT Mystery Solved

Interesting find.. thanks for sharing.

I have the same intake and while my fuel trims are pretty stable, I have noticed the illogical behavior of the IAT increasing during a WOT pull. On average, I seem to gain 2 - 4* depending on the duration of the pull. My IAT also seems too much higher than the outside ambient temp than it should be.

Did you notice similar IAT behavior during a WOT pull?

What about the correlation between ambient temps and measured IAT during cruising at a speed that should results in close values? Did it change after your mod?
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Old 10-10-12, 07:43 PM   #9
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Default Re: AEM CAI:LTFT and IAT Mystery Solved

Was this affecting your A/F Learning D? Do you have a learning view before and after?
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Old 10-11-12, 12:01 AM   #10
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Default Re: AEM CAI:LTFT and IAT Mystery Solved

Quote:
Originally Posted by wrxt4cy View Post
[...] I have noticed the illogical behavior of the IAT increasing during a WOT pull. On average, I seem to gain 2 - 4* depending on the duration of the pull. My IAT also seems too much higher than the outside ambient temp than it should be.

Did you notice similar IAT behavior during a WOT pull?
I saw that with the AEM CAI that I had in my LGT, and with the KSTech CAI that's in there now.

I think it's because the exhaust manifold heats the area down by where the CAI gets its air supply. (So, it's not as much of a CAI as you might think.) It was really bad the last time I went to a dyno, and they only had cooling fans for the radiator and intercooler so I couldn't do anything about it. But I'm not sure an extra fan would helpful anyway, it's a hard spot to get air to.

I still have a short-ram intake, and I'm going to use that for my next dyno session (with the corresponding MAF scaling, of course) because I'm pretty sure that will get much cooler air. I might even keep it on permanently, depending on what my logs say about driving around with it.
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