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Old 12-22-2006, 08:17 PM   #1
problemcat13
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Default question regarding stock ECU

I'm posting in EM because my question is regarding EM--stock EM! The only mod I have is snorkus delete w/ 3" pipe down the fender well and a K&N drop in. No big deal.

99 Forester L 5-speed.

With the temps up and down so much, I've noticed that when the temps cool down for several days I will have a stumble. At about 4100 RPMs the acceleration abruptly fails to pull like it did down low, then picks back up around 5000 RPMs. Not a miss, no audible sound, but just a feeling like I suddenly hit a puddle of water that slowed me down. Between those RPMs every time.

So, is this related to stock EM or cams, or a combination? The main thing I'm wondering is if I'm leaning out too much (since air is colder) until the ECU learns for the cold air? And it's not like I can do much about that anyhow, right?

So, what are the thoughts from the people who actually know how this technical stuff works? Thanks!
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Old 12-23-2006, 04:45 PM   #2
hondaeater69
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the ecu will not 'lean out' when the ambient air is colder. all else equal, colder air needs less fuel, and being your ECU uses closed loop look up tables, your AFR if anything is richer than it needs to be.

but colder air will not cause the stumble, look elsewhere.

i would start with normal maintence items, plugs, air/fuel filters, clean the MAF etc.
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Old 12-23-2006, 05:06 PM   #3
LateApex31
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Quote:
Originally Posted by hondaeater69 View Post
all else equal, colder air needs less fuel

It does?
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Old 12-24-2006, 12:03 AM   #4
problemcat13
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Yeah, I would think colder air needs more fuel (it has more oxygen). BTW, plugs and wires are only 20k miles old, filter looks fine, but I have not cleaned or examined the MAF.

After doing this for several days while the temperatures have dropped into the 50s or so, it will eventually even out. But lately the temps have been up and down and so it's doing this a lot. It's driving me batty.
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Old 12-24-2006, 12:34 AM   #5
bofh
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It could just be condensation. With the temp going nuts water can get into odd places.
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Old 12-24-2006, 12:35 AM   #6
hondaeater69
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OK, your ECU does have an intake air temp sensor, however it's function is not for when air gets colder, but hotter. In other words, when IAT>* (i think it's around 115F) then the ECU will pull timing, and or add fuel IIRC.

the hotter the air, the more fuel you need. fueling in our cars past 12.5 is used to cool the charge, nothing more. that's why NA vehicles get away with running much leaner AFR's than turbo charged cars.

the colder the air, the leaner you can run it, but the ECU will not lean out your AFR for you just because the intake air is colder...
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Old 12-24-2006, 12:49 AM   #7
lancelucas
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Quote:
Originally Posted by hondaeater69 View Post
OK, your ECU does have an intake air temp sensor, however it's function is not for when air gets colder, but hotter. In other words, when IAT>* (i think it's around 115F) then the ECU will pull timing, and or add fuel IIRC.

the hotter the air, the more fuel you need. fueling in our cars past 12.5 is used to cool the charge, nothing more. that's why NA vehicles get away with running much leaner AFR's than turbo charged cars.

the colder the air, the leaner you can run it, but the ECU will not lean out your AFR for you just because the intake air is colder...
I think you are referring in air temp in different respects.

The first is with regards to det/knock. A hotter charge (warm air) is more prone to det/knock, so generally, more fuel is added (or timing removed), which helps cool the charge and help prevent det/knock. The cooler the charge, the less prone to bad stuff, so less fuel needed for preventing it.

The second is with regards to AFR. Cooler air is denser, thus providing more oxygen...so more fuel needed to achieve same AFR than when warmer.

Feel free to flame on if I'm all wet.

Last edited by lancelucas; 12-24-2006 at 04:25 AM.
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Old 12-24-2006, 01:11 AM   #8
hondaeater69
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Quote:
Originally Posted by lancelucas View Post
A hotter charge (warm air) is less prone to det/knock, so generally, more fuel is added (or timing removed),

the hotter the charge, the more prone to det...
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Old 12-24-2006, 04:25 AM   #9
lancelucas
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Quote:
Originally Posted by hondaeater69 View Post
the hotter the charge, the more prone to det...
Yeah sorry, I got ahead of myself. Edited
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