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AEM Infinity idle motor control -GM drive by cable

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Good morning, hoping to get in touch with someone that has LS/Infinity experience

We have a super charged LSX car that is having IAC issues (drive by cable throttle body).

Once running we can get the car to idle great and everything is fine, every key cycle it seems to want different base idle settings. Car has a truck TB with stock 4pin gm iac, checked wiring, new iac etc. Seems like the iac isn't finding home, it sweeps when key is turned on but not when key is turned off. It also only seems to sweep the amount out and in that the ecu is commanding under start conditions (ie lower number less movement at key cycle). Is the infinity only trying to find home when key is powered on? We have adjusted max step etc but no luck.

Thanks for your time. Attached is the tune file.

Attached Files

The Infinity ECU begins parking the stepper valve about 1 second after the ignition key turns off. For that park process to work, the ECU needs to be controlling the main EFI relay and that main EFI relay needs to be sending power to the ECU when it's active. The parking process takes about 3-6 seconds depending on your stepper valve and your ECU settings, and if you interrupt the parking process by turning the key back on that can prevent the valve from parking correctly. If the max number of steps was not configured correctly for your vehicle in the first place, that can cause all sorts of idle problems, especially when cold because that is when the idle valve is most likely to be travelling its full range.

From what you're describing, my suggestion is check that the ECU is wired to control the main EFI relay. For your Inf-708 ECU, Pin C1-29 sends ground to trigger the relay, and the ECU should receive power at pin C1-64 when the relay is active.

Hope that helps,


That's exactly it, our IACV is not parking at key off, the ECU IS staying powered for a few seconds after ignition is keyed off.

Key on the IACV seems to sweep the amount commanded under our start up conditions but there is absolutely no moment when key turned off.

The ECU will keep itself powered after key-off using the battery perm power, pin C1-10. Use a multimeter to back-probe voltage at pin C1-64 or C1-61, those are the pins that power the stepper idle chip on the circuit board. If those pins immediately go to 0V when the key turns off, the idle stepper chip can't move the IAC motor. I'm not aware of anything else that would prevent the valve from parking when the key turns off, aside from the IdlePosition already being at zero (the fully closed stop) before the key turned off.

Hope that helps,


I am seeing the same issue, with an Evo application, Infinity 506. The stepper is not parking at Key off. I am using the AEM plug and play harness. Fourfunction, I see you have not been active here in a while. Did you get your issue resolved?

I see where it is possible to extend the time before the ecu cuts power to the main relay. It is currently at 3 secs. I will try extending that time to see if I get better results.

I resolved my issue. It turns out that someone in the past wired the ecu relay to the ignition switch and disconnected the ecu control of the relay. My issue was what Scott Borg, above, suggested.

Just wanted to post up that I was having a similar issue with my 575sci marine engine running off Infinity 308, and was having high idle clear 'vac leak' sound at idle and traced this back to the ECU not being wired with perm bat going to 12v. An easy way to check wiring is to pull the IACV out but leave it plugged into the harness and you see it fluctuate on key on which seemed correct but watch after you key off. If wired properly you will see the valve cycle 1-2seconds after key is off. Anyways, thanks again for post, helped me figure out my issue.


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