AFR fluctuations at cruise - Miata Turbo Forum - Boost cars, acquire cats.

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Old 03-18-2013, 01:18 AM   #1
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Default AFR fluctuations at cruise

I'm having some fluctuation of my AFR when cruising around. Data log looks like a jagged sine wave. It will fluctuate from about 14.0 to around 16.2. My injector duty cycle remains constant. The idle fluctuation is only +-.4. At wot the afr is very steady.

The fluctuation during cruise seems a little excessive. Or is this normal?

Particulars about the car
2000
Begi cast manifold
Begi separated gas downpipe
Stock exhaust
SR20 turbo running 6psi
AEM FIC
AEM Uego afr guage
RX8 injectors
no intercooler

Thanks for any info.
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Old 03-19-2013, 03:25 PM   #2
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That's pretty normal.
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Old 03-19-2013, 03:30 PM   #3
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Some fluctuation is normal, but that seems a bit excessive to me.


Or am i the only one that thinks that? The awful shitty "tuned" daily driver of mine fluctuates within 1 point, not over 2 like he's seeing.
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Old 03-19-2013, 03:33 PM   #4
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You May have a slight vacuum leak. I had this problem. I would have lean spike under constant light load. It is usually a very small leak from a hose that just is not very tight but not actually diconnected.
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Old 03-20-2013, 12:12 AM   #5
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thanks for the feed back guys. Is there anything else i should look at to get the AFR tighter?
Sensors? Coil pack?
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Old 06-05-2013, 12:36 PM   #6
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Have you looked at your fuel trims with an OBD-II scan tool? Part throttle cruise is still in Closed loop. This is the hardest time for the FIC mostly on the tune side. You need to have the fuel table correct to achieve the correct AFRs and you need to have the O2 modification table correct to keep the trims in check so the ECU does not learn out the fueling changes you made in the table. If you have either fuel or O2 mod table wrong you will have very inconsistent ARFs. Vacuum leaks will make a big difference as well.

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