Honda idle problems seem to be all over all the Honda chat forms so I thought I would clear some things up about how Honda controls its idle. Lets just start out by naming all the parts involved and what thy do.
IAC (idle air control valve)
This valve is a 2 or 3 wire electronically controlled solenoid valve. It can be easy identified by the 2/3 wire going into it and 2 coolant hoses. The coolant hoses are their to keep the valve from freezing. If you have ever let the air out of a tire and watched the valve stem freeze you know why the iac must be kept hot (all has to do with pressure changes because you are going from atmospheric to a vacuum). This valve is controlled by the ecu by a digital signals/analog signal which is basically turning the valve on and off very fast we are talking milliseconds hear. This is measured in duty cycle.
This would be represented as a 0% duty cycle. See how there is no voltage applied to the iac valve.
This would be represented as somewhere around 10-30% duty cycle. See the 12volts applied every couple of milliseconds.
This would be 70-90% duty cycle. See how the iac valve has more on time then off time.
This would be 100% duty cycle. See how the iac is supplied a steady 12 volts from the computer.
The deferents between a digital signal and an analog signal is the analog is not square its rounded looks like when you where a kid and drew shitty mountains you know what I am talking about.
The 2 wire valves are designed with a return spring so at one end you have a spring and the other an electronic solenoid thy fight each other giving you a very precise control of the valve by the ecu via duty cycle (just for note fuel injectors,egr valves,purge control solenoids all work this way too). You can have an opening any where from 0-100%.
The 3 wire valve are designed differently thy rotate instead of pulsate back and forth. Thy are controlled by a small electronic motor. But still controlled by duty cycle To give you a very precise opening controlled by the ecu.
All an IAC valve dose is lets air past the throttle body the more air the higher the idle the less air the lower the idle.
FIT (fast idle thermo valve)
This valve is not electronically controlled. It is easily identified by the 2 cooling hoses going into it it can be under your throttle body or on your intake manifold. This valve is all mechanical. This valve is basically used for cold start up. When you start your car cold it is a good Idle to have a higher idle then normal so your engine will not stall. The oil is still sludge and the gas is not atomizing very well do to cold pistons, valves, cylinder head, intake manifold. It is actually activated by the coolant running threw it if its cold its open if its hot its closed. Same thing hear it lets more air around the throttle. More air higher idle less air lower idle.
Air bleed screw
This is a little screw usually on the top of your throttle body its a flat head. It may be covered with hondabond and for good reason (so it will not spin out).
This little screw controls what is called your base idle. The more you screw it out the more air it lets past the throttle. The more you screw it in the less air it lets past the throttle. Same thing again more air higher idle less air lower idle. The fit,iac and the air bleed screw get their air from the 2 little holes in the throttle body. The top one usually feed the air bleed and the iac the bottom one usually feed the fit. This will come in handy later.
TPS (throttle positing sensor)
This seasor is on the side of your throttle body 3 wires. It is used to tell the ecu how open or closed the throttle is. Lets take a look at how it work in a little bit more detail so I don't just leave you hanging. It has 3 wires a 5volt reverence a signal and a ground. Some more of my priceless paint picks.
The more you move your throttle the more it moves the little blue arrow. This is pressed against a resistor. So if the little blue arrow is at the far left it is sending a signal of about 4.5 volts to the ecu. If the little blue arrow is to the far right it is sending a signal of about .5 volts to the ecu. The resistor is eating up voltage. The ecu is just looking at this voltage. Simple. On a honda or acura if your throttle is closed the signal should be .5 volts or less if you are a wide open throttle the voltage is usually around 4.5 volts.
Engine speed sensors
These sensors are just engine speed sensors. Thy may be located in your distributor or on the crank shaft timing belt pulley. The ecu must know where the engine rpm is to get it all rite. Idle is usually anywhere from 600 to 900 depending on the car. If the ecu dose not know where the rpm is at how is it going to open or close the iac to keep a clean idle. There are 3 engine speed sensors the ckp crank shaft positing, the tdc top dead center and the cyp the cylinder position sensor. Thy all work of magnetic induction if you want to learn more
http://en.wikipedia.org/wiki/Electromagnetic_induction . Basically if you pass a magnet around a coper wire it induces voltage. The faster you pass the magnet the faster the analog wave form. Remember the shitty mountains you drew as a kid. Why instead of the ecu outputting them the rpm sensors are making them and outputting them to the ecu. You can see the sensor wheels hear. This is the shaft spinning inside your distributor. The middle wheel is the crank sensor its the main on transmitting engine rpm. The top on with only one blade is the cylinder position sensor every time #1 flies by is out puts a signal and the bottom one is the tdc sensor every time you are at tdc in any cylinder it outputs a signal.
Note all wheel speed sensors,transmission shaft sensors, power sliding door sensor any sensor sensing rotation work like this. Ok if you just read all this yes I did dabble into sensor logic a bit but I am staying on course with the idle set up.
How it all ties together.
First off the ecu must see .5 volts or less from the tps to know it need to control the engine rpm. Usually you are in control of the engine rpm with you right foot. It watches the engine rpm and adjusts it as it sees fit threw the iac valve. Lets say the ecu want to idle an engine at 800 it opens or closes the iac till the engine idles at 800 this is called idle learn the ecu need to learn the iac positing it likes best and save the info for next time it need to idle the engine. If you disconnect the battery or ecu you will need to do another idle learn if you tweak the air bleed screw, put in a new fit or iac you need to reset the ecu and do a new idle learn. If it can not get the engine to idle at 800 with the iac valve fully closed it gets mad. If the idle gets above 1200 the ecu knows it is out of control of the idle it knows the iac is fully closed because it is not supplying voltage to the iac it begins cutting fuel and spark to keep the idle under 1200 (notorious idle surge problem). Didn't think your ecu was so smart did you?
MY HONDA
Linear Mode
