V8 Idle control problem...

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V8 Idle control problem...

Postby Ritch » Mon 15 Dec 2008 4:16 pm

Symptoms are as follows. Start vehicle cold and revs jump to just above 1000 rpm, once warm idles nicely about 750 rpm on gas, slightly lower on petrol. It is a "little" lumpy but nothing to worry about and has always done it for last 20k miles.

However just recently, as driving along it kind of pulls itself along at 1500 rpm even with no throttle. If I slip it into neutral whilst rolling it sits at about 1500 rpm. However, and this is the bit thats got me stumped is soon as the vehicle is drawn to a halt the revs drop in kind of 3 stages. Down to say 1200, holds 1 sec, then 1000, holds 1 sec then drops to its correct idle speed??? Set off again cruising up the road and it will do this whole thing over again.

I have cleaned out the IACV and have checked for physical restriction in acc cable or throttle link. All seems good.

Its been doing it long enough now for it not to be "one of those" Disco things that is gonna fix itself.

Any ideas chaps?
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Postby paintman » Mon 15 Dec 2008 4:27 pm

Build up of cr8p interfering with the closure of the throttle butterfly?
Blocked breathers?
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Postby vougese39 » Mon 15 Dec 2008 4:59 pm

3.5 or 3.9?
look on the forum fixs under range rover i have listed how to set the base idle which if the throttle pot is clean and the idle valve is cleaned out and you have had the ecu read for faults then it would be set idle then retry
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Postby Ritch » Mon 15 Dec 2008 10:07 pm

paintman wrote:Build up of cr8p interfering with the closure of the throttle butterfly?
Blocked breathers?


I'll give em a clean out at the weekend but its so rhythmic the way it kind of steps down its almost like its programmed in. Its the fact the revs won't drop until the car is brought to a halt that has me confused. Is there a manual "home user" way to read fault codes from these?
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Postby Ritch » Mon 15 Dec 2008 10:09 pm

vougese39 wrote:3.5 or 3.9?
look on the forum fixs under range rover i have listed how to set the base idle which if the throttle pot is clean and the idle valve is cleaned out and you have had the ecu read for faults then it would be set idle then retry


Its a 3.9. Do you mean the throttle potentiometer clean and if so how do I go about this? Same Q about reading fault codes? Is this a dealer thing or can I get a manual read?
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Postby JonV8 » Mon 15 Dec 2008 10:40 pm

I think you are worrying about nothing here,the engine ecu will not let the engine down to its base idle while its moving,to lessen the chance of stalling in the event of a heavy load being applied.The stepping down to base idleonce its stopped moving is exactly what I would expect.If you have a lumpy idle you need to look at idle mixture,oxygen sensors,vac leaks,plugs,leads etc,etc.
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Postby Ritch » Mon 15 Dec 2008 11:41 pm

JonV8 wrote:I think you are worrying about nothing here,the engine ecu will not let the engine down to its base idle while its moving,to lessen the chance of stalling in the event of a heavy load being applied.The stepping down to base idleonce its stopped moving is exactly what I would expect.If you have a lumpy idle you need to look at idle mixture,oxygen sensors,vac leaks,plugs,leads etc,etc.


I think i probably described it badly. Basically the car has driven as expected for the last 20k miles but recently it is now as if the throttle is being gradually applied of its own accord to the tune of 1500 rpm. It can be quite disconserting approaching junctions etc as it requires more braking effort than if the engine was merely idling. On start up its is fine and once at a halt and N or P is selected it then steps down a couple of hundred revs at a time over a pattern of a few seconds until resuming normal idle. Off you go again down the road and it does it all over again. This is why I was looking for a physical restriction but all seems fine.

The complicating factor is if I cruise on a long straight at say 60 and select neutral (no throttle) it maintains 1500 to 1750 rpm until I bring it to a stop. Then once stationary it drops down in stages to normal idle.

Something I omitted from the first post is (possibly coincidental) that it seems to hunt between 3rd and D more than usual. It feels just like the throttle is being squeezed slightly of its own accord before taking into account any of my request?

Sorry for the noddy description but trying to get it across as clear as possible.
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Postby craigmidz » Tue 16 Dec 2008 12:19 am

has it got cruise control as i had a fault on my 3.9 disco virtually the same as what u describin but mine was a manual got told to disconnect my cruise control an problem stopped as instantly as it started
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Postby s1simon » Tue 16 Dec 2008 1:21 pm

Hi

Check throttle cable is adjusted properly as this has had a big effect on gear changing on my d2 v8. It was changing up at 3000rpm no matter how much throttle I used, adjusting the cable made it change gear at higher revs if using full throttle. As to idling problem try cleaning out the throttle housing and idle control valve with carb cleaner. On my old 3.5efi disco the idle used to come down in stages especially when on gas, try resetting the base idle for gas but make sure you have the idle control valve blanked off not just disconnected.
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Postby Ritch » Tue 16 Dec 2008 8:12 pm

Cheers all, will have a go at all this over the weekend and see how it gets on. I'll let you know how it goes :D

Ritchie.
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