how does the ecotec engine idle?

[vh-holden]

How does the ecotec (or really, any EFI engine) idle?

I pulled the intake pipe off to clean out the throttle body in an attempt to make the car run properly, and noticed that when the butterfly is closed, no air can get past it.

How does the engine idle then?

[greenfoam]

There's a thing called thr IAC Idle Air Control Valve that opens and closes to let air in that hole in the side of the throttle body

[vh-holden]

ahhh, thanks for that.

how do i know if it is working? the car is having real issues starting and idling. the throttle body isn't that dirty, so i think it might be somethign else.

[Don Sutton]

Idle speed is controlled by the crankshaft sensor determining counts or pulses from the crankshaft which is fed to the ECU. The ECU will then control the IAC valve when the ECU senses that the throttle is closed (switch on the throttle shaft). The IAC is a servo valve with a stepper motor that opens or closes a bypass valve which bypasses the butterfly.
What can go wrong?
The IAC valve can get clogged with engine vent fumes solidifying in the passage and causing the IAC valve to stick.
The throttle switch can become faulty (intermitent or not work at all).
The crankshaft sensor can become faulty which misses some of the counts from the crankshaft pulley.
An air leak into the manifold from a broken hose or such like.

Usually if everfything is working OK, holding the revs at about 3000 for about 30 secs should allow the idle valve to reset and if the engine is brought back to idle it should be OK. If not check out the items above and if they are OK check wiring connectors. It is also possible for the ECU to be faulty. The Gregory's manual has a good section on trouble shooting this area.

[vh-holden]

hrmmm. the car is weird. It idles fine on petrol, but switching to gas it dies. If i keep the revs up, it is fine, it just wont' idle on gas. It is a factory gas install. I noticed there is a screw on the gas mixer with idle written on it.

should i touch it?

[futronix]

gas system might need a tune.... if there is such a thing.....

[vh-holden]

gas system might need a tune.... if there is such a thing.....

yep, gas tune.

anyone know how to do one!!??

[shaggerz]

I imagine when messing with gas it would be a better idea to take it to a specialist...

[noah_73]

That screw is for mixture adjustment which should be done with the aid of a gas analsyser. good chance your convertor or mixer is stuffed.