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Honda IACV explained
This thread will deluge you with more info than you've ever wanted to know about the IACV in your Honda. This information was copied out of the Honda OBD training manual as I didn't feel like paraphrasing it all.
23.1 General Overview Idle control, along with fuel control are two of the most important sub-systems of the PGMFI fuel injection system. These two systems have a large impact on the driveability of a car. If the idle control system is not operating correctly, many driveability symptoms can occur, some of which are:
The main output device that controls engine idle is the idle air control (IAC) valve. This valve was added to all models in 1988 and is controlled by at least the following inputs:
The Honda PGMFI system is a speed/density type fuel injection system. It does not measure actual mass airflow, but calculates it from the engine RPM and the manifold absolute pressure (MAP) sensor input. On speed/density fuel injection systems, idle is controlled by simply controlling the air that bypasses the throttle plate. The idle control system is one of the PGMFI sub systems that has seen a lot of change since the first 1985 models. The early idle control systems were primitive by today's standards. The curb idle was set by an air by-pass screw in the throttle body. This screw controlled how much air bypassed the throttle blade. Except for the fast idle controls the only other feature was a vacuum diaphragm that opened the throttle to offset the air conditioning compressor load. The idle control systems of these earlier models had the characteristics of a carburetor. The engine control module (ECM) had no control over the idle other than the air conditioning vacuum diaphragm. To help make the idle more stable, the ECM would typically widen the injector pulse width (PW) slightly, and increase timing, when any type of load was sensed. 23.3.1 Idle Speed Control The idle control systems on the earliest PGMFI systems were very basic. Other than a fast idle system and an air conditioning idle boost, the idle was totally controlled by how much air bypassed the throttle plate. This bypass air was controlled by the idle air bypass screw. The idle air bypass screw is a large brass screw with a flat slot and is located on the throttle body. The idle air bypass screw in Image 23-2 is shown inside the circle. http://thelic.org/users/tomo/IACV/Image%2023-2.png To adjust the idle follow the instructions from the underhood label or the service manual. The technique used to set idle for models with an IAC valve is different than the earlier models. In 1988 Honda added an IAC valve to the PGMFI system. By adding the IAC valve the ECM now had full control over the idle. The IAC valve is an electrically activated valve that controls the amount of air that bypasses the throttle blade. Now the ECM could increase the idle to offset the load from any event that occurred. An IAC valve is shown at the left most arrow, in Image 23-1. http://thelic.org/users/tomo/IACV/Image%2023-1.png 23.3.5 Load Events The Honda engines run so lean at idle that virtually any event will cause the idle to "bobble". When a load event occurs, more than just the IAC valve makes a correction. Small changes are also made to the PW and the ignition timing. http://thelic.org/users/tomo/IACV/Sc...ure%2023-1.png Screen Capture 23-1 shows the effect of an electrical load on the PW and ignition timing. This technique was used prior to the IAC valve in an attempt to help stabilize the idle when loads occurred. Even with the addition of the IAC valve, this strategy continued. When a load occurs on a late model car, the IAC valve opens, the PW widens, and the timing is increased. The ECM monitors the idle and will compensate for any load that attempts to pull the idle down. The disadvantage with this approach is that the idle must start dropping before the ECM can make a correction. This would result in the idle "bobbling" on every load event. To help prevent this, the ECM monitors most components that could have a significant effect on the base idle. It can then begin making a correction before the idle has a chance to drop. The PCM is programmed to apply a specific package of adjustments to the IAC valve, PW, and ignition timing for each load event signal. Some of the inputs that are used by the ECM to anticipate a load are:
