Failed the NCT on Lambda...

JohnCleary

I failed my NCT on the Lambda. The reading was 1.033, and the limit is 1.02

Now, the fella in the centre said the reason it failed was because theres 2 leaks in the exhaust. The first leak is between the manifold and the flexi pipe (Where 2 pipes join into 1). On a cold morning, you can see the smoke escaping out there.

The second one is the gasket between teh backbox and the mid-section.

I have since got a new gasket and got the other leak welded up, so the exhaust is leak free. My question is, would these leaks be the reason for the high Lambda reading, or could it be something else? I have the re-test next Sunday, on the last test the CO was normal (Meaning the CAT is fine I think) so I don't think the Lambda sensor is actually borked. It's a 1999 Rover 214. The car is on 93,000 miles... wondering if the Lambda sensor could still be good?

IDMUD

Yup, having a leak will mess with your o2 sensor. I'd say if you were right on the passing limit and you fixed the leaks you'll be fine.

JohnCleary

Would have thought so. I welded the leak up so fingers crossed it'll pass the re-test.. Then im selling it and getting a TDI

Gerry

I think it depends on how big the leaks were, and whether there are 2 o2 sensors on your car or one. Hopefully it will pass next time, if it doesn't, it is running a bit lean. Perhaps the fuel filter needs changing, or if the car has a lot of miles, fuel pump could be on the way out.

greglo23

the reason for this is the exhaust pulls in air as it passes the holes and changes the oxygen content in the exhaust which gives a failing lambda reading.

JohnCleary

greglo23 wrote:

the reason for this is the exhaust pulls in air as it passes the holes and changes the oxygen content in the exhaust which gives a failing lambda reading.

The weird thing is the Sensor is before the leaks! (It's at the manifold - The leaks are at the flexi pipe)

The fuel filter was changed not so long ago. Aswell was the fuel pump as it packed in.

I'm just going to put it through the re-test next week and hope for the best! If it still fails ill start looking into it further

As always, thanks for the comments

AMurphy

Exactly what units of measure are attached to that 1.02 limit?.

Is the O2 sensor even tested during the NCT?. It is not an easy item to test, so I wouldn't expect them to perform any test on it.

So I suspect the item it actually failed was emmissions, HC, or CO or some pollutant was registering too high.
Whether the O2 sensor is the cause of that or not is debatable.

I'd expect to fail any emmissions test is the pipe is leaking.

You do mention the O2 sensor is ahead of the leaks. The I'd expect the leaks not to be uspetting the sensor readings and either the O2 sensor is good and the tuning is bar or the sensor is bad and the emmissions are as you would expect with a failed sensor.
And a leak after the CAT is not going to effect anything other than noise.

The Onboard self tests should enable any mechanic to check the O2 sensor and if it is generating any fault conditions, like staying high/low too long, responding slowly, etc.
93K is in teh region when O2 sensors are most likely to fail. Typically failure occurs between 60 and 120K, depending on quality of fuel and tuning, etc.

20 Times 20 Times

JohnCleary wrote:

Would have thought so. I welded the leak up so fingers crossed it'll pass the re-test.. Then im selling it and getting a TDI

Im looking to sell my GT TDI in the next few weeks , will post pictures and details in the coming weeks.

JohnCleary

Sarge wrote:

Im looking to sell my GT TDI in the next few weeks , will post pictures and details in the coming weeks.

PM'ed you my email address

greglo23

as i said above an exhaust leak is enough to change the composition of the gases. it gives an elevated oxygen content even if the leak is in the back box. this gives a failing lambda reading. even a perfect o2 sensor will have no effect on that.

JohnCleary

greglo23 wrote:

as i said above an exhaust leak is enough to change the composition of the gases. it gives an elevated oxygen content even if the leak is in the back box. this gives a failing lambda reading. even a perfect o2 sensor will have no effect on that.

Aye, the lad in the NCT just said to get the leaks sorted and put it through again.

On the sheet it said it failed because of the leaks, and to repeat the emmissions test

AMurphy

greglo23 wrote:

as i said above an exhaust leak is enough to change the composition of the gases. it gives an elevated oxygen content even if the leak is in the back box. this gives a failing lambda reading. even a perfect o2 sensor will have no effect on that.

