3.5 V8 Disco cuts out/dies when stopping at traffic lights etc...?

Hello.

Yeah, so this has happened once or twice over the past few weeks, when i've been going to offroad days (the only road mileage it really does) apart from tonight when i went out for a 50 mile round trip and it cut out every time i slowed down.

So, it's like this:

Driving along merrily, making excellent power and not a hint of a misfire or splutter.

Slow down in gear for traffic lights, dip clutch and engine will simply fade away unles you catch it with the throttle. So then change down and roll in gear a little more, and then dip clutch to stop, and again, without fail, every time, the engine would silently stop firing and just 'turn itself off'

if you give it a few blips just as you're about to stop, it seems to be able to keep running, and return to idle.

it has recent plugs, excellent leads, and a recent cap and rotor arm. i have spent many many hours waterproofing the ignition system, and with the combination of quality parts and newness, it works impeccably, even underwater.

the only thing which has changed is that i have been driving the car around for the past few weeks with a snapped speedo cable, so the speed transducer for the ecu was not registering that the car was moving, which seemed to result in a lack of power. i have now fixed this just yesterday, so i wonder if that is making a difference?

it only seems to have this symptom when hot by the way.

SO, any ideas?

It has a K&N oiled filter in it, the maf was recently cleaned though, and it ran well just 2 weeks ago.

any one else experienced this???

thanks, sam

Mr Noisy said:

the only thing which has changed is that i have been driving the car around for the past few weeks with a snapped speedo cable, so the speed transducer for the ecu was not registering that the car was moving, which seemed to result in a lack of power. i have now fixed this just yesterday, so i wonder if that is making a difference?

it only seems to have this symptom when hot by the way.



thanks, sam

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Check and clean your stepper motor. :searchlenty on stepper.
Having no speed input might affect the idle because as far asI am aware, the speed input tells the ecu to turn off idle stabilisation so driving around with no speed input definately causes an efi fault and possibly, the engine quits before the stepper motor can be wound to a position where it can do its' job.

Get a new speed transducer!

Thanks for your help kev!

Thing is, car is worse now the speed transducer is "working" than when it was not being driven.

I rebuilt the transducer and spun it with a drill and it did output a voltage, so I thought happy days.

But yeah it is like the idle control is not activating early enough.

I'll remove the stepper motor, I wonder what I'll find!!

where can I buy a decent transducer or stepper? Are any patterns available?

Thanks, sam

My old 3.5 didn't have a stepper motor but the 3.9 has, but when the 3.5 engine had a similar problem (more fun with an auto) I removed the cankcase breather and pipe and it was full of muck, cleaned it with petrol and refitted and all was ok, you may also have a foam filter on the other rocker cover that may requiring renewing.
I now clean the 3.9 breather at every service so no engine stopping probs now.

Mr Noisy said:

I'll remove the stepper motor, I wonder what I'll find!!

where can I buy a decent transducer or stepper? Are any patterns available?

Thanks, sam

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Stepper probably just needs cleaning. I am fairly sure if you do a search you will find a few posts about how to clean it. As for transducer, best bet is surf the net. As a new OEM part i think they are quite pricey. I also seem to think there are two or three different types. I know the one for an RRC was expensive so I got a second hand one about 18 months ago off e-bay for £10.00.

discool said:

My old 3.5 didn't have a stepper motor but the 3.9 has, but when the 3.5 engine had a similar problem (more fun with an auto) I removed the cankcase breather and pipe and it was full of muck, cleaned it with petrol and refitted and all was ok, you may also have a foam filter on the other rocker cover that may requiring renewing.
I now clean the 3.9 breather at every service so no engine stopping probs now.

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Sensible advice and it has to be said the breathers are often overlooked. Nice one.

Well, unfortunately chaps, I removed the rockers, plenum, and all related pipes etc a few months ago and removed 20 years of crud! Literally an inch thick in the rocker covers

anyway, I'm sure they will still be fine, so that's not conclusive!

Not yet checked the stepper motor... Tomorrow's job!

After I went for my drive last night I decided the transducer is no longer sticking cos the needleis smooth, I just hope it's working!

Anyway, thanks for your help guys, I'll try to update tomorrow!

Cheers

I would try the stepper first (dont twist the plunger when cleaning it or it will break off)but if it doesnt work I would change the throttle postions sensor.

Mine was always idling way too low and stalled occasionally , stuart couldnt find any faults with his computer and we changed almost everything by the I had had it for 6 months trying to sort that amoungst other things.

Anyway you may have seen my post about it playing up a few weeks ago and I had stuart down , we plugged in the computer and it was the throttle positon sensor, swapped it for a good used one and its running better than ever and no more stalling. We think it wasnt quite bad enough to register the fault untill now.

I think there only about £20 from the stealer so its worth a try!

Mr Noisy said:

Well, unfortunately chaps, I removed the rockers, plenum, and all related pipes etc a few months ago and removed 20 years of crud! Literally an inch thick in the rocker covers

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That doesn't sound too good! My Rangie is 19 years old and is clean as a whistle (O.K. so it's only done just coming up to 89k) but a build up of crud suggests servicing has been a little lacking.

Check and clean the engine breathers and for teh future make sure the oil and oil filter are changed very regularly say 3,000 - 5,000 miles depending on where and how its' driven.

I took the sump and rocker covers off mine and it was just as bad loads of crud and carbon buld up , its taken me 6 oil changes in a year to get it clean even after doing that.

