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overrun fuel cut is driving me nuts

Old 07-09-2010, 04:21 PM
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Try taking some % away on your decel enrichment and lean out the lower cells in your table a good bit. I bet you'll stop getting backfires. It'll at least get rid of that huge AFR drop. I had this happen when I was tuning, then I realized you can't keep a high AFR during decel unless maybe if you have a huge spark gap and strong coils. I think there's just not enough oxygen to burn the fuel. High vac has been the hardest part for me to tune and I still haven't gotten it perfect. You also may need to play with spark timing so it's under enough compression to ignite.

Also, as an FYI, if the fuel isn't burned, you will have oxygen in your exhaust, which looks like a high AFR to the WB sensor. It detect the amount of oxygen still in the gases. Let autotune do its work and just slowly lift off the gas from high RPM until your foot is completely off the gas and see what it figures out. That way it'll tune the cells above and the WB won't all the sudden start thinking it's lean when fuel stops being burned.
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Old 07-09-2010, 04:25 PM
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I never backfire or have poppy exhaust between shifts...



Actually, notice at sec: 1044, the AFRs start to richen up during decel...looks like overrun still hasn't come on and you're getting to a better spot on your VE table to get back to stoich, but the map was too lean prior. Overun never kicks on there, you have it set to come on below 20 kPa and after 2 sec, and RPM greater than 1750, but you don't even pass those triggers.

I have mine set to above 1700rpm and active below 29.5kPa (I idle at 33kPa, so anything below is decel) and only under 0.8% TPS, only returning fuel once it's around 1100rpm. I bet your pops will go away if you actually get overrun to activate. in fact, now that I've seen MS3 has the return fuel at X rpm, might be why in this heat i've stalled onve or twice, looking at logs, it's returning fuel a little too late and not getting fuel back in time to stabilze idle.
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Old 07-09-2010, 05:21 PM
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lots of good info, thx guys.
Overrun was turned off in that log to make sure it wasn't related to it. Forgot to mention that, sorry. There's actually less backfire with overrun turned of.
I agree the logs look as if I need more fuel in my 20 kPa row, but I cannot grasp that I would need more fuel in my 20kPa than in my 29kPa row. I noticed the weird rich / lean thing as well. Each time I shift, it goes very rich for a split second than very lean. I was thinking of lowering decel fuel and addding fuel in the 1400 to 2800rpm cells.
TrickerZ's explanation makes sense as well though. It would explain the rich / lean thing.
I'll play with it some more using more extreme numbers (add / remove 10 points or so) and do some more logging.
I haven't turned on EAE because I figure I need to solve this first.

Last edited by WestfieldMX5; 07-09-2010 at 05:46 PM.
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Old 07-09-2010, 06:20 PM
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it'll always go a little rich when you lift from residual fuel and what not vs. closed throttle.


you may want to turn EAE on and see if tuning the sucked-from-walls correction table helps in this situation. This curve assumes that some fuel gets stuck to the cylinder walls when you lift so you can control how fast the ECU pulls fuel when you lift, and you can control it vs rpm and clt temp as well.
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