How does base idle work

Hello

I've been looking over the base idle screw today trying to fix mu cold idle problem on my 93 4.2lre LSE. One thing I could not figure out was, how does the base idle screw work? I read that turning it counterclockwise speeds up the idle and clockwise decreases it, but I couldn't see what it actually affected. Can anyone explain?

Thanks
NKG

The idle is normally controlled by the engine ECU. I presume your LSE is Fuel injection?

NKG said:

Hello

I've been looking over the base idle screw today trying to fix mu cold idle problem on my 93 4.2lre LSE. One thing I could not figure out was, how does the base idle screw work? I read that turning it counterclockwise speeds up the idle and clockwise decreases it, but I couldn't see what it actually affected. Can anyone explain?

Thanks
NKG

Click to expand...

O.K. then, here goes - 4.2LSE has Lucas 14 CUX ECU and hot wire AFM; the same system as on the 3.9. As Irishrover said, the normal idle speed is controlled by the ECU which senses engine speed and load and then adjusts the idle speed with the idle air by-pass valve (stepper motor).

If you have been doing work on the engine like say head gaskets or you've had the plenum and ram housing off, you would be wise to check the BASE IDLE.

Base idle is supposed to be 500rpm with the by-pass air hose removed and the valve disconected. The base idle screw you refer to is, as I understand it what might be termed a mixture screw. Depending on which way you turn it, you increase or decrease the amount of air going into the engine and thus influencing idle speed.

Once you can get the engine at a smooth idle at 500rpm, reconnect the idle air by-pass hose and valve, the ECU should then lift idle speed to about 700 - 750 rpm. If you can't get base idle at the specified engine speed you may need to look for intake leaks, check ignition timing and also make sure the throttle butterfly is correctly set and that the throttle shaft isn't worn. Make sure you clean you plenum.

You say you have a cold idle speed problem but don't say what is wrong. Is not idling fast enough or is it too low? Does it hesitate /stall when you gas it? If so, would say check timing first, also air filter then the usual suspects - rotor arm, dizzy cap, plugs and leads.

BTW you might also want to check the stepper motor is working properly as well - I think it can be checked with a DVM acrcoss the relevant terminals on the main plug into the ECU.

take the stepper motor out, and give a good clean, with carb cleaner. sounds to me as though it is clogged up. and also as above leads rotor etc. but try cleaning stepper first. always works for my LSE.