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falkster

Member Since: 12 Nov 2023
Location: Sutton Coldfield
Posts: 64

1988 Defender 110 V8 Petrol CSW Trident Green

V8 idle all over the place

hi All,

excuse my wording as I don't know what the right word is to describe this, which is probably why the search function was of much help neither...

My V8 is changing revs on idle up to the point where it sometiumes cuts out when stopping at a traffic light.
It drives perfectly normal and when I have my foot on the accelerator to only raie the revs ever so slightly, it is also perfectly fine.
it sounds like a boyracer on a traffic light but the other way round if that makes sense. so instead of rhythmically increasing revs, I am losing revs and it most of the times juuuust recovers the revs before cutting out...

Any ideas where I could start with the fault finding?
And can anone tell me how you would describe the rhythmic down and up of the revs on idle with proper words??

Thanks.

1988 Land Rover OneTen County SW, 3.9l Petrol V8 (serpentine), original "Santana" LT85 gearbox

10th Aug 2024 9:56am

TexasRover

Member Since: 24 Nov 2022
Location: Paris
Posts: 1050

2002 Defender 110 Td5 DCPU Chawton White

We are assuming here it is a 3.5 with the dual stromberg Carbs (1988)?

10th Aug 2024 10:07am

BaronDefenders

Member Since: 28 Jun 2019
Location: London/Cotswolds
Posts: 896

2004 Defender 90 Td5 CSW Santorini Black

Is your 3.9 on carbs or injection? Charlie

1949 Series 1 80 (SOLD)
2002 Td5 90 (SOLD)
2008 Freelander 2 (SOLD)

1958 Series 2 88 Pastel Green (2019 LR Legends Best Restored)
1983 V8 110 Limestone (Previously owned by Tom Sheppard MBE)
2004 Td5 90 Santorini Black (Td5INSIDE Powered & Rebuilt by CSK)
2012 Puma 110 Zermatt Silver (Overlanding Build)

Instagram: @BaronDefenders

10th Aug 2024 10:27am

falkster

Member Since: 12 Nov 2023
Location: Sutton Coldfield
Posts: 64

sorry, i wasn't aware there was a carb version of the 3.9.

it's on injection.

thanks

1988 Land Rover OneTen County SW, 3.9l Petrol V8 (serpentine), original "Santana" LT85 gearbox

10th Aug 2024 2:34pm

TexasRover

Member Since: 24 Nov 2022
Location: Paris
Posts: 1050

I am not sure how that vintage 3.9 Injection looks but, did you check your IAV (idle air valve), this should be a canister looking thing that bypasses the throttlebody (TB).

The ECU (Engine Control Unit - computer) will use it to regulate the idle air. The TB should be closed with the air only passing through the IAV where the air flow is controlled. Check and clean both the TB and IAV.

10th Aug 2024 2:45pm

falkster

Member Since: 12 Nov 2023
Location: Sutton Coldfield
Posts: 64

I'll check that...

Thanks.

1988 Land Rover OneTen County SW, 3.9l Petrol V8 (serpentine), original "Santana" LT85 gearbox

10th Aug 2024 2:56pm

falkster

Member Since: 12 Nov 2023
Location: Sutton Coldfield
Posts: 64

as a first step, I cleaned the IAV and this seems to have sorted it but I will assess in real driving conditions on Monday when I go to work to see if i need further looking into the throttle body as well.

To be honest, with this being on the intake side of the engine, how comes that there is quite a bit of black gunk on the IAV? I would have expected the air coming into the system being rather clean after the air filter...?

1988 Land Rover OneTen County SW, 3.9l Petrol V8 (serpentine), original "Santana" LT85 gearbox

11th Aug 2024 11:17am

diesel_jim

Member Since: 13 Oct 2008
Location: hiding
Posts: 6092

Also is the speed transducer connected at the transfer box? this raises the idle a bit when you stop so it doesn't cut out.

12th Aug 2024 6:31am

falkster

Member Since: 12 Nov 2023
Location: Sutton Coldfield
Posts: 64

diesel_jim wrote:
Also is the speed transducer connected at the transfer box? this raises the idle a bit when you stop so it doesn't cut out.

erm...?? time for some research. Never heard of a speed transducer to be honest, but I will find out.