Really annoys me when people dont change there oil - I can feel the engines pain!

befroe you spend a fortune on guessing and changin parts which don't have any influence start with cleaning the stepper motor reset the base idle and retry also check for air leaks and replace the cap leads and rotor as well clean the plug on the afm too!

road speed transducer only tells the ecu to switch off the idle circut this coupled with the postion switch work as a pair if one fails it just defaults but as fett found out it will not log an ecu fault but throttle switchs are rare even rarer are the transducers!

get the ecu read

Hello,

first thing, yes, I was thinking about going on diagnostics to check the sensors, but who has the correct lead for a 14CUX ?!?!

Regards the crud, looking at receipts the engine has been regularly serviced, but not regular enough!

I have scraped the sump and rockers, the rest of the engine appeared dirty but not crudded when i did that, and now I change the oil as soon as it gets dirty, just to get things anywhere near normal again.

The thought of a strip and rebuild makes me very excited though

ANYWAY

yesterday i removed the stepper, and firstly, all the pipes and castings were quite clean!

I cleaned off the cone and lubes the stepper, then plugged it in and turned the ignition on, and it does appear to move - but I don't know what it should be doing!!

I have another 3.5 at the minute so I will borrow a stepper.

I did about 50 mile yesterday running about and again the engine is fine when cool, but as soon as warm it simply dies as the revs drop when slowing down from speed at the lights etc.

Really bad as I was heading home, i couldn't even get it to idle, i was having to keep catching It with the throttle or it would have cut out.

However, turn off and restart and it idles spot on, until you slow down again!!

Regards a TPS, I can try that aswell off the other car.

Thanks for your help guys!

If my stepper unit was making a buzzing noise at all, does that mean it could be faulty?!

Mr Noisy said:

Hello,

first thing, yes, I was thinking about going on diagnostics to check the sensors, but who has the correct lead for a 14CUX ?!?!

yesterday i removed the stepper, and firstly, all the pipes and castings were quite clean!

I cleaned off the cone and lubes the stepper, then plugged it in and turned the ignition on, and it does appear to move - but I don't know what it should be doing!!

I have another 3.5 at the minute so I will borrow a stepper.

I did about 50 mile yesterday running about and again the engine is fine when cool, but as soon as warm it simply dies as the revs drop when slowing down from speed at the lights etc.

Really bad as I was heading home, i couldn't even get it to idle, i was having to keep catching It with the throttle or it would have cut out.

However, turn off and restart and it idles spot on, until you slow down again!!

Regards a TPS, I can try that aswell off the other car.

Thanks for your help guys!

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O.K. you don't necessarily need a lead to check the EFI system. A good multi-meter will do all the sensors. If there is an ECU fault then you would need to plug it in - best done on the car!

The symptoms you describe could be a couple of things: Coolant thermistor, possibly vac advance, possibly stretched timing chain or possibly throttle potentiometer.

If you swap the throttle potentiometer make sure you set it correctly, I believe there is a small amount of adjustment but it needs to be right.

Have a check that you don't have any vacuum leaks as well.

You say the stepper is winding itself in/out; sounds O.K. but not sure about the buzzing noise - that doesn't sound quite right.

Thanks kev

I wish I had some spec on the sensor values I should be looking for!

Haynes manual diagram is good enough, then I could check all the circuits against spec, cos I don't know what range the transducer or TPS should be working in.

Of course it could just be an air leak!

I'm gonna keep at it, cheers!

I/stuart/ we didnt have to set up my tps its just goes straight in.

If you have a spare give it a try , you have nothing to loose!!!!

ok, i think i found the problem! actually im pretty sure, just not able to be sure until i can drive the car! (tomorrow)

anyway, found a great website called britishv8 which had a LOT of info about every part of the hotwire system, including info on test voltages and resistances.

so, proceeded with the multimeter to check everything at the ecu plug, everything came up nice and correct in range etc.

unfortunately i was not able to test the speed sensor, because the old classic which they used to demonstration was not fitted with one!! stroke of bad luck!

anyway, luckily i have this other disco here at the minute so i checked the speed sensor operation on that, which gave me a regular(ish) pulse at pin 6 on the ecu plug, at about 12 volts or so.

mine was much different, about 3 volts.

stripped mine down again and found a very poor solder joint which broke as soon a i moved in to inspect.

i resoldered that, added another jumper wire for another PCB track that looked a bit shaky, and put it back together, and now i have a regular(ish) 12v ish pulse at pin 6 when my bro rotates the speedo cable which we pulled out of th transfer case.

it doesnt seem to raise the idle at all when we spun up the speedo cable in a drill (which some people seem to think it should) but then there is a small possibility that it did raise a couple hundred but i just couldnt tell.

gonna check the base idle shortly, but for now im pretty confident!

thanks for your help again guys, and wish me luck for tomorrow!

p.s. checked lrseries.co.uk for prices, 100 quid for a new speed sensor!!

Mr Noisy said:

it doesnt seem to raise the idle at all when we spun up the speedo cable in a drill (which some people seem to think it should) but then there is a small possibility that it did raise a couple hundred but i just couldnt tell.

gonna check the base idle shortly, but for now im pretty confident!

thanks for your help again guys, and wish me luck for tomorrow!

p.s. checked lrseries.co.uk for prices, 100 quid for a new speed sensor!!

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The speed transducer itself doesn't raise or lower idle. It only tells the ecu to either switch idle stabilisation ON or OFF.

As for the price of a new one - Told ya look on e-bay get one for £10.00.

classic kev said:

The speed transducer itself doesn't raise or lower idle. It only tells the ecu to either switch idle stabilisation ON or OFF.

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But if it thinks that you are not moving and the thing is doing 2,000rpm, it may try slowing things down. Then when you get to the lights it has shut things right off. Just a theory.

The TPS only affects fueling when the throttle moves. A bit like the power pump on an old carbie. The ECU does not really care much about where the throttle is, only whether it is moving up or down.

I would think that the fault is with idle stepper motor or base idle.

Speed transducer solved it!