dodgy lambda sensor

[v8bobber]

HI all,

I posted a question a while back about my 3.9 not running particularly well at idle. It is not as bad as a misfire but is an intermitent "roughness". On the advice of Fridge I have interrogated the Tech archive which led me to do all the diagnostic tests which were fine except for the one for the lambda sensor.

Further investigation into testing Lambda sensors have shown that the drivers side one is pulsing between 0.1v and 0.9v when putting a test meter between the signal wire (black) and battery earth, apparently as it should be. However, when I have done the same test on the passenger side sensor the reading seems constant at about 0.45 volts maybe dropping to 0.35 volts occasionally, not fluctuating between the above values as I am led to believe is correct.

I have also changed the plugs recently and they seemed quite wet as if to suggest that the engine is running a bit rich.

My question is, what are the symptoms of a failed / dodgy lambda sensor? Is it likely to be the cause of my slightly rough idle.

Oh, and I forgot to mention, the engine had also started "surging" on start up. ie, i start the engine and it runs fast and then goes down to near stall speed or sometimes infact does stall and then revs up again unti after about 10-15 seconds it finds a constant speed and it's then fine again after that.

Can I safely assume then that the lambda sensor is faulty or not, and if it is duff, would it account for the problems that I am having? What are the symptoms of a faulty lambda sensor anyway?

Hope someone can advise

Dave

[Highway_Star]

If it's giving a constant or near constant signal, then yes I would say it's goosed. I've had one go like that, IIRC it gave 0.45V DC no matter what.

As to the rough idle, to be honest I doubt that one Lambda probe out would make it that bad, after all EFi V8's without Lambda probes can idle perfectly well. Usually it shows up at MOT emissions time.

Have a close, and I mean close look for air leaks, especially around the MAF sensor, also whip the idle control valve out and test that it works, and give it, it's housing and pipework a thorough clean. The diagnostic tests, whilst thorough will not tell you about air leaks or a dodgy idle control valve. there is also throttle flap/lever position to check and adjust, and base idle speed/mixture.

One other thing, ignition! Check the amount of slop in the distributor, sometimes they wear so badly you get 'scatter timing' whihc doesn't do the idle any favours. Check also the operation of the advance retard weights and springs. Throwing a genuine parts cap and arm won't do it any harm either.

My own 3.9 (1990) started life with Lambdas, Cats & all that. I ditched the cats and probes, changed the tune resistor to the non cat version and it ran a lot happier. I do have a single Lambda probe fitted but that's for my LPG system. It actually runs cleaner without the cats etc than it did with, passing the MOT emissions no bother, as opposed to struggling every time.

Mine also suffered rough idling, stalling at junctions, surging on startup and all that. With every thing I did it got a little better, but the final cure was finding pinholes in the induction flexi pipe form the filter to the MAF. Wrapped said pipe in self amalgamating tape and it's been A1 ever since.

[v8bobber]

Thanks for that,

I had kind of dismissed air leaks as I assumed that this would have given a lean mixture. The plugs clearly show that it has been running rich for a while. Saying that though, it has only recently started doing the surging since I started fiddling with it!

How do i test for a dodgy idle control valve other than to take it out and turn the ignition on and off to make sure it isnt sticking?

Also, flap/lever position checking and adjusting, base idle speed/mixture, is that in the workshop manual somewhere. I haven't seen anything this morning after having a quick flick through. I seem to remember that there was something about the TPS but with big letters saying - DO NOT ADJUST next to it. lol.

Theres nothing to say that the previous owner has fiddled with it though i guess. THe vehicle is new to me and i am only getting the engine running sweet so it can go into my new project.

I can see what you are saying and it gives me some more avenues to explore which is great.

Hope you can assist in pointing me in the general direction of the above tests.

Thanks again

Dave

[Highway_Star]

Basically the idle control valve should sweep through it's travel and come to a datum rest point when you power up the ignition. They are notorious for sh1tting up and sticking, or the stepper motor packing up. Also the housing and pipework developing leaks and blockages.

Air leaks... It takes tiny, tiny leak to make the idle rough. You won't see a weak mixture as the Lambda probes would see the leaning off and the ECU would richen the mixture to compensate, which will alter the idle speed slightly, the ECU then trys to correct the idle speed, by doing that the mixture alters due to the air leak, the Lambda probe sees that, the ECU corrects.... See where this is going? Hence it's constantly fighting to maintain mixture and idle speed.

