Why does it appear that just cleaning my IAC does not solve my idle hesitation speed problems? Is it possible that the solenoid stepper motor wears out over time? Or is it the needle valve that wears out? Do the rubber valves wear out over time?

I've tested the solenoid and it passes the ford "good or bad" tests electrically. 9 ohms across the leads and is not shorted to the case. I just want to understand the theory of this stepper motor. Do the windings begin to burn out? Any physicists in our audience?

 

First, the IAC valve does not use a stepper motor, but a solonoid to quickly open and close the valve. Variations in the duty cycle change the amount of air that is bypassed around the throttle plate. It's kind of like a fuel injector.

Any part can wear out and fail. They all have designed ranges of conditions under which they can operate. When they are operated closer to extreme conditions, they will wear out faster.

The IAC solonoid consists of a coil of wire around a spring loaded plunger. When the coil is energized, a magnetic field is generated, which pulls the plunger in, which opens the valve. The coil uses insulated wire, so if it is exposed to too much heat, the insulation can break down, causing shorts. You've checked continuity of the coil already, so the coil is still functional.

The plunger is a steel rod that partially extends into the coil, and a spring returns it to the closed position when current going to the coil is switched off. This on/off action can occur tens of times a second, and like any moving part, will wear out eventually. You can check this action by quickly applying 12 volts to the coil. The plunger should pull into the body very quickly, and should pop out just as quickly when you remove the 12 volts.

The valve is attached to the end of the plunger, and, like the valves in the engine, can also wear out, and lose seal. Or it can simply jam, and never open. You can check this with a little screwdriver.

Some of the air passages may also be obstructed. There is a vent tube from the valve cover that goes to the air intake hose which is supposed to suck gasses out of the crank case. This is part of the PVC system. If there is a lot of blow-by in the engine, there would be a lot of partially burned fuel and oil vapors that get sucked in. This gunk can gum up parts like the throttle plate and the IAC valve. So occassional cleaning with something like Gumout should help. Make sure to hit the passages in the IAC valve, throttle body, and the manifold.

Is your only problem maintaining a steady idle?

 

No, the idle is fairly steady - it appears to be slow responsiveness of the valve when it is cold. The engine chokes until the valve closes again - 2 or 3 seconds. When it warms up it works fine. Maybe the return spring has lost it's springiness?

Thanks for the excellent description of what's happening in there... I haven't actually cleaned the IAC holes in the intake yet either. I'm going to try that tonight in case that's gummed up and not allowing enough air to pass effectively. The mess might be as a result of a kinked return hose in the summer and drove for quite a while with it that way... lots of unburnt gas into the intake via the vacuum line. Where else would that gas have gone? I guess anywhere through the vacuum tree, huh?

 

After much fiddling it looks as though the solenoid loses it's responsiveness. I'm pretty sure the valve itself is working fine. I changed only the solenoid on it (have two old ones) and it works a little differently but still malfunctions. Gonna go for a new one.

 

can someone tell me where is this IAC located? Or if there is a picture, that would be great.

Thanks

 

If you look at the engine from the front of the car, the IAC valve is bolted to the right side of the intake manifold, behind the throttle body. It is a cylinder shaped thing with two wires coming out of the back of it.

 

cool, got it. Thanks a lot.

 

Actually turned out to be my EEC engine temperature sensor - replaced the iac first to no avail.