23.4 Idle Air Control (IAC) Valve Overview Honda used two different types of IAC valves. The most popular is the 2-wire unit. The two wire IAC valve uses current through a winding to open a plunger against a spring. On some of the later model Hondas, a 3-wire IAC valve is used. The 3-wire is a rotary type valve. The following information applies to the 2-wire IAC valve. The 3-wire IAC valve is covered at the end to this section. The models with IAC valves still have an idle air bypass screw that allows the base idle to be set. The base idle speed is set by unplugging the IAC valve and adjusting the idle air bypass screw. Unplugging the IAC valve will set a diagnostic trouble code (DTC). It is best to clear the DTC with scan tool so that adaptive learning will not be lost. The base idle speed is important. If the base idle is set too high, the IAC valve will not be able to bring down the idle speed by reducing the bypass air. If the base idle is set too low, the IAC valve will not be able to add enough air to compensate for large loads. When the base idle speed is set correctly, the IAC valve will need to open some to supply additional air for the engine to reach correct idle speed. 23.5 Two Wire IAC Valve The 2-wire IAC valve is supplied battery voltage on one side of its winding and the ECM supplies an electronically simulated ground on the other side, as shown in Illustration 23-1. http://thelic.org/users/tomo/IACV/Il...ion%2023-1.png The ECM controls the current in the IAC valve winding by controlling the amount of time the ground is supplied to the IAC valve. The amount of current that flows through the IAC valve windings control how strong the magnetic force is that opens the plunger against spring tension. On OBD-II equipped Hondas, the functionality of the IAC valve is checked by an output state monitor (OSM) inside the ECM. The current needed to operate the IAC valve is compared to a standard. If the current requirements deviate significantly from the normal level a DTC could be set. The ground signal supplied by the ECM is a duty cycle type signal. The current is controlled in the IAC valve winding by controlling the amount of time the ground is supplied. The valve does not fully open and close, but is held open a certain amount by the current in the valve windings. The ground is turned on and off so fast that the plunger does not have enough time to fully closed. http://thelic.org/users/tomo/IACV/Sc...ure%2023-2.pnghttp://thelic.org/users/tomo/IACV/Sc...ure%2023-3.pnghttp://thelic.org/users/tomo/IACV/Sc...ure%2023-4.png In Screen Capture 23-2 you can see the voltage reading of an IAC valve ground wire taken with a digital storage oscilloscope (DSO). When the volt- age is at 0v (at the bottom of the signal) a ground is being supplied to the IAC valve and current is flowing through the IAC valve winding. The DSO Screen Capture 23-2 was taken on a cold engine. You can see that the ground time (0 volts) is more than the no ground time (battery voltage). This causes the IAC valve to be more open and increases the idle. Screen Capture 23-3 is the same car but just warmed up to normal operating temperature. You will notice now that the on/off time is virtually the same. Screen Capture 23-4 shows the IAC valve current as loads are created. Turning on the defrosters made the first "hump" and turning on the air conditioning created the second one. This is the information that is available by using a Mastertech with the Honda/Acura software on some models as early as 1992. Many scan tools cannot provide this information prior to OBD-II. Since "IAC Command" is an OBD-II defined parameter, it is available on all OBD-II equipped models with a generic scan tool (GST). When you use the Mastertech unit with the Honda/Acura software on pre OBD-II Hondas, the unit of measure is in amperes. The unit of measure for IAC valves since OBD-II is in "counts". It is a relative number that increases when the IAC valve current increases and decreases when the IAC valve current decreases. The IAC valve is supplied heated water, via two small coolant hoses, to keep the valve from developing ice during cold operation. The IAC valve is not sensitive to the temperature increase from the heated water. The position of the IAC valve's plunger is totally dependent upon the amount of time the ground is on, which is controlled by the ECM. IAC valves traditionally give very little trouble. You can test them for full open and full close operation as follows: 23.5.1 To Cause the IAC Valve to Fully Close If you want to test the IAC valve to make sure it will fully close down, you can temporarily unplug it. When you unplug the IAC valve the idle should drop to the base idle. When the car is fully warmed up, and is on base idle, all the air is being controlled on the idle bypass screw. If the idle air bypass screw does not seem to have total control over the idle, the IAC valve may not be shutting down completely, or there is another source of unmetered air. 23.5.2 To Cause the IAC Valve to Fully Open If you want to test the IAC valve to make sure it will fully open, you can temporarily ground the wire that goes from the IAC valve to the ECM. One wire will read steady battery voltage, and one will have varying volts. The varying volts wire is the one you should ground. When you ground this wire, the idle should increase significantly. You can also test the 2-wire IAC valve winding with an ohmmeter. Make sure there is no continuity between the winding and the case. Check for approximately 11.5 ohms on the winding itself. 