IACV explained

[Ulvetanna_imp]

Well some months ago i had no idea that i even HAD one until cormanus put me straight, so i guessed others may have the same question.

We were hoping to prove that a particular part was at fault just so all involved could just go ahead and repeat the procedure, so a lot of reasoning went on and after all that we now know how it works but don't yet know which part is faulty, all we know is that the 1600+ dollars replacement clears the problem and provides the owner with the bike that he paid for, that seems a bit steep for a small part or even a poor contact, so we did all these tests.

The question is;
a; does the iacv do what the ecm tells it or
b; does it get told to do the right thing but is unable to do so from time to time.

We are still checking wiring but that is just to be sure all the possibilities have been covered, we don't know.

On a more dramatic note; the bike has developed a fault, was it caused by the rider or the manufacturer?

Don't answer that. max

The IACV is a very simple mechanical part; all it does is adjust the idle mechanically based on what the ECU has calculated.

There are several sensors at play here, and the combined values from those sensors are used to actively adjust the IACV in real time whenever the throttle is closed.

I don't have my manual any more, or I could look this control circuit up myself.

However, it doesn't much matter which sensors are involved; we need to find them, find out what parameters are being observed, and then what calculations and algorithm are used to send the final signal for adjustment to the IACV.

This is directly related to emissions control compliance so it is very likely the process is pretty complex. But it can be corrected. Most bikes don't have this problem. It is fixable. Somewhere in the chain, something needs to be modified, replaced, or adjusted.

QUESTION #1

Why does the idle need to be constantly adjusted in real time? Why not just have a set screw you can reach down and turn?

ANSWER

Idle is the time when the engine is producing the most emissions, under the worst circumstances. Most idling is done in urban traffic at a stop, where the pollutants are just pumping into the air and doing the most damage. So the engine's fuel mixture and idle speed need to be very carefully controlled under those conditions.

Also the ECU monitors electrical loads and the charge state of the battery. The idle will temporarily increase to provide more current from the alternator if necessary.

QUESTION #2

Which sensors are involved?

ANSWER

Typical sensors are air intake temperature, throttle position, airbox pressure, engine temperature (oil temperature is critical as it's air/oil-cooled), exhaust gas temperature, and oxygen sensor values. I don't know which are involved with the CB1100 but that is a typical array.

If any of those sensors are faulty the signal sent to the IACV is going to be erroneous and cause an issue.

The sensors usually provide a voltage of 0-5 volts to the ECU which then converts this voltage to an output reflecting the actual value of the units. It combines these various sensor channels and then uses a calculation based on math channels or algorithms or some other process.

If any of these processes are flawed within the ECU programming, THAT could be the issue right there. In that case a reflash/reprogram of the ECU with a program proven to work is all that's needed.

If we have sensors with intermittent issues, or bad conversions of analog to digital values, or any corruption or hysteresis in the system (that is a fancy work for uneven response) that can cause a problem as well.

Having worked with some fairly complex systems that used this kind of architecture, I'd say the problem is NOT likely to be the IACV. It's a simple electro-mechanical device. It's more likely a sensor, or bad calculation in the ECU.

However, it's possible Honda had a bad run of IACV units and some of those got into a few bikes.

The IACV would, therefore, be the FIRST place to look since it's the simplest solution. Then proceed to the various sensors.

But if those all check out, the ECU programming -- or the ECU itself -- could well be the culprit.

Heat has a way of really messing with computers. So does vibration. There are lots of both in a motorcycle.

Lastly, if the owner hasn't modified the bike in any way, and just rode it normally, there's no way the owner could do anything to disrupt this function. Changing intake/exhaust features might do it, but should not. Again, this is going to be based on whether the ECU can correct for the these changes, or will produce erratic/erroneous values.

Riding the bike hard, bouncing it off the rev limiter, none of that is going to hurt this Honda. One thing I know for sure, since I've done did all that. I never worried about the reliability of that bike. It loves to be "rode hard and put away wet".
Just an addendum, "IACV" stands for "[url=https://en.wikipedia.org/wiki/Idle_air_control_actuator]Idle Air Control Valve".

Click the link for more.