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Old 01-31-2008, 02:20 PM   #1
wittmer25
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Join Date: Dec 2005
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Location: Waco, TX
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2004 STi gt30r
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Default getting misfire cel's on all 4 cyl

I need some help or ideas.

I just installed a built ej257 w/ 100mm pistons with a compression ratio close to 9.0:1 & cosworth cams & mildly ported heads. My car seems to be running well except that I get a lot of misfire cel's especially at low rpms. I haven't run the car hard at all because I am still breaking it in (<4500 rpms and boost below 5 psi). Mostly mild city driving. I have low ltft & stft correction percentages to by the way. My car seems idle well at 1000 rpms when its warm. I changed the idle maps so that it idles at higher rpms at each coolant temp range.

I haven't changed my timing maps yet and I am thinking this may be the problem. I was hoping that the increased compression ratio would offset loss of volumetric efficency due to the high lift cams at low rpm so that my timing maps might still be close enough in that rpm range. I am guessing the knock sensor is what detects the misfires like it does detonation. How does it know which one is occuring? Or are they considered the same? Should the base timing be reduced say across the board when compression ratio increases? I realize since I have the cams that the volumetric efficiency should slightly drop at low to mid rpm and rise from mid to high rpm. Based off volumetric effeciency, timing should be changed accordingly, right?
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Old 01-31-2008, 04:15 PM   #2
Jon [in CT]
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The ECU watches crank rotation speed very closely and if it decides the speed is too slow after a cylinder fires, it concludes that incomplete combustion has occurred and declares a misfire. The knock sensor is uninvolved with misfire detection.

Maybe you should try adjusting your AVCS intake timing.
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Old 01-31-2008, 04:24 PM   #3
wittmer25
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2004 STi gt30r
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I also forgot to mention that I have a act streetlite flywheel and ATI dampener.... I guess these could also have some effect then on misfires. Can you give me an example of which direction to adjust the AVCS? More advance at low rpm, low load or less advance at low rpm, low load? Maybe a trend would be helpful. Before I installed my engine, I had my AVCS tuned to give me max advance at low rpm high load...tapering to zero AVCS advance at around 5500 rpm. I left the AVCS map stock below approximately 1.0 g/rev. This seemed to help spool my gt30r nicely. Like I said earlier, the engine "feels" smooth. It doesn't seem to hunt at all. But I logged cylinder roughness for each cylinder... it counted many misfires in each cylidner in a matter of minutes.
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