7.4 Startup / Idle 7 ADDITIONAL ITEMS: BEYOND BASIC FUEL AND IGNITION CONTROL
• PID ramp to target time - Once PID engages, the MS3Pro will gradually move the target value from the
current RPM to the target speed.This can be used to help larger P-values be used, making it easier to tune
PID to catch sudden drops in idle speed.
• PID Control Interval - This controls how often the PID control code runs. Setting this too high will make the
code respond too slowly to sudden changes in load, possibly allowing the engine to stall. Setting it too low
will make the engine unstable and likely to have surge issues.
7.4.7.1 Tuning closed loop idle Before trying to tune closed loop idle speed control, be sure to try tuning
warmup only idle speed control. With warmup only control, a higher step-count or duty should yield higher RPM.
Make sure that this is the case, and that smooth idle speed can be attained with warmup only before moving on to
closed loop control.
There are two main things to tune when tuning closed-loop idle speed control:
• PID gains
• Conditions for entering PID control
We recommend tuning these in stages. For example, PID cannot be tuned if the code is never entering the PID
loop. Because of this, it is a good idea to start by tuning the conditions for entering PID control, then come back
and tune the PID gains.
This includes both closed loop settings and engine state settings:
• VSS threshold - Using VSS in engine states will make it so that the PID loop activates after the vehicle speed
drops below the VSS threshold. This can prevent closed loop idle from activating while decelerating if using
an automatic transmission that freewheels on deceleration or if coasting with your foot on the clutch.
• Throttle closed TPS threshold - Set as low as possible. If the TPS has a bit of noise, set it to around 1%;
otherwise set it to 0.3%-0.5%.
• PID delay - This should be set so that the RPM dropping on throttle lift can come to a rest slightly higher than
the target RPM, and become stable there. Between three and five seconds normally works the best.
• Crank to Run Taper - This setting controls how long after starting the code will delay before entering PID.
Between three and five seconds works well for this setting.
• Slow deceleration threshold - This are the first of the PID lockout detection settings. Use this setting so that
the code can tell the difference between decelerating with closed throttle (engine braking) and sitting at one
RPM. This should be lower than the rate of deceleration in top gear.
• Overrun MAP threshold - This is the second PID lockout detection setting. The code assumes that if MAP
is lower than this setting, the driver must be decelerating, and not "locked out" of the PID loop. Set this to a
value lower than the MAP reading at idle.
• Slow acceleration threshold - A good value for this setting will typically be in the 200-400 RPM/sec range.
If the engine speed suddenly accelerates with no throttle input (like if the clutch is engaged while the car
is rolling and in gear), it must accelerate at a rate greater than this setting before the PID code will be
disengaged. Setting this value too high can lead to stalls after engaging the clutch in this manner.
To tell whether the code is entering PID idle control, use the "CL Idle" indicator in TunerStudio. If the current gauge
cluster in TunerStudio does not include this indicator, temporarily switch to a cluster that does.
Most modern OEM cars enter idle speed regulation in a very similar manner. The MS3 idle speed control
algorithm was emulates this behavior. The sequence of events that the code was designed to follow are listed
below:
1. Throttle Lift - On throttle lift, the code opens the valve to the value learned in the last iteration of the PID loop
(or the initial value table) + the dashpot adder. The logic here is that the last learned value should result in an
RPM close to the target RPM. The dashpot adder is added so that when RPM settles, it settles to an RPM
slightly higher than the target. This is in case the air conditioning was turned on or IAT increased or anything
else that might make RPM lower than the last time the PID code ran.
AMP EFI MS3Pro manual version 1.202, firmware 1.5.0, 4/21/2017 Page 186
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