23.7 Service Issues The idle control systems on Hondas are more prone to malfunction since they are a blend of mechanical and electrical devices. Also, older Hondas may use many of the different idle control methods on one car. Later model Hondas are less prone to idle control problems since virtually all the idle control is done with one component, the IAC valve. Following is some of the more common service problems that you may experience in the field. 23.7.1 IAC Valve Not Responding to a Load If you have a Honda that is equipped with an IAC valve and it does not appear to be responding to a load, check for a blocked inlet screen (shown inside the circle in Image 23-8). The inlet screen can become clogged, and stop bypass air even if the IAC valve is open. http://thelic.org/users/tomo/IACV/Image%2023-8.png The inlet screen had been used on some models, but there seems to be no pattern. Most IAC valves do not have this screen, but you should be aware if it. 23.7.2 Engine RPM "Dips" When a Load Event Occurs If the RPM dips or bobbles when a certain load is introduced, it is probably not getting an "advanced warning" about the load. For instance, if every time you turn the steering wheel the idle fluctuates, the ECM is probably not getting a signal from the PSP switch. 23.7.3 Idle Speed Too High This seems to be one of the more common problems with the idle control system. Since the PGMFI is a speed/density fuel injection system, any additional air will cause the idle to increase. Some of the sources of additional air could be:
The most common reason for a high idle is a defective fast idle thermovalve. The testing of this valve was covered earlier in this module. (VXs do NOT have a FITV, so this will not be a cause for fluctuating idle on a VX) The second most common problem is that the throttle plate is not shut. The PGMFI system is designed to run with the throttle shut and all the air that enters the engine at idle bypassing the throttle plate. The first thing to check for is a tight throttle cable. This is fairly common, especially on Civics. Make sure the cable has slack when the car is at idle. If the cable is not too tight, but you suspect the throttle is being held open, check the throttle stop screw. The throttle has an external stop that keeps the throttle blade from actually hitting the bore of the throttle body. It is set at the factory and should never need setting. Image 23-9 shows a typical throttle stop screw on a multi port injection system and a dual point injection system. The screws are usually hard to find since you should not be adjusting on them. Look and see if the yellow paint has been broken. It is not uncommon to find that somebody has used this screw to set the idle. http://thelic.org/users/tomo/IACV/Image%2023-9.png Honda does not offer an adjustment procedure for this screw since it is a factory setting. If the throttle stop needs setting simply unscrew it until the throttle is resting against the bore. Then turn the stop screw in until it starts moving the throttle. Go an additional 1/2 turn or so. 23.7.4 Idle Fluctuates Wildly This situation is also common. This is not really a malfunction, but more of a symptom. It is symptom of an idle that is too high after the car is fully warmed. http://thelic.org/users/tomo/IACV/Sc...ure%2023-5.png What is actually going on is the ECM's fuel cut on deceleration strategy is cutting the injectors off at 1100 RPM. You can see this from the scan tool Screen Capture 23-5. When the solid line (RPM) goes up to 1100, the dotted line (PW) goes to zero. When the RPM drops to below 1100 the injector turns back on. This cycle will repeat until the idle is brought below 1100 RPM. The ECM determines that the car is decelerating if it sees the throttle closed and the RPM above 1100 RPM. During deceleration (at normal operating temperature) the ECM cuts the injectors off for fuel economy and emissions control. This same situation occurs if the idle RPM is too high (throttle closed and over 1100 RPM). To correct this problem look for the source of air that is causing the engine to idle so high. |
Holy cow. Thanks for all the great info! Is the OBD training manual available online?
