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Old 12-13-2008, 09:50 AM   #1
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Default idle gayness v.confusion

I have a 99 TB and IAC valve.

Apparently the stock valve was scoped at 24.5hz. When I put the valve there in MT, it wants to idle at 4k rpm. When I put it at 65hz, it is close to normal, but I still have oscillations which tells me that I need high-res code.

Where should I go from here on bringing down the idle?

I have about 3 hours to get this taken care of before I drop the car off at BEGi for tuning, so I have a lot of work to do, fast.
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Old 12-13-2008, 05:05 PM   #2
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Oscillations in the idle are often best addressed by adjusting the recovery speeds, limits, and deadband.
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Old 12-13-2008, 05:50 PM   #3
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Idle Control valves
As long as the MS ECU has been upgraded internally (see the main MSnS Extra Website) with a high powered transitor in place of the
standard idle transistor it can control most idle valves as long as they are NOT stepper motors (these have 4 wires/connectors) Most 2 or 3
wire valves (e.g. Bosch 0280 140 505)
Idle control can be used in B&G on/off, Warm up, or Closed-Loop Modes.
B&G on/off simply opens or closes a valve to provide additional air below a temperature threshold
Warm up mode linearly varies the duty cycle of a 2 or 3 wire solenoid actuated valve to provide additional air during cold startup, while
gradually scaling down the duty cycle until the engine is warm.
Closed-loop mode attempts to keep the engine idling at a set rpm by using PWM with a 2 wire solenoid actuated valve, and actively varying
the duty cycle to maintain rpm.
Warmup Settings:
Lower temp idle frequency is the higher duty cycle required to start and run a cold engine. (115 is a good starting point)
Upper temp idle frequency is the duty cycle the idle valve will be scaled down to as the engine warms up. ( try zero here)
These two values are linearly interpolated from one to the other starting from the lower engine temp to the higher engine temp (fast idle temp,
and slow idle temp.)
Idle valve frequency This is generally something most people will not need to mess with, however, different idle valves are designed to work
best at specific operational frequencies. Note that the Frequency of actuation is 1000 / this value (255 is a good starting point)
Please Note: true DUTY CYCLE% = Lower or Upper dc * 100 / Idle Valve Frequency value example Freq value - 200 = 50 hz operation, a
DC value of 80 = 80*100/200 = 40% true duty cycle
While most valves operate at 100 hertz, some older style 2-wire valves will work better at frequencies as low as 39 hertz. There is an inverse
relationship between frequency of operation and control precision because of the algorithm Used to control the PWM output. Lower
frequencies have more steps between dc values. 100hz operation has 100 steps and therefore has 1% control steps. 50hz operation has 200
steps and has 0.5% duty cycle steps and that is why the duty cycle values put into MT need to be double the true DC% value.
Some valves may emit a buzzing sound which can be eliminated by increasing the frequency above 100 hertz.
There is a formula for modifying the idle valve frequency. The formula is '10000/desired frequency = x where x is is the variable you input in
megatune. For example '100' would be 100 hertz, and '50' would be 200 hertz
Closed Loop Settings:
This is fairly experimental code, but with careful manipulation of variables it should be possible to produce a stable idle.
Cranking (dc) is the duty cycle required at cranking. (try Zero here)
Minimum (dc) is the duty cycle slightly lower than a warm idle dc when regulated.
Closed (dc) is the duty cycle where the valve closes.
The values here follow the rule above and are related to the frequency selected and are not true DC except when the idlefreq values is 100.
Some valves are normally closed (use 0 here) others require a certain DC to clsoe and will regulate above this value. Bosch 2 wire valves are
normally open, close at 25% and regulate at about 40%.
Fast Idle RPM is the idle target for the engine at, or below the fast idle temperature.
Slow Idle RPM is the idle target for the engine at, or above the slow idle temperature (fully warmed up).
The two RPM set points are linearly interpolated between the two temperatures to provide a smooth transition during warmup
TPS Threshold is the point above which the idle valve closes in ADC (as it is no longer needed).
Idle Activation (rpm) is 'how many rpm' above idle that the idle valve returns to operation after the throttle has been opened and closed again.
This follows the idle speed as it is interpolated from Fast Idle RPM to Slow Idle RPM
Dashpot Settle is the wait time required after a dashpot event for the rpms to settle below the idle activation point. After a dashpot it is
possible that rpms will overshoot the activation point and this is the time required to settle below activation point.
Dashpot Adder (dc) is the dashpot duty cycle added to the last controlled idle duty cycle which allows it to recover to a nice idle with
minimal overshoot. These values should be low: 1-5 DC
Deadband range This is a range of rpm that you do not want it to bother changing duty cycle to regulate idle.
30 to 50 rpm seems to be good here try smaller values unitl it seems stable.
Adaptive idle control This controls the time between idle control events. The goal is to have a Slow Recovery which is tuned to the engine's
time constant at idle. You do not want it to hunt nor have falling idle speed. The Fast Recovery is how fast you want it to recover to a decent
idle when the idle speed is higher or lower than the target this can be tuned for startup and to see what is needed to recover if idle speed drops
significantly below target for example when the A/C or electric fans kick on and the idle drops below target.
The rpm values determine the curve. The lower value should be close to the dead band value and the upper one should be a bit further away,
maybe 100 or 200 rpm the next 2 values are time constants needed for startup and idle valve closure
Closure speed controls the speed at which the idle valve closes. It is the time step delay between each as it steps to a closed dc value
Startup delay is a wait time right after the engine is started that the system stabilizes before it attempts to find an idle speed

Last edited by hustler; 12-13-2008 at 06:12 PM.
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