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Old 10-21-2014, 07:09 PM   #1
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Default Engine misses at idle and at light load when hot, AFR seems unstable

Hello, the car is a 1994 1.8L with GT2560 turbo, MS3X ECU, 650cc FIC injectors. I just recently bought the car and I'm still getting my bearings with the current calibration. Here's the whole situation:

After the first autocross a few weeks ago, it developed a bad miss when hot. The spark plug wires looked old and the cylinder 4 wire was touching the valve cover. I installed new Toyota COP's in place of the old coil/plug wire setup. I adjusted the dwell to 3.5ms cranking and 2.5ms max and installed new spark plugs gapped at 0.045" at the same time. This seemed to fix the miss but it still idled poorly. I'm fairly certain the IAC valve is shot since it dies at idle as soon as I put the A/C on. While driving the A/C works fine. Watching the IAC valve duty cycle change seems to have no real effect on the idle speed as well.

Yesterday, after another autocross, it started to miss again and this time not just at idle. It wasn't nearly as bad as the previous time though (pre COP install). Under load I do not hear any miss, but the AFR fluctuates between roughly 11.8 and 12.8 at full load. The fluctuation has a period of about 20ms and does not vary with engine speed. I'm not sure how much fluctuation is normal for this engine.

I logged the signal from the CAS this morning as it was stumbling around at idle and did not see any missing pulses. Assuming the idle stumble has the same root cause as the miss during cruise, that would rule out the CAS, correct?

I'm starting to run out of ideas here and any help would be appreciated. My current tune is attached and I'll try to get some logs as well. (UPDATE: logs added to post #6)
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File Type: msq CurrentTune.msq (157.8 KB, 46 views)

Last edited by AlwaysReamin; 10-24-2014 at 11:13 AM.
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Old 10-23-2014, 07:37 AM   #2
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Where is your IAT sensor located?
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Old 10-23-2014, 08:25 AM   #3
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Originally Posted by sixshooter View Post
Where is your IAT sensor located?
MAT is located right at the outlet of the FMIC. That's the only air temp sensor I can find.
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Old 10-23-2014, 09:43 AM   #4
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That's a good place for it. I was worried you may have been encountering heat soak of the sensor but that shouldn't be the problem with your location.
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Old 10-23-2014, 02:43 PM   #5
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Does the injector dead-time of 1.2ms @ 13.2V seem right? I would like to measure it using the method described in the sticky, but I don't think my idle is stable enough to get accurate data.
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Old 10-23-2014, 07:29 PM   #6
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Now with datalogs.

The missing while driving one doesn't get interesting until the last bit of acceleration.
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File Type: msl bad idle 2.msl (32.4 KB, 45 views)
File Type: msl miss while driving.msl (588.5 KB, 41 views)
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Old 10-24-2014, 10:43 AM   #7
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I noticed in the "bad idle 2" log that MAP increases randomly, causing it to go lean, miss, and stumble around. That seems to me like a clear indication of the IAC valve failure. It must be opening erratically.

Could the miss at cruise be related as well? I don't see the same MAP fluctuation in that log.
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