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westonben

3.9 Bad Idle

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It's an old one I know, but I'm at my wits end with it.

I've got a gulf spec, 3.9 V8 EFi 1999 Series 1 Auto (non Cat). I've had a 6 month long-running problem with the idle being erratic (pull up at lights to have the revs dip, and usually recovery, also, low speed driving sometimes a problem with low revs.

It's got the Lucas 14CUX hot-wire and now most of that is new.....

I have replaced:

Fuel pump

Fuel Filter

Leads - Magnecore

Rotor Arm, Dizzy Cap, - Genuine

TPS

IAC (Stepper)

MAF

Coolant Temp Sensor

I'm sick to death with the problem and just can't diagnose the problem properly here. I'm in Qatar and the local main dealer hasn't been able to fix the problem, they have reset everything and whilst it's certainly running smoothly, it just doesn't always idle right.

My last remaining items to try (in order) are:

Fuel Temp Sensor

Coil

VSS

Anyone else got any other ideas before I drive it into the sea?

Thanks

Ben

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I don't have time to look for a link but I think there is V8 diagnostic info in our Tech Archive forum

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I don't have time to look for a link but I think there is V8 diagnostic info in our Tech Archive forum

Yeah, been reading it with interest. Will have a tinker over the w/e

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Sounds like ignition. Wires, plugs to start with, coil? Oh, sorry, no coil on EFI as far as I know.

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One of the problems I had when chasing this problem was the Genuine Parts dizzy cap breaking down through the insulation at the clip recess & shorting to the rear spring clip. Weird but true.

:blink:

Quick fix was clean the area with electronic contact cleaner & give it 3 coats of clear nail polish inside & out.

That fixed the mofo. :P There was no visible blemish even through a magnifying glass.

Eventually ( read a long time later), the MAF was replaced & it's been 100% since.

I suggest you pull leads one at a time & see if that breaks down near it's own dizzy terminal. That's how mine was found & with it normally connected, it looked fine.

The other thing I found that helped a lot, was to remove & clean the plug pins at the MAF. I also gave each one a slight twist, checked behind the rubber boot for loose or broken wires & ran it again. Beautiful. :P

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Have you replaced the plugs? You dont mention it.

Yes, sorry, new plugs too.

Have checked the MAF connections and had it recently tuned for emmissions (helped).

No Coil??

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Sounds like ignition. Wires, plugs to start with, coil? Oh, sorry, no coil on EFI as far as I know.

Hi fern, the 3.9 has a coil. :P

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Hi,

Sorry no easy answers, but a couple of extra questions if you don't mind:

Does this happen every time or just randomly?

Does it happen more when hot than cold or about the same?

Does it still happen if the air-con is switched off?

To me this sounds like the stepper is not functioning correctly (either because it's not working or because it's not being told to do anything). I know you have replaced it, but I am still drawn to this, for the following reasons: Firstly it is the stepper which controls tickover speed... And secondly, when performing the procedure for setting the base-idle (which effectively removes the stepper from the system) and therefore preventing the ECU controlling the tickover, then you will get very much the same effect that you describe.

And so to another question: When started, does it rev to (say) 2000rpm and then slowly decline (in steps) to normal tickover? If it does then we know that the stepper is operational.

It might be worth having a look at all of the vacuum hoses - I previously had a problem with mine that was down to a split vacuum hose and coincidentally a loose sensor connection - which the advice given above, cleaning and SLIGHTLY twisting contact pins, eventually fixed. Unfortunately I don't know which pin it was since I did them all at the same time :( Oh, and I did all the pins on the ECU and on all of the various sensors too. Incidentally - prior to this I had been through the whole of the diagnostic described in the tech archive and all the readings were ok.

TTFN

Roger

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Also wondering if you have dodgy Vehicle Speed Sensor, but that would probably show up as a speedo fault as well. When slowing down, that puts a signal through to the Idle Control Valve to keep the revs up.

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Does it do it ONLY as you slow down and stop to allow tickover, and also the harder you deaccelerate the worse it can be ?

If so suspect Rioad speed transducer and or air leak plenum area, one way re air leak is to CAREFULLY squirt WD40 around the joints of the plenum / inlet manifold / trumpet to inlet base on tickover if the tickover raises considerably you've a air leak.

Nige

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Hi,

I have the exact same model and the same fault, but its cured now hopefully.

With the A/C on, reversing out of a tight space it would die, but if the A/C cycled the revs would hit 1500rpm, and then crash to 500rpm, just to stabilise.

I stripped all the vacuum / manifold hoses off, used loads of seafoam & carb cleaner, cleaned out the flame trap with petrol, and smeared all joints with RTV sealer to stop the fine sand from getting in.