The stuff about setting the throttle levers, base mixture, idle speed etc is in the Land Rover workshop manual, I don't think it's in the Haynes.

A couple of other things spring to mind too.

TPS setting, if this is off the ECU won't know when the throttle is fully closed.

Speed transducer. Some of them have a fully electronic speedo, some have a mechanical speedo, however all 3.9 Hot wire EFi vehicles have a speed transducer, it tells the ECU when the car is moving or not, so it goes in or out of idle control mode. If the speed transducer isn't working the ECU will not know the car is stationary or moving, IIRC it defaults out of idle mode....... There's a simple test procedure in the book.

Engine breathers & flame traps, these need to be clean and clear.

[FridgeFreezer]

Never underestimate the fussiness of Hotwire.

[cooper]

i'd have to agree with looking for air leaks, also idle control vales are noutorious for causing irractic idles, as this happened on my omega. best way to tell if theres a fault with your oxygen sensor is to take it to a garage and ask them to check the emmisions for you, as this will then give you a definitive answer, also are there any leaks in the exhaust system as this can cause uneven idle

Cheers

Shaun

[FridgeFreezer]

best way to tell if theres a fault with your oxygen sensor is to take it to a garage and ask them to check the emmisions for you, as this will then give you a definitive answer

I think you might hit a problem there - the O2 sensors are per bank yet the emissions from the tailpipe are the combination of the two banks, so the good one will mask the bad to a degree.

Also I'm not really sure what the garage are going to tell you that's of any use, you're better off with a multimeter and a brain.

[cooper]

I think you might hit a problem there - the O2 sensors are per bank yet the emissions from the tailpipe are the combination of the two banks, so the good one will mask the bad to a degree.

Also I'm not really sure what the garage are going to tell you that's of any use, you're better off with a multimeter and a brain.

True but due to the information you have to put into the computer it knows that its a V8 and will to a degree compensate for the fact of there being 2 oxygen sensors so should still show up

[FridgeFreezer]

Sorry, I can't guess your level of experience here so I have to ask exactly what computer you're talking about and what readings you're hoping to get, and what you're going to do with that information?

I would say the initial test with a multimeter is fairly conclusive that one sensor is unresponsive. I'd try sourcing a non-cat tune resistor to see if that makes it behave, total outlay maybe 5p max.

[cooper]

Sorry, I can't guess your level of experience here so I have to ask exactly what computer you're talking about and what readings you're hoping to get, and what you're going to do with that information?

I would say the initial test with a multimeter is fairly conclusive that one sensor is unresponsive. I'd try sourcing a non-cat tune resistor to see if that makes it behave, total outlay maybe 5p max.

i work in a garage and use the ministry approved emmisions machines for mot purposes and they give lambda readings as part of a mot cat test, also co2 hcppm at idle fast idle and 2nd fast idle

P.S. Sorry for the high jack V8bobber

[FridgeFreezer]

OK, can you tell me how paying a garage to read the lambda and HC from the exhaust is going to help diagnose a faulty lambda sensor when he's already measured the two outputs with a multimeter and can see one sensor not responding?

If it were a bank fault, EG stuck injector(s) or driver fault, I would expect the reading to rail either low or high. Sticking in the middle is incredibly unlikely to be anything but a duff sensor.

[v8bobber]

Right this is where I am so far.

I am not in a position to double check the possibly dodgy lambda sensor at the moment as I have decided on a minor stripdown in order check the injectors which is my job for the morning.

I think the IAC valve may be playing silly buggers as I took it out and cleaned it and put it back and seemed much better. I was under the impression that with the thing out, and the ignition turned on and off the little plunger should move in or out but just stayed where it was, however when I turned the ignition off the thing just kind of vibrated, so not sure if thats working properly or not. It's been 15 years since i've been involved with petrol engines!

Anyhoo, I spent an hour checking all the hoses and there aren't any leaks. I was very thorough with regards to that, there was one tiny split on the out casing of the hose that runs between the breather and the plenum chamber but did not appear to have gone through, plenty of tape has sorted that though as a precaution.

