Microsquirt tuning and surging idle problems

I'm running a Ford 5.0L with a microsquirt computer in a Miata. Basically a 1991 Mustang with Miata skin. The engine has been over bored 0.04" and has a Crane cam, which one I'm not sure. I purchased the motor from some else and the info is not available. It has Trickflow heads and Intake with a 75mm TB, 24 psi injectors, 14point7 wideband O2 sensor. When I first started the motor it had open headers and no O2 sensor and it ran fine. I didn't capture any data or make any changes to the tune that came in the computer. I had a custom duel exhaust installed which has high flow through catalytic converters and mufflers. After the exhaust installation I can't get it to idle without surging. I have gone through the timing setup, TPS and sensor cals and adjusted the idle set screw that will allow the engine, although surging, to warm up and the surging will stop and idle at around 1100 RPMs. I've watched all of the A91what videos and I tried to collect data for the IAC test but when I change the IAC above 30% it starts surging and I can't collect any good data. I tested the IAC and it appears to work fine. I'm attaching my setup and data I collected while it was warming up. Midway through the warmup I changed the timing to fixed at 20 deg. Please Help.


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No I don't know what the cam is. I just know that it is a Crane cam. The guy I bought the motor from is in the military and I lost contact with him over a year ago. It's a hydraulic roller. My best guess is the PowerMax Hydraulic Roller 214/220.
I loaded your @a91what file and the surging stopped if I start it up with closed loop idle. I had a little trouble collecting the data files and found that I couldn't change the IAC duty cycle unless I start it in open loop idle. I couldn't start it in close loop and then change it to open loop with out turning it off and restarting it. It still surges some in open loop but I was able to collect data. Thanks for your help as I was getting very frustrated and didn't think I was ever going to get to drive it.


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  • 2020-05-18_14.30.37_Warmup.mlg
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Mod Dude
Apr 6, 2011
Hillsborough county
well yeah, the changes affect how the ecu deals with the output requiring a power cycle. if you need to manually test the idle valve there is the test modes all the way on the right.

I dont understand the 3 datalogs, did you make changes during these logs?
I didn't make any changes. The warmup data was just data collected during warmup with the file you gave me. I had to start it 3 times before it ran. The IAC data was me trying to collect PWM duty cycle data with the advanced set at 16 degrees and PWM duty from 20% through 100%. When I set it at 40% I heard the fans come on, I don't have manual control of the fans, so after I collected data for 100% I started over at 20% and went through 60%. I probably should have just started over. The timing data was with the PWM duty cycle set at 40% and varied the advanced from 8 - 28 degrees. After looking at the data I think I should collect the PWM duty with the timing set to around 20 degrees and the timing data with the PWM duty set at 60%.