How to: Tune for a smooth idle
"How do I get a smooth idle like
my stock ECU had?" It's a common enough question to warrent a
small article with tips for how to get the idle in tune. The
first key may surprise many novice tuners:
The idle control settings are not
the most important thing to adjust to get a smooth idle.
This may seem counter-intuitive,
but consider this: If the engine is idling at constant load with
the throttle held at a constant opening, shouldn't you have a
constant idle speed? If your idle surges or hunts under these
conditions, and your idle control valve is staying at one
opening value, the idle control settings are not the problem. To
get a steady idle, you need three things in place:
correct fuel, correct timing, and correct
airflow. These problems should be addressed
individually.
Correct Fuel
Engines don't like to idle lean; a
lean air/fuel ratio is one of the biggest causes of a surging
idle. On an engine with sequential injection, this is simple
enough - get the engine idling at 14.7:1 for gasoline, and
you're good to go. Batch fire engines, however, have issues at
idle with intake pulse reversion sending some of the fuel into
an adjacent cylinder. So, if your engine is batch fire, you'll
need to add a little more fuel so all the cylinders can stay at
14.7:1 or richer. Batch fire engines typically idle best in the
mid 13's, so if you're leaner than that, put in some more fuel
and see if that stabilizes the idle.
You'll need to make sure the
air/fuel ratio is maintained during cold start. If your idle
hunts when the engine is cold, but stabilizes when it warms up,
try more warm up enrichment.
Correct Timing
Timing is a very useful tool for
controlling your idle speed. Ideally, you'll want the timing to
be set to slightly below the timing that would give it the most
power - on most piston engines, this typically works out to
somewhere in the 10 to 18 degree range. Then you can add a row around
300 RPM below your target idle speed if you find the idle to be
unstable. The timing in this row can be bumped up by
around 2 to 4 degees over the idle timing. The 3d view below
shows a spark table set up this way.
Setting up your timing like this
has a stabilizing effect on the idle. When the idle speed drops,
the timing advances, increasing power and moving the idle speed
back up. The result is an automatic feedback loop that helps
maintain a constant idle speed. Not every engine really needs
this. However, you need to look out for timing curves that do
the opposite. If you've specified less timing below the target
idle speed than you have at the idle speed, this often makes for
an unstable idle as the ECU pulls timing while the idle speed
falls. This can make a slight disturbance in idle RPM amplify
itself into an idle speed that cycles up and down rapidly.
Too much timing can be trouble. If
the timing at your regular idle speed already puts you at the
most power 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.
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. If you don't feel much like adjusting
closed loop idle, 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 this on many early '90s cars.
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. MS1 has a two point curve, while MS2 and later lets
you set a more detailed curve of IAC valve versus temperature.
Closed loop mode tries to maintain a target
idle speed. While not every car needs closed loop idle, it can
help compensate for major changes in load. For example, if the
idle drops an unacceptable amount when you turn the air
conditioning on, you can use the closed loop control to
compensate. MS1/Extra has a fairly crude algorithm, while
MS2/Extra and MS3 use a more sophisticated (and easier to tune)
method called a PID loop.
The MS/Extra code variants provide a
closed loop feature that allows targeting a specific idle speed.
I recommend leaving the closed loop control
off until you have the engine
idling well at a steady load. Both badly tuned closed loop
idle and badly tuned fuel or spark settings can cause an
oscillating idle. Leaving the idle in open loop (warmup)
mode will let you rule out one cause of an oscillating idle and
let you get just the fuel and ignition side straightened out.
As a rule of thumb, trying to use closed loop settings to
compensate for poor fuel or timing settings will have the ECU
chasing its tail, but never really catch it. You may find
the engine runs perfectly well in open loop and you don't need
closed loop, particularly on a race car with no air conditioning
and few electrical loads.
The closed loop settings are best
used to compensate for load changes, such as turning the A/C on.
If your car is equipped with working A/C, a good way to judge
the effectiveness of your closed loop settings is to click the
A/C on and see how quickly it stops the idle speed from
dropping.
The following links provide
detailed information about tuning closed loop idle.
MS1/Extra closed loop idle settings
MS2/Extra and MS3 closed loop idle settings
Note that there have been
improvements to the idle code as new firmware releases come out.
As a general rule, the newer the firmware, the better the closed
loop idle control. If you're running an older firmware and
having trouble making closed loop idle work for you, check if
there's a newer release, or even a new alpha code being tested
with improvements to the closed loop idle.
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