Speed transducer has been checked as it is part of the 24 stage diagnostic test as outlined in the workshop manual.

I have the injector rail out and will be testing the injectors shortly, then it will be a case of putting everything back together and testing that lambda sensor again to check that it is faulty. I happened to speak to a mate of mine last night who is in my 4x4 club and works in a garage and tells me that he has seen a couple of 3.9s with a slightly lumpy idle which have been down to a dodgy lambda sensor.

Highway, I hadn't thought about the engine breathers, I will take those off and clean them while i'm doing the injector tests. I'll post a picture later if I can work out how.

Cheers,

Dave

[v8bobber]

further update.

Injectors seem to be fine. Interesting to watch them working. Must have certainly flushed em out a bit anyway!

I've just had a thought, how is it possible to get rid of the lambda sensors bearing in mind that this engine will be going in an offroad only trialler?

Dave

[Pete Attryde]

further update.

Injectors seem to be fine. Interesting to watch them working. Must have certainly flushed em out a bit anyway!

I've just had a thought, how is it possible to get rid of the lambda sensors bearing in mind that this engine will be going in an offroad only trialler?

Dave

Fridge supplied the solution earlier. Sorry I don't know the resistor value or part number.

Pete.

<snip> I'd try sourcing a non-cat tune resistor to see if that makes it behave, total outlay maybe 5p max.<snip>

[SteveRK]

If the voltage on the suspect Lambda is fixed at 0.45v and does not fluctuate at all then this would strongly suggest a faulty Lambda sensor as the design of the system is such that the mixture is constantly adjusted between rich/lean due to the time lag between the sensor and ECUs ability to correct the mixture, thereby resulting in the Lambda signal swinging between 0 - 0.9volts every half second or so.

Rough idle is subjective but my thoughts on the Lucas ECU is that the dwell time is rather long resulting in an idle that whilst smooth constantly fluctuates (albeit very very subtly) as the ECU corrects the Lambda inputs.

I initially replaced with cheap Lambdas (sold as orignal manufacture without plugs on the end) from USA and then replaced with UK sourced OEM types with correct plugs and these were much better (and more expensive)

[v8bobber]

Well it seems I am back to just about square one.

I have had the injectors out from the block and have fired them "manually" from the ecu, they all seem to be giving the correct flow rate and also the pattern seems to be alright.

Air leaks seem to be non-existent and everything is alot cleaner than it was before.

Which brings me back to the lambda sensor malarky. I am hearing hints that the lambda sensors can be by-passed so I don't need them. I would like to find this out sooner rather than later, especially as i'm looking at about 60quid for a new one.

Cost wouldn't normally be a problem but it might be excessive seeing as the machine is only going to be used off road

So Fridge, if you're listening, or anyone else for that matter, knows how to do away with the lambda sensors and cares to let me know how - i would be grateful. I have included some pics, but am aware that i'm probably going to get shouted at as they may be a bit large.

Dave

This is what the 3.9 will end up in

in bits

testing

back together

[Highway_Star]

All you have to do is locate the tune resistor in the loom and replace it with a non-cat one. Part numbers are in the green bible. The ECU will then ignore the Lambda probes. You then need to get the engine set up on Sun tuner or similar.

The IAC valve should sweep through it's range then come to datum when powered up/off. So if all it's doing is shoogling than that's not right, and won't be helping your idle any.

That injector picture looks more like a squirt than a spray to me. I'm more used to seeing them give a spray, like in this photo http://www.airpowersystems.com/falcon/inje..._movie_logo.jpg

[v8bobber]

I did think about the spray pattern at the time but thought the pattern was this way because the injector was permanently "open" ie not pulsing for however many milliseconds. Perhaps I just don't want to believe that as all 8 were the same. I don't fancy purchasing 8 new injectors!

I can safely assume that the IAC valve is cream crackered what with that and the duff lambda sensor I'm surprised that the bloody thing is working at all and should be happy that the thing idles to some degree at least. It's trial day tomorrow and I will speak to a mate of mine who did away with his lambda sensors. I'll ask him how he did it, and if it's not too much hassle I'll take that route. I always like to stay standard if possible but the exhausts on the trialler are tubular ones and I would have to fabricate the mountings for them which is asking for trouble!

