Fiesta idle air control valve

It'll not disassemble. Clean it out with carb cleaner or even give it liberal sprayings of WD40. Cleaning it is pretty much a temporary fix, though. A replacement should be sought as soon as possible before it gums up again.

Yes, that didn't look possible.

Why will it gum up again (quicker than it did the first time I mean) if I clean it out properly?

Thanks,

Peter.

To clean it out properly would be very hard, if not possible. The nooks and crannies that the crud get into often don't give up what they've accumulated very easily. I suppose if you spent a great deal of time and gave it a good, thorough, proper cleaning, it may survive for a fair amount of time.

OK, fair enough... I'll give it a shot (without spending ages on it) and see how long it lasts for :)

Thanks,

Peter.

My vectra needed cleaning out every couple of months. I fitted an oil separator, and it was fine for another couple of years.

Bob

I got a garage to do it for 20 or 30 quid; on two different fords a year or

2 ago. They done a great job, all has been fine since. I wondered at the cost, but they did actually show me what they cleaned, was pretty impressed at their effort.

Yes, seems quite difficult to clean it out properly. I gave it a good blast with carb cleaner, let it soak a bit, blew everything out with compressed air and let it dry a while before reattaching. Symptoms have changed a bit, it's stopped dropping revs down to 500 (resulting in the occasional stall) when I suddenly depress the clutch. Now it's either fine (most of the time) but sometimes just conks right out when it's time for the valve to open! Is that because it hasn't dried out properly yet and is sticky? Is it time for a new valve?

Oh yes, out of interest..... Why isn't the engine throttled by restricting fuel flow instead of restricting the air supply? Or is the function of the throttle position sensor to let the ECU know how much fuel to pump through?

Cheers,

Peter.

After I cleaned mine, I put a bit of oil in it.

Bob

It is. The engine is only getting a certain amount of air, thus injects a proportionate amount of fuel to keep it ticking over. Without the valve, no air gets in, engine dies. Other side of the coin is, too much air gets in, it causes lean mixture and the engine grenades itself very quickly.

Another one is cutting the spark, causing the engine to miss every so often. This causes unburnt fuel to leave the cylinders and shag the cat quick style.

Yes, what I guess I'm asking is why does the mixture have to go rich when idling? I appreciate that lean mixtures burn better but why can't idle be controlled by finely regulating the fuel supply rather than the air supply? Seems to be a waste of fuel!

Is it that the fuel required to idle when there's plenty of air available is so small it can't be accurately controlled?

Yes, that doesn't seem like a good option :)

Peter.

I'm not sure I'm following you, but ... ECU's inject fuel based on how much air it thinks is entering the cylinders (based on readings from the air flow meter or manifold pressure sensor). It doesn't let in air proportionate to the fuel it wants to inject. When you stamp on the go pedal, you're opening a butteryfly valve to allow more air in. Basically, the only way to regulate engine speed is to regulate the air supply.

Nup. Not only does it ruin the catalyst, but is a gross waste of fuel.

OK..... so what you are saying is that having the throttle regulate the fuel usage and leave the airways uncontrolled isn't possible because there would be too much air available when it idles and hence it'd be too lean and the engine would run too fast? My point is that they presumably should be a sweet spot where there is just enough fuel to keep the engine alive but not enough to cause the revs to run away.

Also, as the air intake is restricted doesn't that mean that when the piston is sucking air/fuel into the cylinder on it's down stroke it's fighting a developing vacuum? Sounds efficient :)

Just trying to understand here :)

Thanks,

Peter.

Yes re: air thing. In theory, you're right. There would be a fixed size hole to allow a fixed amount of air, thus requiring a fixed amount of fuel for a fixed engine speed. What happens when you flick on the head lights? All of a sudden, that amount of fuel isn't enough, as the engine needs to work harder. Flick the air con on, then stick the fan on full and you've burdened the engine even more. Where is the extra air going to come from to help compensate for the extra load? With the idle valve, the engine can open the throttle (if you like) to make more power as not to stall under the load.

Correct. Although, that vacuum isn't totally wasted. It assists your brakes and helps draw in excess fuel tank vapour pressure etc.

Quite right.

Ah... I'm not thinking having a fixed air supply but have it unrestricted so the ECU can just increase the fuel flow if the revs start to drop due to a new load being put on the engine. Presumably that won't work for some reason but I'd like to know why! :)

Right.

:)

Peter.

It certainly wouldn't work. Petrol engines make power by burning an oxygen and petrol mixture. This mixture should be as close to stoic as possible (stoichiometric is about 14 parts air, to 1 part fuel). The more of this mixture the engine can ingest, the more power it can create. If you have an unrestricted airflow, the engine will be free to inhale as much air as the cylinder will hold (at atmospheric pressure, not getting into forced induction, yet). The problem with this, is that if you're only injecting a tiny amount of fuel, you're going to have way too much air, and not enough fuel, resulting in a lean mixture situation. Problems with lean mixtures are caused by the excessive heat generated. It causes melted pistons, detonation, and various other nasty engine destroying results.

ECUs inject fuel according to the air it thinks will land in the cylinders. If you had this unrestricted air flow, the ECU would try and fuel to match this and you'll have a runaway engine that will rev til it grenades itself. Therefore, the only way to regulate engine output, is to regulate it's air intake.

That's how a diesel works. Try it with a petrol engine & it'll just stop. Never mind dismally failing the emission regs.