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where did u get so much info.for years I have been seraching and never found anything like this.u hit the nail on the head(concise).u hav a contact need as much info as possible.automechanic.
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Thanks for the geat info! I needed this.
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You all are welcome for the info. I actually have a copy of the OBD Training Manual that I source the info from when I have time to read it.
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I have to say, the PGM-FI system on my '90 Accord is far better than a lot of newer vehicles I've driven. Only the A/C toggle makes any appreciable change in the idle, and then only for a fraction of a second. The fact that I can crank the steering wheel as fast as possible and the idle doesn't even bobble bests even my OBD-II '96 Ford Ranger.
Is there anything in there that indicates a sensor that I can interrupt that will kill the engine instantly without either powering off the computer or triggering the Check Engine light? |
Hi. TomO - once again great info! Thanks!
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Great information! Just wondering if any of this applies to the HCH II. I have an bobble issue at idle. Appears to be the a/c compressor cycling. Not a major problem for me, but annoying to other drivers of the car.
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bumping this from the dead.
I have a VX engine D15Z1 i am working on getting into an 89 civic wagon. I do not have a IAC valve. Is the IACV very specific for a vx? I see that the vx does not have the fast idle thermovalve. It does have a bolt pattern similar to the one shown in image 23-8 shown above. I am thinking about going to a yard and finding something with a similar bolt pattern and 2 wire plug. Any one else run into an issue replacing the IACV on a vx? |
Thanks a lot, this is exactly what I have been looking for. I have an issue with my Honda, and I suspect that it has something to do with the fuel injection system, but I'm not sure. This information should be very helpful in helping me to isolate the problem.
on delay timer |
WARNING
Long post. If I wrote someting incorrect, straighten me please. For quick info go to Chapter 2. Chapter 1. Some time ago I had iddle issue. 93' Civic VX, 250000km. Briefly: - cleaned EACV - adjusted iddle but the real couse was Purge Control Selenoid Valve (PCS) vacuum pipe was cut a bit, PCS had oslo some small crack causing air leak. Constant vacum control valve (CVC) had some crack hidden close to manifold vacuum pipe (someone had to pull the pipe causing crack or someting). After some time the iddle problems came back to me :mad:. I started digging forums again and I've found a lot of useful info and long stories about iddle issues. Every case looks a bit different so I'm pretty far away from saying "check this and that, it will fix you problem" but I'd like to share some thoughts and observations. Temp went down from 15'C to 2'C (arround -5'C at night) during last weeks. After starting 1500rpm's - acceptable, no fluctuations. Once it got warm iddle went down to about 600rpm's. When I stopped on lights the iddle fluctuations appear (from arround 400rpm to 1100rpm) :confused:. I always try to think what has changed: -maybe some valves got dirty too much to work as before -maybe some sensor died -outside temp. went down Chapter 2. Again I've checked all the piping for vacuum leaks - no leaks :confused:. I looked at vacuum diagrams for long time, I even created some kind of Ishikawa diagram :D. I knew my Electronic Air Control Valve (EACV or EAC) called sometimes Idle Air Control Valve (IACV or IAC) is clean, PCS and CVC is repaired, i don't have vacuum leaks for this part of the system. I went to other parts of the VX system. I've read about PCV valve which is located deep under intake manifold. I took a look (it's not easy to find it for the first time) and reach it with pliers. PCV was clogged (which seems like common problem) but I noticed PCV rubber is very hard (can't blame 93' car) and there were oil leakages arround the PCV hole. I've cleaned PCV and decided to swap hollow elbow with PCV, put new long pipe from elbow to PCV. Hollow elbow rubber was also hard so I had to use some thin sticky rubber tape to seal it. The best way would be to change all rubber parts for sure (that's the plan). Last thing was iddle adjustment (standard well known precedure: