Idle Air Control (IAC) Valve Replacement.

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Idle Air Control (IAC) Valve Replacement.

Post by Rednaxs60 »

Just replaced the IAC valve on my '85 LTD. Have had idle issues at start, excessive fuel smell, having to keep the RPM higher than normal for some 45 to 60 seconds then the system would idle, and disconnected the IAC valve from the system with no change in RPM. Read everyhting I could regarding the operation of this valve and the issues with a faulty one. Did the resistance check and when warm it was out of spec, and at the high end of the spec when cold.

Received it today and installed. Had to drain the coolant because of the coolant hoses going to the valve.

Socrace over on the NGW forum found this description and it is exactly how the valve works: "There are valves that need no control other than a 12V supply when the fuel pump is on. These heat up a bi-metalic strip that rotates a plate inside the valve and shuts the air down as the engine heats up, these are known as Extra Air Valves. They are becoming rare and hard to get hold of, they were fitted to various early efi engines. The Idle Speed has to be controlled with the throttle stop when using these type of valves, as the valve is only used to add air when the engine is cold." The stock Honda unit works the same as this, except it has additional coolant warming that this one doesn't need. These coolant lines are to prevent icing up in colder weather. The FI system is based on the auto industry and hence the same prinicples were applied to the early Honda FI systems. I would also surmise that this was a valve that worked in the Honda FI cars and so it was used in the '85 LTD and '86 SE-i.

Here is a look inside the working end of this valve. Pim205GTI over on the CX500-650 forum posted this picture of the internals:

Here is a diagram of how the valve works:

After I installed the new IAC valve, started the bike without changing any idle settings. I put the shelter and others items I removed to do this work and let the bike warm up to operating temperature. Interestingly, the idle climbed to 1450 RPM whereas before it would stay at 1040ish RPM. This has lead me to conclude I was having to dump a lot of fuel into the engine at idle, cold or operating temperature - hence the excessive fuel smell. I adjusted the idle, went for a good road test and have now adjusted the idle to suit. The telling of how well the idle system is working will be tomorrow morning when I do a cold start.

There are similar IAC valves available for FI bikes. This one from SMP: Standard Motor Products AC356 Idle Air Control Valve Here is a site for it: ... ingdocs-20 Don't forget to look at your local auto wrecker as well, lots of good used products available that may fit the bill.

I do not believe the coolant hookup is necessary because the coolant is to prevent icing and I for one will not be doing any riding in weather that would test this.

Will update tomorrow after starting in the morning.


"When you write the story of your life, don't let anyone else hold the pen"
"My '85 FI model is not a project bike, it's my retirement career"
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