True, but only if the Leak is UPSTREAM from the sensor. or very close if downstream, like 2" at most.

JC indicated the leak is downstream of the sensor, though did not indicate how far the two were seperated.
And the leak between the CAT and silencer/back-box has didly squat effect on the sensor.
If that were the case, the opening at the very end (tailpipe), which is a very big leak woud prevent any O2 sensor from working correctly. And many cars have a vapor drip hole in the bottom of the back box, to prevent water collecting in it when cold and rusting them out.

The only way you'll get a downstream leak to effect the sensor is if there is backflow, and with your foot on the throttle, there is little is any backflow.
And with your foot off, the TPS would be informing the ECU the throttle is closed and pretty much ignore the condition.

greglo23

if you read my post you would see that i'm saying that the leak affects the gases flowing into the test equipment and not the o2 sensor. extra oxygen in the exhaust gases gives a failing reading on lambda. with a perfectly operating emissions system e.g. o2 sensor, catalytic converter, egr valve et al, any leak in the exhaust pipe will tend to draw in a small amount of ambient air and this, when mixed with the exhaust gases, changes the readings taken at the tailpipe by the test equipment. a very simple way of checking is to put on a leather or other strong glove, hold your hand over the end of the exhaust pipe and you should not be able to hold the pressure in for long.

AMurphy

OK, NOW I see what you are saying.
However, given Eu typically call the O2 sensors the Lambda Sensor, I thought you and the originator were refering to the Lambda (Sensor).

I'd have simply put it as "failing the emmissions test due to a leak" and indicating which components, HC, CO, CO2, NoX, etc were out of spec and by how much.

Anyway, you are then correct. A leak will mess up the readings for the ex gasses in the test machine as u describe....later.

JohnCleary

Hey guys, Thanks for the feedbank..

The lad in the NCT said I failed on the leak in the exhaust. He said as a result I failed on the Lambda, which read 1.033, 1.02 being the limit

He told me to fix the leak in the exhaust (mainly the one between flexi pipe and manifold) and then come back. I did the NCT 5 weeks ago, and the car was fine on emmissions (failed on track rod end) so I presume its the leak thats causing the problem

AMurphy

Based on the revised POV, yes, I'd agree it is the Leak that played a part in the failure. However, It still would be very nice of them to provide you with a list of statistics, if assuming you did have a leak somehwere, the balance of gasses would be upset, either you have too much Oxygen or too little of something else, hence driving the balance of gasses out of kilter for normal combustion.
Thus, "Lambda" as used would appear to be a determined or calculated Fuel/Air mix based on the analysis of the available or sampled gasses. It does not really indicate what you are high or low in, ie, HC, Co, CO2, NOX, etc.
Nonetheless to arrive at that F/A Lambda value, it must be using the concentrations of the sampled gasses against a theoritical "good" value.

So Given that Lambda >1 means excess air and you have leaks... I might expect the leak upstream of the CAT to leak out due to backpressure, but a leak downstream.... could go either way, depending on circumstance. You've heard of the Venturi effect I'm sure.
Anyway, I'd fix them both, were I you.

Actually, I fail to see why they even bothered to test it given it had leaks. Bit like why bother to test a car that has no operational lights?.

JohnCleary

AMurphy wrote:

You've heard of the Venturi effect I'm sure.
Anyway, I'd fix them both, were I you.

Actually, I fail to see why they even bothered to test it given it had leaks. Bit like why bother to test a car that has no operational lights?.

1) No, never heard of it
2) They'll test anything thats half road-worthy for the money.

How do they test the lambda reading? Does it be tested with the probe thats stuck in the exhaust? Or do they connect something to the engine bay and get a voltage reading? I Presume my results "1.033" was a voltage reading, the limit being 1.02 for my car?

AMurphy

Well I did write you a reply, but some settint in this site logged me out while in progress....which cause the reply to evaporate on "submit". We'er back to that sheeet again. used to happed long time ago, stopped happening, and now back again.
Therefore to save all that trouble again.

"Lambda" is the Air/Fuel ratio. and your figure of 1.3 is a ratio of your sample to the theoritical. Also called the "Excess Air Ratio".

Look up Venturi in Google.

Look up Lambda here;
http://www.forparts.com/BoswidebandO2.htm