How to Tune Your Engine’s Idle Speed

Now that the engine has been mapped under most stable operating conditions, it is possible to move on to a less stable condition: idle. Because of the slow engine speed, there is a relatively long time between possible corrections the PCM can make at each TDC event. The longer pause before the next feedback signal makes it easy to over- or under-correct at idle speeds. Following trends found under stable operating conditions at medium engine speeds and loads downward gives a better estimate of what the engine wants at idle speed. Remembering that idle speed is the result of the delicate balance between engine torque and engine drag from loads, small changes in engine torque can yield relatively large changes in idle speed.

 


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A couple layers of control are required for this delicate balance at the lowest acceptable speed without stalling. The largest controlling factor to idle speed is engine throttling. Throttling can come from either the throttle blade itself or the IAC motor. While very large changes can be made to engine torque by changing throttling, actual changes take a relatively long time to fill the manifold and allow the engine to react.

To prevent stalling in the mean- time, a faster correction to engine torque can be made by changing spark advance. This is why it is desirable to set base ignition timing lower than MBT at idle, leaving room to adjust timing when necessary to reduce speed fluctuations and prevent stalling. Idle spark advance values can be surprisingly small, especially in engines with more efficient combustion chamber designs. Many modern DOHC engines with stock cams idle with single digit spark advance.

If a larger than stock camshaft design simply does not allow stable idle at the stock speed, adding 100 rpm at a time can quickly bring it into stable operation. Once stable, reducing an additional 50 rpm may help quiet engine operation at idle. This is where carefully choosing load and speed set points in a speed density application or MAF scaling come in very handy. There is no reason to simply increase idle speed from 800 rpm to 1,000 rpm after a camshaft change when a little extra calibration time allows the engine to be stable at 850 rpm. Careful control of fuel delivery, airflow mapping, and spark compensation are the keys.

 

A datalog of engine behavior at idle shows relatively low calculated engine loads and airflows. Notice how spark advance moves rapidly to help maintain a steady engine speed.

A datalog of engine behavior at idle shows relatively low calculated engine loads and airflows. Notice how spark advance moves rapidly to help maintain a steady engine speed.

 

The MAF on this engine is installed on the airbox. The hook-shaped appendage on the inlet tube is a Helmholtz resonator, used to cancel out standing wave vibrations that would otherwise corrupt the MAF signal at low speeds as well as make an audible noise.

The MAF on this engine is installed on the airbox. The hook-shaped appendage on the inlet tube is a Helmholtz resonator, used to cancel out standing wave vibrations that would otherwise corrupt the MAF signal at low speeds as well as make an audible noise.

 

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This engine uses two Helmholtz resonators (arrows) to further dampen intake tract oscillations. Each chamber is tuned to a different frequency. The PCM is hiding beneath the right arrow.

This engine uses two Helmholtz resonators (arrows) to further dampen intake tract oscillations. Each chamber is tuned to a different frequency. The PCM is hiding beneath the right arrow.

 

Little by little, engine speed can be reduced to continue mapping air-flow from the stable areas mapped earlier to speeds approaching desired idle. As long as the fuel injector properties have been modeled correctly, it is possible to continue with airflow mapping for progressively slower speeds and lower loads. With each progressively lower engine speed, the airflow model can be adjusted as before. The same process of checking actual lambda against the target value applies. With proper airflow modeling, the engine is likely to exhibit loads at idle of 10 to 18%.

Changes in injector offset (dead time) show up most prominently at low speed where actual pulsewidths are smallest and dead time represents a larger percentage of total injection time. If the injector flow was not modeled accurately before starting, this is where the calibrator sees issues.

