MOT failure: emissions

[Andychim400]

Hi
My 1999 400 has just failed MOT on emissions. Sailed thru last yr at the same testing station, 3k miles later and no changes to the car other than a new coil and it's failed as follows:

Fast Idle @ 2500 - 3000 rpm
CO : <= 0.300% Actual 3.923% Fail
Lambda: 0.970 - 1.030 Actual 0.870 Fail
HC: Pass

Second Fast Idle @ 2500 3000 rpm
CO : <= 0.300% Actual 3.779% Fail
Lambda: 0.970 - 1.030 Actual 0.874 Fail
HC: Pass

Natural Idle
CC : <=0.500% Actual 3.261% Fail

The car is running well, no misfires....I was actually just thinking how well it was running with the new coil...pulls like a train.

Any ideas? Possible lambda sensors?

It's in HHC next week for a service, just wondered what the likely outcome will be...easy on the wallet or pants down?

Cheers
Andy

[rev-erend]

As the lambda is mis reading - it looks like it's shot.

[That Daddy]

Looks like Lambda probe/s with those readings,check coolant sensor 1st though(its expensive if you get it wrong)

[blitzracing]

rev-erend said:

As the lambda is mis reading - it looks like it's shot.

Be careful about the Lambda reading from a 4 gas analyser. Its not a just a direct result of the lambda probe output or the excess oxygen in the exhaust, but takes all the gasses into account to produce a figure that represents a perfect burn. Having said that however at its most basic, its a ratio figure of how much oxygen is left in the exhaust after the burn has taken place. This is a ratio of the remaining oxygen left against the value for a perfect value of 14.7:1 air/fuel that gives Lambda 1. This is why you dont have a parts per million or % value attached to it as its a ratio. More to the point the CO value is high, so the engine is likely overfuelling. It could be the ECU is in get you home mode due to a sensor failure (you dont really notice often). You need to take some voltage readings from the back of the lambda probes on both sides (white and black wires) and check they are constantly switching between about .3 and .7 volts (can be a bit higher.) If they are stuck at .7 or more then it shows over rich. If it does not switch at all, it could be a failed probe.


Mark

Edited by blitzracing on Wednesday 28th May 13:40

Edited by blitzracing on Wednesday 28th May 14:55

Edited by blitzracing on Thursday 29th May 09:21

[blitzracing]

Just been doing my homework. A low Lambda reading from an MOT means over rich and a higher number (greater than1) indicates a lean condition.

12.6:1 - max power - lambda of 0.86
14.7:1 - ideal combustion (chemically) - lambda of 1.00
15.4:1 - max economy - lambda of 1.05


However, the voltage from the back of probe does in fact rise when more fuel is in the mixture. (in reverse to the MOT test results).

So a low or no voltage from a probe, will give a high MOT Lambda and a high CO reading as the ECU tries to correct the condition.

[BryanWilton]

If you need Lambda sensors, go on to Ebay.com (usa site,not uk site)
Do a search for - land rover oxygen sensor 1993-1997 defender

Company that sells them is called Global-Automotive, priced at $28.50 each
That works out at around $70 for 2 with postage to UK, thats about £35 total, which is an absolute bargain!

I have bought these myself & they are genuine NTK & are the right size, fitted mine about 12 months ago & all is well
Only small downside is that they don't come with the plug on them

You just need to cut your old cables & through crimp them together with a crimp tool (think the through crimps come with them)
Take about 7-10 days to arrive

Hope this helps....

[Andychim400]

Thanks for the replies chaps, really helpful and much appreciated.

Like I said it's in HHC on Monday so we have something to go on.

Cheers!

[Andychim400]

blitzracing said:

Just been doing my homework. A low Lambda reading from an MOT means over rich and a higher number (greater than1) indicates a lean condition.

12.6:1 - max power - lambda of 0.86
14.7:1 - ideal combustion (chemically) - lambda of 1.00
15.4:1 - max economy - lambda of 1.05


However, the voltage from the back of probe does in fact rise when more fuel is in the mixture. (in reverse to the MOT test results).

So a low or no voltage from a probe, will give a high MOT Lambda and a high CO reading as the ECU tries to correct the condition.

New Lamda sensor fitted yesterday (only one had failed) and it flew through the emissions re-test.

Thanks for the advice.

Cheers
Andy

[Earl Grey]

Andychim400 said:

blitzracing said:

Just been doing my homework. A low Lambda reading from an MOT means over rich and a higher number (greater than1) indicates a lean condition.

12.6:1 - max power - lambda of 0.86
14.7:1 - ideal combustion (chemically) - lambda of 1.00
15.4:1 - max economy - lambda of 1.05


However, the voltage from the back of probe does in fact rise when more fuel is in the mixture. (in reverse to the MOT test results).

So a low or no voltage from a probe, will give a high MOT Lambda and a high CO reading as the ECU tries to correct the condition.

New Lamda sensor fitted yesterday (only one had failed) and it flew through the emissions re-test.

Thanks for the advice.

Cheers
Andy

I have the same problem, what was the buggerd sensor reading ? I have 0.7 v on one side and 1.10v on the other what one would you say need replacing ?

[blitzracing]

That means a very rich mixture, that the ECU cant control. If you can get some air into the plenum (take a vacume pipe off or remove a spare blanking plug) you might see the reading drop to show the sensors are still working,(its likely to rev high as you do) but you really need to borrow a fault code reader to see what ECU input has gone wrong,( like the temperature sensor is open circuit). It might be in fall back mode, but 1.2 volts is very high, even for that.

Mark