The PCV valve was stripped of the back of the chamber BUT be careful not to go at it hammer & tong style, spayed with carb cleaner, replaced with a new gasket & RTV.

The hose from the MAF to the throttle body, spayed inside the throttle body with carb cleaner so the throttle assembly seats properly.

I did change the plugs, HT leads, fuel filter, put a K&N filter on, reset the ECU I was told by Al Tayer the local stealer, exercisers! To unplug the MAF leave it un plugged for 20 secs or more, turn ignition on, switch off & replace wires.

The only thing I never did, was to remove the injectors and have them serviced, I just filled up with a full tank of 98, and put two bottles of STP injector cleaner in, and went for a blast in the sand, it cured it!

As a back up check a couple of weeks ago I was losing water, so I carried out a head check, only to find the coupling on my heater matrix U/S, so I know the heads are good.

You’ll get there in the end, I’m sure it’s only a Land Rover!

Jim

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Thanks for the great suggestions, I've a few things to look at there. Should have mentioned I've been right round her with carb cleaner and found nothing to suspect vacuum.

Answers:

Does this happen every time or just randomly?

Sometimes I think it's fixed it runs so well, then it starts acting up.

Does it happen more when hot than cold or about the same?

Got to say more when hot

Does it still happen if the air-con is switched off?

Yes, but I can hear a clicking sometimes (relay) then the revs bounce up (650rpm) before slowly dropping down again (500rpm).

When started, does it rev to (say) 2000rpm and then slowly decline (in steps) to normal tickover?

Not every time. It sometimes starts and immediately dies......

... sounds like you're onto something

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Does it do it ONLY as you slow down and stop to allow tickover, and also the harder you deaccelerate the worse it can be ?

Yes. Once it's at idle it seems ok, just when slowing to a halt or going into reverse.

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QUOTE

When started, does it rev to (say) 2000rpm and then slowly decline (in steps) to normal tickover?

Not every time. It sometimes starts and immediately dies......

To me this is the tell-tale. Background: When you switch off the motor, the ECU sends a series of pulses to the stepper to open it right up ready for the next start, and then when you start it will close the valve in steps - each time assessing the stability of the tickover before closing it a bit further, until finally it gets to 700rpm.. I would assume from the above that yours is not always opening up properly. Normally this would be a dirty or sticking valve, a bad connection etc, but as you have already changed the valve, then I would suspect wiring. To the best of my knowledge this initial fast-idle is not dependent on temperature.

Hope this helps!

Roger

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I think you're onto it Roger.

Took it out last night and it was acting up horribly, so I got SWMBO to hold it running in Drive while I pulled the plug on the stepper motor.

Guess what?

Nothing changed. So in the past I have seen it retracting when turning the ignition to the on position, so it was working. I will do this check again. Also I've got the old stepper to try...

So, my question now is what (other than wiring) could be the fault? Presumably the stepper motor only starts to work on certain signals? VSS being one I imagine?

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I have a sneaking suspicion that if you pull the plug, the pintle remains in the position it is in as the power source has gone, so it may not necessarily indicate a lot yet.

I reckon this would have to be the biggest common headache on a V8i from what I have read & experienced. :unsure:

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Took it out last night and it was acting up horribly, so I got SWMBO to hold it running in Drive while I pulled the plug on the stepper motor.

Guess what?

Nothing changed. So in the past I have seen it retracting when turning the ignition to the on position, so it was working. I will do this check again. Also I've got the old stepper to try...

So, my question now is what (other than wiring) could be the fault? Presumably the stepper motor only starts to work on certain signals? VSS being one I imagine?

I have a sneaking suspicion that if you pull the plug, the pintle remains in the position it is in as the power source has gone, so it may not necessarily indicate a lot yet.
Yes, that's right /\

Additionally, something to watch out for - and I am not 100% sure on this, but I think that if you remove the connector from the stepper with the ignition on then the 14CUX will register a fault code and may even go into limp-home mode. Certainly if I am messing with any wiring to sensors etc I always finish up by disconnecting the battery (ignition off first!) for 30secs which re-sets any fault codes.

There is an interesting 'resource' at this web site, which is well worth working through, but which has the following interesting snippet on the initial start up sequence:

The sequence to restart the engine actually starts as you turn off the ignition switch. As the ignition voltage is removed from the ECU, the unit sends out a signal to the stepper motor attached to the plenum chamber to wind it fully backwards and allow maximum air into the plenum chamber. This can be heard as a buzz from the stepper motor as the engine dies. As the ignition voltage has now gone, the engine simply stops with the stepper motor in maximum air position.

