3.9 V8i engine idle speed

[Big Red]

My series 1 Disco is a standard 3.9 V8i petrol with auto gearbox and I've been trying to adjust the engine idle speed without much success  :?

When first started, it initially revs straight upto 1500 rpm on the tacho, and then after a few seconds it reduces to a steady 1000 rpm at idle which I consider too high, even when the engine is warmed up and at normal operating temperature. Somwhere around 600-700 rpm is where I want it to be.

So far, I've tried the following without success:

Checked the throttle cable is adjusted so the throttle closes against the stop & lubed the cable

Visually checked all hoses for any signs of splits / air ingress

Thoroughly cleaned Idle Air Control valve (aka stepper motor) and aperture into which is fits with carb cleaner - but still don't know if it's working properly.

Clamped the hose from the IAC valve & adjusted idle speed to approx 600 rpm, however as soon as I release the clamp on the hose the engine races to 1500 rpm and then gradually falls to the same 900 rpm after a few blips on the throttle.

Added fuel injection system cleaner to a full tank of fuel to help remove any carbon build up

Replaced the air filter

It's driving me crazy now and I've run out of ideas - have I missed something obvious? What exactly does the Mass Airflow Sensor (MAF) do, and how can it be checked?

Any advice would be much appreciated.

[Damonski]

I have a similar issue with my Mpi.

This weekend Im hoping to clean that "stepper motor" and all the linkages.

[Range Rover Blues]

It's working fine, leave it alone.  The ECU knows the engine speed and adjusts the extra air valve to maintain what it thinks is correct, irespective of throttle idle position, which it can work out anyway.

When cold it fast-idles to prevent missfiring and keep things smooth, plus heat up a little faster.  In a cold engine the fuel can condense which leads to bore-washing (this is BAD).

After a warm start the ECU will self-check and bring the engine slowly to idle, it also allows a fast burst of charging to replace the power used by the starter.  It's probably better for emissions too.

[Big Red]

RR Blues, I hear what you say about leaving well alone and I will probably take your advice, however I'm still worried that when the engine first starts from cold and initially races upto 1500 rpm the oil pump won't have had chance to lubricate the camshafts and upper cylinders thus causing premature wear. Is this normal for a 3.9 V8?

I still don't know what the Mass Air Flow device (MAF) does and whether it's working properly - could this have any affect on idle speed?

Damo - I grew up in Goostrey, just down the road from you, but have seen a bit more of the world since.

[Damonski]

YUP!  Just down round those 'S' bends at up the top of the hill :)

Cant advise on the v8i but the Mpi does similar, I suppose when thinking about it, any petrol car with a choke of any type would "race" in the same way.

[Range Rover Blues]

It's the lesser of evels I'm afraid, when cold there are lots os things getting damaged, including your pistons not being the right shape!  The engine needs to warm up quickly, and TBH I think the oil pressure is higher at fast-idle anyway so the oil gets there a lot faster.  The initial start-up relies on the oil to resist draining back to the sump when you last switched off.

The MAF is a hot-wire device, it uses a by-pass orifice to measure a callibrated percentage of the air flow.  It works using a wheatstone's bridge where 2 of the resitors are fixed, one is a reference thermistor for ambient air temp and the last is a hot-wire in the air stream.  the current heats the wire, the air cools it.  By comparing the cooling effect on the wire (the wheatstone's bridge) one could calclutate the mass air flow, not volumetric air flow as in the flapper EFi fitted to 3.5 angines.  I'n not sure if the MAF unit has it's own brain or not as there are 4 wires to it but I think it does.

Anyway, the Haynes manual list a series of checks for the hot-wire system inlcuding the MAF, I'll have a look for you.