engine on, unplug EACV, make short circuit on diagnostic plug, adjust screw on TB (set iddle - fot VX 420 +/-50 rpm), turn the engine off, unplug short circuit on diagnostic plug, plug in EACV, take out back-up fuse for a minute, wait to cool down, start engine, wait until the radiator fan starts, take a ride). So far it works but what makes me think is the rubber parts and vacuum leaks. They are also crucial for the iddle issues and pretty "cheap" and easy to verify at the first place. I could change plugs, wiring, check grounding, clean TB, change ECU, try to adjust iddle etc. without any success. Chapter 3. I totaly agree that sensors fail, good ground is crucial for all electrical devices, welds are important, vibrations can wear down the wiring insulation. Although I think at the beginning it's worth to remember what is the sytem based on - air, electrical signals, rubber, plastic, metal etc. As we all know most of the holes made in engines (e.g. aluminum alloy) are getting bigger while hot. Rubber could get more flexible but it's not expanding like aluminum alloy. Everytime I'm looking for causes of failure I try: - start from basic things (like is there vacuum, power etc. at all) - to go back (step by step), - think what has changed, - when exactly it started, - what was the exact situation, - who was fixing IT (who? how? where? when? with what?) - how all of this could influcence my problem Unfortunately lots of failures appear as resultant of different small failures and that's why they are so hard to solve. The only way is to trace them separately and try to eliminate them each after another. The End - sorry for long post. I'm also sorry if I sound like I knew more than I know ;) Thank you to all who shares the knowledge - I would be in dark forest now without YOU! I wish you all patience and time. |
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IAVC two wire to three wire mod that works?
Ok I dug into how the the three wire IAVC works. It appears that the third wire on the IAVC is a solenoid that closes the valve. Basically it replaces the spring in the two wire version. It is just the inverted signal of the other. So lets say if the signal on wire one is on for 1 second and off for 2 seconds then the signal on the wire 3 is on for 2 seconds and off for 1 second (you would never see pmw like this but its a example). So if you my guess is that if invert the signal on 1 and applied it to 3 (not sure which wire is common) you should get this value to work. So slap on a PWM inverter on wire 3 and walla... 2 wire signal controlling a three wire IAVC. This is just a educated guess but is anyone up to trying this out? I am still waiting for my motor so it could be awhile before I can try this. I am will dig around on Newark for some thing that should work.
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Im debating on buying a new iacv.the reason for my idle not dropping normally was the tps was wayyyy off on the calibration. 1.45 at closed throttle. Now the iddle fluctuates up and down a little over 1k rpms but stops at a complete stop.iv taken it out and cleaned it several times.my radiator is leaking .cud off coolant pressure thru the iacv cause wierd idling issues as well?
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What can you tell us about i-VTEC and i-Dsi? I swear under certain conditions it feels like its running on 2 vs 4 cylinders.
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Seafoam deep creep got a lot of gunk out of my iacv.
First make sure the gasket is good and the filter is clean.spray brake cleaner like crazy in it and let it dry out.then spray some seafoam deep creep in there and let it sit for a while.recleaned it out again and got a lot more gunky black build up.also running seafoam thru the brake booster vacuum line every so often has been keeping it clean as well.last resort you should ever have to buy a new one if these don't work |
Did I miss something for the 3 wire IAC valves? 23.4 States "The 3-wire IAC valve is covered at the end to this section." but I don't see how to test it or any info for it. I'm working on a 2003 Honda Element and just want to make sure it works or not (i.e. ohm/voltage reading on which pin). I did see in another post that the 3 pin is like the two pin plus the 3rd wire acts as the spring. I do have the manual for the car but it just states to do a IACV test on the Honda brand OBD II sensor and I just have an off brand one.
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