IAT heat-soak in IC end tank - Help me understand this. - Miata Turbo Forum - Boost cars, acquire cats.

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Old 09-18-2011, 09:11 PM   #1
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Default IAT heat-soak in IC end tank - Help me understand this.

So, like many MS users, I deal with IAT heat-soak and lean, hot restarts. I started with the IAT sensor in the charge pipe (FM pipe std. location just before TB). In this spot the car would barley start after sitting for short periods and run very lean (16:1 -ish) but the condition would clear up after a couple stabs at the throttle and moving just a few feet; even on the hottest day.

As an experiment, I hung the sensor in free air in front of the radiator, near the headlight. It would still heat-soak but not as bad. The car started fine and would be back to normal AFRs in a block. This was a longer recovery period, but less heat overall.

I finally got off my lazy *** yesterday and did the accepted fix for this. I drilled and tapped my IC end tank at a point near the horizontal centerline of the mouth; getting the sensor out of the engine bay. Dig this: The thing now runs at least as lean on hot re-starts as in the first location near the TB, and takes longer to run normally than either of the other locations. The sensor is physically as cool as a cucumber to the touch. What gives? Any theories?

I'm guessing if I could put it in a similar spot to Brain's, then it would get cool air more quickly, but my charge pipes are silicone so it's not convenient. Still, I thought it was the physical temp of the thing that caused this issue in the first place. I'd guess my sensor was bad, but it seems fine after a half mile or so.

Last edited by soloracer; 09-19-2011 at 05:53 PM. Reason: sp
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Old 09-19-2011, 04:47 AM   #2
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What gives? Any theories?
Sensor now gives the correct readings, but you now need to tune the car correctly to those.
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Old 09-19-2011, 01:54 PM   #3
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Hot restarts are the only problem area. Idles fine when cold or hot. Already ran VEAL to make adjustments for new location, but the hot restart issue just doesn't seem to make sense.
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Old 09-19-2011, 01:56 PM   #4
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so increase the MAT corrections at low rpms....
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Old 09-19-2011, 02:27 PM   #5
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Yeah, I will. I just thought it was odd. I expected it to be nearly a non issue; especially since the sensor itself is cool to the touch.
Is it that the whole system gets so hot while sitting that the air coming into the intercooler is really hot?
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Old 09-19-2011, 02:31 PM   #6
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the problem is more that there's not enough airflow over the AIT sensor, but the endtank itself will be warm to the touch, so the sensor reports a bit higher. but this should easily be able to be tuned out but being more aggresive with your ait-based air density table curve.
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Old 09-19-2011, 03:22 PM   #7
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Makes sense. I did blow air from a compressor into my IC when I had it out and noticed there wasn't a lot of flow over the end tank where the sensor is. Weird thing though is I drove the car hard yesterday for a couple hours and the cold side of the IC was still very cool in 70* ambient temps.

So the sensor apparently gets better airflow in the stock FM location, but heat soaks worse there. So really it's kind of a toss up in my case as to which is better. Maybe I'll try a short length of tube on the IC outlet and put it where you have yours. How does it fair there in your experience?
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Old 09-19-2011, 03:26 PM   #8
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it definitely better in this location now. just tune your enrichments.
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Old 09-19-2011, 03:28 PM   #9
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Originally Posted by Braineack View Post
the problem is more that there's not enough airflow over the AIT sensor, but the endtank itself will be warm to the touch, so the sensor reports a bit higher. but this should easily be able to be tuned out but being more aggresive with your ait-based air density table curve.
This is one of the things holding me back from going MS3. Well, that and $300....
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Old 09-19-2011, 03:29 PM   #10
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you could upgrade your msI for $190...
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Old 09-19-2011, 03:34 PM   #11
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Yeah, + $40 for the case, + MS3X for sequential fuel, which I REALLY want. I'll probably do it over the winter when the Miata hibernates. I really wish they would fix the code so you could end the MAT corrections at higher revs with MS3.

Edit:

Oh, and I would still need to make or bribe someone to make me a standalone harness, then I get to re-tune, again.....sounds painful!
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Old 09-19-2011, 03:41 PM   #12
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James has more or less refused my request for that. The best I could get was disabling AIt enrichments during ASE, but I honestly didn't use it...I didn't like the long timer for ASE i was required to use. I just canceled out the ideal gas law and moved on.
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Old 09-19-2011, 03:44 PM   #13
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Quote:
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This is one of the things holding me back from going MS3. Well, that and $300....
You can get one of my units, which has said issue fixed. There's a group buy going on right now, and all the units have several pretty awesome features as well.
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Old 09-19-2011, 03:49 PM   #14
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Aren't yours DIYPNP units? MSII. I was hoping to just upgrade what I have to MS3 in order to have sequential fuel and good AC control. Besides, I don't have the cash without first selling the MSI I have. I'm buying another guitar I don't need this week. ;-) Too many expensive hobbies....
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Old 09-19-2011, 03:50 PM   #15
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That seems just goofy that it couldn't be put in the code as an option. Are there risks to canceling the ideal gas law?
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Old 09-19-2011, 04:38 PM   #16
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Quote:
Originally Posted by soloracer View Post
Aren't yours DIYPNP units? MSII. I was hoping to just upgrade what I have to MS3 in order to have sequential fuel and good AC control. Besides, I don't have the cash without first selling the MSI I have. I'm buying another guitar I don't need this week. ;-) Too many expensive hobbies....
The units in the group buy offer sequential fueling AND better A/C control than the MS3.
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Old 09-19-2011, 05:03 PM   #17
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The units in the group buy offer sequential fueling AND better A/C control than the MS3.
Thanks. I looked it up. Sounds like a nice unit. Just don't have the cash right now. May go this way in the future though, depending on how / if MS3 shapes up.
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Old 09-19-2011, 05:35 PM   #18
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The air velocity at the ic pipe, or at the top of the end tank, is greater than the middle of the end tank, and lowest at the bottom of the end tank.

The lack of velocity in the middle of the end tank is why your "recovery period" is longer.

However, the fact that it reads hot when the tank is cool, makes no sense.
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Old 09-19-2011, 05:46 PM   #19
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I like others had lean restart issues, I've used the IAT in various locations, final spot being in FMIC endtank. I've also dialed out the ideal gas law corrections using the MAT correction table so it doesn't effect it.

I still get hot restarts and believe it to be more than just to do with the IAT and more to do with fuel on the intake walls and heat soak at the head\end of intake manifold runners, etc. because I've seen the same commanded fuel pw at these restarts as at normally running idle just fine.

I was just gonna throw more fuel at startup at it. This is on a ms2, pcb v3, ms2e 3.1.0 of course.
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Old 09-19-2011, 05:51 PM   #20
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Originally Posted by JasonC SBB View Post
The air velocity at the ic pipe, or at the top of the end tank, is greater than the middle of the end tank, and lowest at the bottom of the end tank.

The lack of velocity in the middle of the end tank is why your "recovery period" is longer.

However, the fact that it reads hot when the tank is cool, makes no sense.
Makes sense. Well, I have yet to check in TS / MLV if it actually reads hot. I plan to do that if I get another warm day to play. I had the car out and running VEAL and the damned netbook battery died before I got back. I'm only going on the hot start characteristics and uber-lean AFR reading on the LC-1.
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