When starting from scratch on a throttle stop set point without ETC, allow the engine to idle with the blade held slightly open. Remove any IAC control (unplug the motor from the harness) and slowly close the blade position until the engine barely runs near the desired speed. Reinstalling the IAC harness lead should result in a more stable idle. This prevents the IAC from leaning to either a fully open or closed static position. The object is to get the IAC motor in the middle of its adjustment range so that it has maximum flexibility to adjust airflow either up or down. The TPS output should be checked at this time to ensure that the output value is within normal operating range and registers as “closed throttle” to the PCM. In some cases, it is not possible to open the throttle blade enough and remain within the range the PCM considers “closed throttle.” In these rare instances, a small bleed hole can be drilled in the throttle blade to allow slightly more air bypass when closed. Start small and increase in very small increments to avoid scrapping the throttle body. A final check should be made to verify that commanded idle speed in the PCM matches actual engine speed and that the IAC is near the middle of its range.

During idle speed calibration, it is helpful to monitor data in real time for MAF, MAP, lambda, injector pulsewidth, and temperatures. Verify that all signals are within normal operating range for the PCM. Some PCMs have a value for minimum allowed MAF that may need to be reduced when using a scaled meter. Actual MAF or MAP values give an indication of exactly which cells should be modified to complete airflow modeling at idle speeds. Pulsewidth is also useful to monitor since it can show any “hunting” activity in the PCM for target lambda that may be caused by background functions such as evaporative or EGR compensations. Some PCMs also have a lower limit value for pulsewidth that may need to be reduced when using larger fuel injectors.

After finding the stable idle speed at normal operating temperature, a temperature compensation curve can be built. To offset the poor combustion, higher fluid viscosities, and undesirable emissions at cold start, idle speed should be increased at low engine temperatures. If idle is stable at 800 rpm and 190 degrees F, this idle speed is usually fine down to about 160 degrees F. Expect to add up to 400 rpm at freezing to allow for smooth engine operation, with a smooth transition as temperature changes. A side benefit to increased engine speed at cold temperatures is faster warm-up times for components, coolant, and oil. As a measure of precaution, temperatures above 220 degrees F may also be aided by increasing idle speed 100 rpm or so to boost water pump flow during cool-down after extreme operation.

 

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Dashpot

After a stable idle has been found, the PCM needs a way to get to it smoothly. Just like a carburetor, most PCMs have a dashpot function built in that allows for a softer landing to the actual idle speed. Dashpot is usually shown as bypass airflow from the IAC at closed throttle versus engine speed. It is up to the calibrator to contour this function so the engine speed drops quickly once the throttle is closed at higher RPM, and slows its rate of change as idle is approached. Too fast and the engine drops right past idle speed and stalls; too slow and the engine hangs at higher RPM when lifting off the throttle. Changes in throttle body size, camshaft design, and intake manifold design and volume have noticeable effects on required dashpot.

Lower dashpot values at medium to high engine speeds can be used to add engine braking when the throttle is lifted. Care should be taken to make sure this does not interfere with normal cruise throttle position, which may be relatively low for larger throttle bodies.

 

The dashpot function controls the amount of airflow at closed throttle. At higher speeds more air is allowed through the engine to control pumping losses and drag.

The dashpot function controls the amount of airflow at closed throttle. At higher speeds more air is allowed through the engine to control pumping losses and drag.

 

The actual amount of dashpot airflow must be reduced over time for the engine to return to idle. This table slows the rate of reduction as speeds decrease to provide a gentle return to idle speed without stalling.

The actual amount of dashpot airflow must be reduced over time for the engine to return to idle. This table slows the rate of reduction as speeds decrease to provide a gentle return to idle speed without stalling.

 

When adding a supercharger to a naturally aspirated engine, it is important to recognize the source of air feeding the IAC mechanism. When the throttle is closed, the intake manifold is under vacuum and higher pressure is present in front of the throttle blade. This pressure differential is exaggerated in supercharged applications. It is desirable to have the IAC draw air from an atmospheric source rather than the pressurized section between the compressor and manifold. This reduces the force-feeding effect on the IAC, keeping its actual flow range low where it belongs, and increases the adjustment precision of the PCM. Dashpot values need to be greatly reduced when the IAC is fed from a pressurized source. The same area of opening in the IAC with higher-pressure differential across it yields greater flow in the exact same manner as increased fuel rail pressure increases injector flow. This also means that each “step” size of adjustment to the IAC position is larger, reducing resolution and control.

 

Written by Greg Banish and Posted with Permission of CarTechBooks

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