On turning the ignition back on, a short pulse (about 1- 3 seconds) is sent to the fuel pump to pressurise the fuel rail. Once the starter motor starts to turn the engine, a 12v pulse is fed back to the ECU from the negative side of the coil as it the ignition amplifier switches. The ECU then turns on the fuel pump and energises the fuel relay that provides a fixed 12 volt supply to all the injectors. The transistors in the ECU starts to ground the injectors with a longer pulse than the normal idle pulse for about 3 seconds. This provides enough fuel to start the engine, combined with the stepper motor still being in its wide open position. Once the engine has fired, the air flow meter then takes over feeding the air flow volume back signal to the ECU and the injector pulse width is reduced to match the fuelling requirements for the engine at tick over. The stepper motor is also wound in to stabilise the idle at around 800 RPM. This system accounts for the short burst of higher RPM at tick over as the stepper goes from wide open to part closed during the start process.

One side effect of this system is if air leak develops any where in the inlet system, the engine will start and run for 3-4 seconds and then die. The initial over rich mixture will allow the engine to run, but once the air flow volume comes into play, (Now reduced because of the air leak) the injector pulse width is reduced to the point where the is insufficient fuel to keep the engine running, so it dies. The whole pipe work and breather system around the plenum chamber is pretty finely balanced and can be easily go out of tolerance should an air leak develop.

As far a I am aware the things that directly cause the ECU to tell the stepper to do something are:

1. Start-up sequence - see above.

2. Throttle movement/position - for example, if (in neutral) you rev it to 3000rpm and suddenly release the throttle, the stepper will control the fall of revs. There are probably other situations too.

3. Load signals - If the Air-con, heated screen etc are on the target idle speed is increased.

4. Vehicle speed - on a closed throttle when the vehicle speed and the revs are both above trigger levels, the stepper will close totally to give max engine braking, which would cause a stall if you dip the clutch, except for the fact that as the engine revs fall quickly the stepper will again kick in to control the fall of revs and bring the engine to a stable idle.

5. Ignition problems - If the tickover is found to be unstable (misfires are picked up by the ECU) then the tickover will be set higher using the stepper.

Additionally, the total range of sensor inputs are: air flow, fuel temperature, engine (water) temperature, throttle position, road speed and on catalytic cars a lambda input. Any of these in combination may affect what the stepper is told to do - so if the stepper is actually ok and it's a control problem, then you are back to systematically checking the sensor inputs as described in the Tech Archive.

Hope I am not just adding to the confusion here :(

Roger

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OK, good stuff. Already read that page on the 14CUX, it's an interesting read...

My thought is that if I pull the plug on the stepper at idle and there's no change then the idle wasn't doing much in the first place? So while no conclusive, still indicates to me that the stepper was not probably moving at idle.

Where's that multimeter?????......

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My thought is that if I pull the plug on the stepper at idle and there's no change then the idle wasn't doing much in the first place?

Not really, it just means that it won't be adjusting the idle when different loads are applied as it should -if you load the engine with it disconnected (fans, heated front/rear windscreen, put it in D with the hand brake on, AC on) it should drop and not recover. With it connected it should maintain the idle speed, or at least something close to it!

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Not really, it just means that it won't be adjusting the idle when different loads are applied as it should -if you load the engine with it disconnected (fans, heated front/rear windscreen, put it in D with the hand brake on, AC on) it should drop and not recover. With it connected it should maintain the idle speed, or at least something close to it!

I disagree.

If it's idling all over the place in with it plugged in and when unplugged it doesn't make any difference then the stepper wasn't doing much to the idle in the first place?

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westonben,

I had a not to disimilar problem with my efi the other week, in my case it was the speed transducer tha was at fault, the speedo completely failed on me, and while it was not working, my idle speed was eratic, and if I slowed down, the revs would dip so much, the car would stall...Once I replaced the speed transducer, it runs sweet as a nut now..the new clutch helps aswell :), but that's another story.

I will say this though, the speed transducer is an absolute pig to swap out on your back, i really needed my 2 year old to be handy with a hex key to get that job done, all in all, it's probably a 20 second job, but when you have to reach around the brake drum and the handbrake cable..it takes a lot longer :)

This may not be helpful, but it was a cure for my problems...oh, and here I had to get it from the main dealer, not even the landrover showroom/parts dept had them in..and it was expensive..for what it is.

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I disagree.

If it's idling all over the place in with it plugged in and when unplugged it doesn't make any difference then the stepper wasn't doing much to the idle in the first place?

Right, mis-reading things here... "doesn't make any difference" should be read as "doesn't make any improvement to the idle".....?

If the idle fluctuations continue with it unplugged, then yes it proves the stepper is probably not at fault, if the idle stabilises then clearly there is a problem with it.

If you run through what I posted above then you will get an idea of whethe the stepper is working to somthing close to correctly... which is what I was posting.

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Sorry - mis-post (the point I was going to make has already been covered)

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