Failed MOT

[jjsnowy]

So,after failing the first MOT,on emmissions,and a couple of other small issues,a new LAMBDA sensor has been fitted(the reading for this at the emmissions test was sky high).
The old one just fell off when the floor pan was removed,so it was definitly faulty. New one in,gave the car a good run,had a look at the rest of the system-manifold,flexi pipe,cat,back box,seemed fine,and i failed my mot again,same problem...anyone had any similar problems?
I dont want to start replacing bits until i know where the problem is.

Any help is hugely appreciated.

[campbell]

Is your engine standard? Or tuned / improved in any way?

[Rich H]

If the lambda has been fooked for a while it could be the cat isn't very well.

Rough loving usually sorts them but it might be too late for that....

Did the lambda make a difference at all?

[robin]

Does the idle speed return to around 800 RPM once the engine is warmed up?

If you leave the engine running for long enough (perhaps you need to hold 2000 RPM in neutral for a while) does the temperature climb on the display? It should climb to 100 +/- 5 degrees then the fan should cut in. Does the fan cut in?

I assume that the car does not have any after market fuel pressure regulators fitted?

Is the vacuum pipe from the fuel regulator connected to the manifold?

Assuming it is NOT a VVC, is the black/white stripy rubber hose connected from the inlet manifold to the ECU, via the vapour trap that is fitted to the clam near where the bootlid crosses the boot wall on the driver's side?

Check this hose is not split (if you pop the hose off the manifold and suck on it you should feel it hold a vacuum - be prepared for an oily/petrol flava in the mouth for a while ).

Does the engine drive well under load - can you get to the rev limiter in 2nd gear (once engine warm, obviously, and somewhere where it is legal and safe to do so - 60mph ish)?

If the answers to all the above is yes then I would take it to somewhere you trust that has the rover/MGF/lotus MEMS ECU reader (this is pre-OBDII so not the cheap ebay kit, but most independent garages should have rover MEMS readers I think).

Get them to do a live sensor reading test - that will show that the ECU sees sane values from all sensors; don't bother taking it anywhere where they don't have a working reader - that's just a waste of time/money.

A failed cat won't give huge emissions readings, though it will obviously fail the test.

Cheers,
Robin

[jjsnowy]

The engine is completely standard.

The temp gauge rises to 1oo,then drops when the fan starts up,as usual.

I will check the hose to the ECU,and the vacuum pipe.

I had it on a diagnostic,but it was a snap-on version,could only find the standard 1.8 k-series rover engine in its menu...
all readings moved as expected,except ambient air temp,which stayed the same throughout,i dont think it should have done that.
This could mean its running slightly rich,which would confuse an emissions machine.
The new LAMBDA sensor brought the reading down slightly,but its still off the scale.

Iv not tried to get to the rev limiter in second,but i have in third,and its fine,even power,no problems.
SO,check hose and vacuum,and find an ecu reader nearby.

Thanks for the suggestions,i think i answered everything.

Jay.

[robin]

The snapon one is probably already OK - the engine is rover 1.8K - no difference at all to the install in MGF and various similar cars.

The ambient temperature won't change as it doesn't change, if you see what I mean ... did it look like it was reading about the right temperature for the day you tested it on (I would guess somewhere between 0 and 20 ambient air temperature pretty much anywhere in Scotland over the last week).

If the idle is smooth and you can reach the rev limiter easily then it's probably not a misfire causing it (misfires dump unburnt fuel into the exhaust which then partially burn with the next load and create a lot of HC and CO content).

I'm guessing Rich is correct ... the cat may well be shagged.

With the lamda replaced you should put it back on the snapon (or other) machine and check that the lambda sensor is now toggling between rich/lean/rich/lean which is what it should do when running closed loop (warmed up on the idle, for example).

As a matter of interest do you have the actual numbers off the emissions fail report?

Cheers,
Robin

[jjsnowy]

I will stick it back on the snap-on set,the lambda should be working.
The car did have a bit of a pop before,which,after the work,is now gone,it doesnt seem to miss-fire at all.
As for the readings,Co was around 350-the acceptable amount for that is 200,and the lambda was around 2000,the limit being 1050.
When the Co goes down,the lambda shot up,every time.

[Gourlay83]

1 lambda is 14.7 afr. The higher the reading the leaner the mixture. I make 2.0 lambda reading at almost 30-1 afr are you sure that's correct?

Check your ecu is reading the lambda correctly, as Robin said. If its ok, chances are your Cat is goosed.

Check your failure slip again, and let us know the readings.

Alan

[robin]

The pop pre-lambda replacement was probably air being drawn into the exhaust through what was left of the original lambda and causing any unburnt fuel to ignite in the cat or the back box.

If the lambda is really 2.000 then Alan is right, it means you have 2x the AFR than you're supposed to have. Given that AFR is Air to Fuel Ratio it means you have twice as much oxygen for the fuel that's being burned. This is bad for your engine (if it's true).

It's hard to see how you could be producing such a lean stream with such a high CO content (CO is normally produced when there's not enough O2 to produce CO2 instead; even then the cat will try hard to oxidize CO to CO2 by using either stored O2 or O2 liberated from NOx reductions).

Is it possible that the gas isn't flowing out of both tail pipes and they're sticking the probe into the stalled pipe? Or perhaps that there is an air hole in the back box or one of the tail pipes?

If you get the car idling and warmed up, get some rags and stuff them into the tail pipes and hold them in with your boot or a well gloved hand. If there's an air leak in the system you'll soon hear it hiss ...

If there is no exhaust leak then your engine is running really lean ... look for air intake leaks (sometimes you can find these with a fine mist water spray - you'll see the swirl of the water droplets head towards the air intake leak) ... and your cat is failing to use the spare O2 to oxidize CO to CO2 so you probably also need a new cat.

You might be able to borrow a cat from somebody on the list as some people have them for MOT use only on their, err, non-road race cars That would allow you to eliminate the cat as the problem without splashing out on a new cat.

Cheers,
Robin

[Rich H]

Good point on the twin tail pipes....
Alternatively find a grarage who are a bit more 'understanding' but don't forget the biscuits for the tester's guide dog....

[jjsnowy]

Okeydoke, i will try those options.
Unfortunatley,its a single peice back box,not the standard twin pipe,so there is only one place for the probe to go.

Thanks again.