I will have to wait a while now though as my P38 has decided that it's time to play silly buggers so I won't be taking the 3.9 out of the disco just yet, incase I have to move it sharpish to get the work done on the road truck. Oh Joy!!

Thanks for your help so far by the way!

Dave

[robp]

Not sure where the tune resistor is situated on the range rover loom but on the disco there is a two wire plug exiting the loom just before the ecu.

I was suffering exactly the same problems on my v8 disco as one of the lambdas was knackered.

The original cat tune is I believe 2300 ohm (not sure if that the correct spelling!) I replaced it with a 470ohm 1w resistor costing 15p from maplins as this is the non cat tune version. Result....ecu no longer looks for lambdas and it runs a whole lot smoother. Got to be worth 15p to try it.

Rob

[v8bobber]

Not sure where the tune resistor is situated on the range rover loom but on the disco there is a two wire plug exiting the loom just before the ecu.

I was suffering exactly the same problems on my v8 disco as one of the lambdas was knackered.

The original cat tune is I believe 2300 ohm (not sure if that the correct spelling!) I replaced it with a 470ohm 1w resistor costing 15p from maplins as this is the non cat tune version. Result....ecu no longer looks for lambdas and it runs a whole lot smoother. Got to be worth 15p to try it.

Rob

Thanks Rob,

As luck would have it, its a disco that I have too. I used the Range Rover part of the forum as I thought that there would be more 3.9 RR owners than 3.9 Disco owners but it seems that it's a disco owner that has solved the problem for me - that'll learn me!

I will go and have a look in the morning and see if I can find it. Can it really be that easy?

What symptoms was your vehicle having. It kind of runs ok but it just seems to have an intermittent and random miss and perhaps fluffs on acceleration now and again. This has all been done in the workshop unfortunately as there is no longer an MOT etc on it.

I am a bit wary of petrol electrics but i am learning slowly.

Dave

[FridgeFreezer]

Just replace the tune resistor with the correct value, as said it'll cost you about 15p from maplins and then the ECU will ignore the lambda sensors.

FFS don't start with injector spray pattern hypochondria or any other over-analytical stuff until you're sorted the very basic and almost dead cert that is the dodgy lambda sensor. EFI isn't complicated but I'm always impressed how difficult some people can make it Also, connecting the injector straight to 12v is as likely cause a fault as find one, they are not designed to be run at more than about 85% duty cycle. Different injectors have different spray patterns depending on the engine configuration, as long as fuel's squirting out with gusto and not leaking when it's off, I'd call it good and stop faffing about with things you don't need to.

[v8bobber]

Just replace the tune resistor with the correct value, as said it'll cost you about 15p from maplins and then the ECU will ignore the lambda sensors.

FFS don't start with injector spray pattern hypochondria or any other over-analytical stuff until you're sorted the very basic and almost dead cert that is the dodgy lambda sensor. EFI isn't complicated but I'm always impressed how difficult some people can make it Also, connecting the injector straight to 12v is as likely cause a fault as find one, they are not designed to be run at more than about 85% duty cycle. Different injectors have different spray patterns depending on the engine configuration, as long as fuel's squirting out with gusto and not leaking when it's off, I'd call it good and stop faffing about with things you don't need to.

Thanks fridge, i'm sure you meant to sound helpful in that post - but failed. I'm sure that you know loads about fuel injection systems, good for you. But it doesn't make you the owner of all rover V8 injection systems either. If I want to run my injectors as per the Land Rover workshop manual then I will, and I will do anything else that I feel like with the vehicles I own, to learn about them or any other reason, that I see fit.

I appreciate your technical advice, I do not appreciate your attitude. Do not believe that you are in any position to tell me, or anybody else what to do.

Not amused to say the least, and will cut this post short before I say something out of character..............

[FridgeFreezer]

You're welcome

I don't mean to sound jaded and cynical, but I am so that's how it comes across.

[paintman]

Just wonder if I'm missing something here?

Asks for advice to fix a fault which is almost certainly the faulty Lambda sensor but won't replace it or fit the resistor as advised.

Instead fiddles about with the injectors in the hope it will make the problem go away.

And then throws a strop.

Weird.

[DEANO3528]

Ok guys, thanks for trying to help.

I'll lock this one before it degrades into a slanging match.