High revs at idle

[vogue]

My V8 Disco is revving too high. When i first start it with no throttle it revs at 2000rpm and then slowly drops to 1000rpm after 10- 20 sec.
When i stop at lights and put it in neutral it is doing 1500rpm. Sometimes it will drop back to 800rpm.
It is an auto and it is much harder to stop as a result.
What are the likely causes?

[Hoges]

My V8 Disco is revving too high. When i first start it with no throttle it revs at 2000rpm and then slowly drops to 1000rpm after 10- 20 sec.
When i stop at lights and put it in neutral it is doing 1500rpm. Sometimes it will drop back to 800rpm.
It is an auto and it is much harder to stop as a result.
What are the likely causes?

First off I'd check for a binding throttle cable, ensuring that the butterfly in the throttle body is properly seated when foot is off throttle...

[Gary S11]

Hi, mines done the same , but only after I put the timing up to 10DEG, BTDC...Gary

[vogue]

I do have the timing set to 10 btdc for the lpg.

Also about to fit a new vacuum advance. Could that have something to do with it?

[bee utey]

You can of course set the base idle lower. Insert a 3/16 allen key in the hole atop the throttle body and turn it inwards a turn or two. Failing that it may have something to do with the road speed input to the ECU being crook but I don't know anything exact about that.

[Gary S11]

I do have the timing set to 10 btdc for the lpg.

Also about to fit a new vacuum advance. Could that have something to do with it?
Let me know how it goes with the new vac advance please...I don't want to alter my base idle as when it does come down it would be to low...I basically learnt to live with it....only prob going down a steep hill in low ..if I need to blip the throttle it takes off ...

[bee utey]

Let me know how it goes with the new vac advance please...I don't want to alter my base idle as when it does come down it would be to low...I basically learnt to live with it....only prob going down a steep hill in low ..if I need to blip the throttle it takes off ...
You do understand the vacuum advance doesn't get vacuum at closed throttle? It will make no effect to the idling speed.

[streaky]

Sounds to me like the stepper motor needs cleaning.

It's the large plastic bolt head thingy at the rear left of the plenum.

Unclip the plug....unscrew the stepper motor and clean everything in sight with contact cleaner. Don't be tempted to mess around with the plunger spring...just extend it very carefully and clean the plunger seats with contact spray.
Screw it back in hand tight plus a quarter turn (max).
Then pull the EFi fuse for ten seconds to reset everything.

[vogue]

Ok, i pulled out the stepper motor, it was black on the tip. I sprayed the tip and spring with contact cleaner (does it come apart?), put it back together and started. The revs still started high, 2000rpm but dropped back to normal within 5-7 secs.
Any ideas?

[Gary S11]

You do understand the vacuum advance doesn't get vacuum at closed throttle? It will make no effect to the idling speed.
Thanks bee utey, I realised that but my probs not at idle, it's above 2000 rpm, the same as vogue, blip the throtle and it sits above 2000 and slowly drops to the correct base idle

[bee utey]

Thanks bee utey, I realised that but my probs not at idle, it's above 2000 rpm, the same as vogue, blip the throtle and it sits above 2000 and slowly drops to the correct base idle

Yes but you see that I said "closed throttle" i.e. your throttle plate isn't open. There is no way vacuum can pass to the vac advance unless the throttle plate is opened past the port hole above it. The idle problem is due to the ECU moving the stepper motor in the back of the plenum chamber. Either the stepper or the ECU aren't working as planned.

The only other adjustment of the idle speed is the base idle, that one that is adjusted by a 3/16 allen key from the top. It too doesn't move the throttle plate. Screwing it down will limit the maximum amount of revs the stepper can apply. It's also worth looking at the air hose to the stepper and see if it has split ends. And if someone has replaced your fumes breather plastic tee (in front of the plenum) with a home-made brass one, it's dollars to donuts they haven't fitted the correct small jet size in the small branch to vacuum (about 3mm max).

[vogue]

Next step: How do i check if the stepper motor is working properly?

[vogue]

Time for an update, I installed a new stepper motor and all is good. Idle revs are a bit high at around 1000rpm but will adjust base idle speed and all will be good. Thanks for everyone's help.