1988 Land Rover OneTen County SW, 3.9l Petrol V8 (serpentine), original "Santana" LT85 gearbox

12th Aug 2024 2:48pm

Devon-Rover

Member Since: 22 Jan 2015
Location: South Devon
Posts: 916

2014 Defender 110 Puma 2.2 USW Aintree Green

The Idle raise with the speed transducer was an Auto thing to prevent stalling when sat in D against the torque convertor.

As with all things petrol start with the base ignition timing. and that the ignition system is sound.

Base idle can be checked with as per the manual.

the IACV can play up, on a cold start does it do the normal high revs then steps down in stages as the engine goes through it's initial warm up?
If it does then it's probably working fine.

The plenum chamber can warp and cause an air leak this will effect the idle running as unmetered air is getting in.
Spaying the joints when the engine is running with brake cleaner will show if anything is leaking.

I'd start there before getting deeper. IMO i've found that Ignition gremlins can upset things enough that people just wind the idle speed up to cover them as the extra ignition timing comes in. Also can be found on Fb, Ytube, Insta & Twitter @4WDSouthwest

12th Aug 2024 5:18pm

falkster

Member Since: 12 Nov 2023
Location: Sutton Coldfield
Posts: 64

Devon_Rover wrote:
The Idle raise with the speed transducer was an Auto thing to prevent stalling when sat in D against the torque convertor.

Meaning a manual gearbox wouldn't have a transducer?

Quote:
As with all things petrol start with the base ignition timing. and that the ignition system is sound.

Base idle can be checked with as per the manual.

I'll have to look that up. Might well be that the previous owner's mechanic has messed with this as they were struggling with getting the emissions right for the MoT before it was sold to me.

Quote:
the IACV can play up, on a cold start does it do the normal high revs then steps down in stages as the engine goes through it's initial warm up?
If it does then it's probably working fine.

Interestingly it does not do this every time but often enough for me to have thought in the past why it is doing this. So thanks for the clarification. Learning a lot of new things at this stage.

Quote:
The plenum chamber can warp and cause an air leak this will effect the idle running as unmetered air is getting in.
Spaying the joints when the engine is running with brake cleaner will show if anything is leaking.

I'll check that. still have half a can of brake cleaner buried somewhere in the garage.

Quote:
I'd start there before getting deeper. IMO i've found that Ignition gremlins can upset things enough that people just wind the idle speed up to cover them as the extra ignition timing comes in.

Thanks for that very nicely structured advice to fault finding.

1988 Land Rover OneTen County SW, 3.9l Petrol V8 (serpentine), original "Santana" LT85 gearbox

14th Aug 2024 2:36pm

diesel_jim

Member Since: 13 Oct 2008
Location: hiding
Posts: 6092

I'm not sure about the American NAS 90 that had the manual box if they used the same 3.9 14CUX ECU (I think they did) so probably would have had the speed transducer.

My story.... I rebuilt a 110 V8 (carbs) and put in an ex rangerover 3.9 with the 14CUX. and an LT85 5 speed manual box.
drove great but every time I slowed to a junction I had to keep feathering the throttle to keep it going otherwise it would conk out

Discovered the speed transducer, fitted it, boom, perfect idle everytime.

It goes down by the gearbox. for the old type cable operated speedos (unlike the electronic Td5 type) there are 2 pieces of the cable. first is about a foot long, goes from the transfer box to the transducer, then the longer piece goes from transducer up to the speed head.

15th Aug 2024 7:39am

falkster

Member Since: 12 Nov 2023
Location: Sutton Coldfield
Posts: 64

more things to look out for...

First I am taking the 110 to Cornwall though.

After my return, your posts (and the other ones in the thread) gave me a bit of homework to do around the engine bay...

Thanks for that. The responses are much appreciated.

1988 Land Rover OneTen County SW, 3.9l Petrol V8 (serpentine), original "Santana" LT85 gearbox

15th Aug 2024 10:29am

diesel_jim

Member Since: 13 Oct 2008
Location: hiding
Posts: 6092

https://rimmerbros.com/Item--i-AMR3386P--S...gJCvPD_BwE

This ^^

15th Aug 2024 1:47pm

falkster

Member Since: 12 Nov 2023
Location: Sutton Coldfield
Posts: 64

thanks for the link.

1988 Land Rover OneTen County SW, 3.9l Petrol V8 (serpentine), original "Santana" LT85 gearbox

16th Aug 2024 9:01am

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