Newbie with disco II V8 problem

[beanmasteruk]

Got a 99 Series 2 disco V8 with LPG conversion. Failed it's MOT on emissions,CO2 is 1.211.

doesn't seem to matter if it is tested running on the gas or petrol, pretty much the same reading. The LPG was fitted around 4 yrs ago and has never ben an issue. (LPGA approved inst)

MOT guy says change O2 sensors. Anyone agree or otherwise?

failure sheet says Co2 0.00%, HC 3 ppm then a strange little symbol(?) says 1.221

[Ally V8]

You could waste money on Oxygen sensors that are OK anyway,or you could get it diagnosed properly,the levels are not far off so it could be just an arcing plug lead causing a misfire etc.Is there a decent independant near you ?

[Aragorn]

Ally: umm what?

The CO and hydro carbons are perfect which means the cats are working fine.

The funny symbol (λ) is the "Lambda" symbol, and looking at those results i'd agree with what the MOT tester says, replace the sensors.

[Ally V8]

Exactly - Um what ? Now I've sat down and re-read the original post I'm just as confused.Whilst gas analyzers read Co2,it is not used in the test. Co,Hc and Lambda are,in the first instance he states Co2 as 1.211 and later on says 0.00%.

So assuming he means Co and not Co2 which reading is right ?

Now I am not eating lunch,ordering parts and answering the phone at the same time I'll rethink my answer.

The two figures which are not repeated and make sense are the HC and Lambda readings.For the Hc to be at 3ppm the engine is obviously burning all its fuel and the Cats finishing off whats left - so its running closed loop off the Oxy sensors.For such a high Lambda reading there is a post Cat air leak present,split silencer seam,leak at a joint etc.

Reason I say this is that if either oxy sensor was inactive - reading 0v all the time the engine ecu would be pushing the injector pulse width all the way wide to get a rich,(0.8v)signal.The Co and Hc readings would soar and the Lambda reading would come down rich - less than 1.

So my flippant comment about the plug lead is wrong in this case,but the oxy sensors are doing their job.Besides two mins work with a decent multimeter connected to the black output wire would soon prove if they were cycling correctly.

It really pi--es me off when expensive sensors containing precious metals are just cast away when an emissions test is failed,with little or no thought - with even less thought given to testing them.

[Aragorn]

He states that Lambda is at 1.211 later on in the post...

If the lambda value is above 1, the engine is running lean.

The HC and CO values are fine, as the engine IS burning all the fuel its being fed.

Either theres an exhaust leak allowing air into the exhaust system, or the engine itself is producing the lean mix, the figure doesnt tell you one or the other for sure.

If its the engine, the lambdas arent doing their job properly, because they should be detecting that its lean and the ECU would then richen things up, UNLESS the lambda is reporting everything is fine, they can get sluggish when old, so the engine is happy, or the lambda is reporting rich, causing the management to lean off the mix as much as it can. On the ECU's have had dealings with in the past (VAG and Vauxhall) the ECU would flag a fault for the latter (reaching its maximum lambda adjustment) but not the former, as it thinks everything is fine.

So ye he should be checking for leaks first, but if its leak-free the lambdas are the next port of call, unless he fancies running some diagnostics on the engine to see if its saying anything is wrong.

[BogMonster]

God I love living in a country with no emissions regs

I don't understand any of the above, I unplugged the immersion heaters in my exhaust pipe the first week I got my Discovery seven years ago and nobody gives a flying F

[Ally V8]

He states that Lambda is at 1.211 later on in the post...

If the lambda value is above 1, the engine is running lean.

The HC and CO values are fine, as the engine IS burning all the fuel its being fed.

Either theres an exhaust leak allowing air into the exhaust system, or the engine itself is producing the lean mix, the figure doesnt tell you one or the other for sure.

If its the engine, the lambdas arent doing their job properly, because they should be detecting that its lean and the ECU would then richen things up, UNLESS the lambda is reporting everything is fine, they can get sluggish when old, so the engine is happy, or the lambda is reporting rich, causing the management to lean off the mix as much as it can. On the ECU's have had dealings with in the past (VAG and Vauxhall) the ECU would flag a fault for the latter (reaching its maximum lambda adjustment) but not the former, as it thinks everything is fine.

So ye he should be checking for leaks first, but if its leak-free the lambdas are the next port of call, unless he fancies running some diagnostics on the engine to see if its saying anything is wrong.

Thats where you are missing the point,if the engine was running lean enough to give a true Lambda reading as high as 1.211 - ie no exhaust leak,no matter if the oxygen sensors were working or not the engine ecu would be seeing a 0v output from them and drive the injector pulse width all the way wide - within seconds.Immediately the Lambda would drop like a stone and the sensors,(if working)would go rich,0.8v to get it cycling ,(And give a decent Lambda reading)or stay at 0v,(not working) and end up with a black smoking rich thing.So the fact that the exhaust is giving decent Co/Hcreadings tell that its running properly down as far as the oxy sensors,but pulling air afterwards.