6.5 Getting a good idle 6 TUNING THE MS3PRO
output, you have nowhere to go but down. Over-advanced timing will hurt power as well - beyond a certain point,
adding more timing will not bump up the idle, and taken to extremes, it can do the exact opposite. So you want to
"hold back" a little timing at your idle RPM and run less than the amount of timing that would give it the most power.
6.5.0.9 Correct Airflow You’ll need to get the right amount of air to maintain a good idle speed. How to do this
depends on what sort of IAC (idle air control) valve you have.
No IAC valve at all? Yes, you can run without an IAC valve. Just use your idle speed set screw to adjust it
open enough that the engine doesn’t stall on cold starts. Sure, your idle speed with the engine warmed up will be
higher than it could be, and you’ll have no way to adjust for stuff like the A/C or electrical loads, but it’ll work.
On/off IAC valves give you two choices: On fast idle, or off it. You’d set the idle speed set screw to your desired
idle speed with the engine warm, and if your on/off valve is big enough, turning it on will keep the engine from
stalling on a cold start. No closed loop control is possible. You can even add a second on/off IAC valve alongside
an existing IAC to kick up the idle when the A/C turns on. Nissan used the on/off A/C valve on many early ’90s
cars, although they paired it with a PWM IAC valve.
PWM IAC valves or stepper motor IAC valves are continuously variable. This lets you adjust the amount of
opening time as a function of temperature. There are two ways these can be run. Open loop mode (also called
warmup mode) simply tells the valve to open a set amount as a function of temperature. Closed loop mode tries
to maintain a target idle speed. We’ll cover closed loop idle tuning and several of the MS3Pro’s other idle control
features, such as increasing the idle speed when the A/C or cooling fan come on, under the Additional Items.
AMP EFI MS3Pro manual version 1.202, firmware 1.5.0, 4/21/2017 Page 123
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