MOT emissions failure

Posted: Mon Jul 02, 2012 9:56 am

Morning all, hopefully someone can offer some advice for this one.

Car failed the MOT on the CO content reading to high on both fast idle tests, though is below limit on natural idle. HC AND Lambda readings are ok, as below.

Fast idle test
CO 0.779 (max 0.30)
HC 46 (max 200)
Lambda 1.018 (0.970 - 1.030)

second fast idle test
CO 0.641
HC 48
Lambda 1.010

Natural idle test
CO 0.068 (max 0.50)

Only thing I can think is at fault is the cat itself, but this is only just under 2 years old, and have not experianced any running faults. Has anyone got any suggestions as I dont want to stat changing things unnecessarily

Regards Alan

Posted: Mon Jul 02, 2012 10:08 am

Was the car cold when you had the mot done? may require an air filter and oil filter and oil change then whacking some injector cleaner into a lowish tank of fuel and running up the dual carriageway/motorway for 30mins before getting it rechecked

Posted: Mon Jul 02, 2012 10:36 am

air filter and plugs had been recently changed, vehicle was hot as had a run up the bypass to make sure. Wife has been using the vehicle for the past few months and shes been doing long runs every day

Posted: Mon Jul 02, 2012 10:38 am

do an oil change and oil filter change and run some decent injector cleaner thru it mate could save the expense of a cat

Posted: Mon Jul 02, 2012 8:23 pm

any vacuum leaks will mess the emissions up aswell, lambda readings are a little off and i think this can mean its compensating for the incorrect air/fuel mix. i had a crack in my exhaust manifold whick gave me slightly high readings, similar to yours. i think alot of the manifolds have weak spots on the different engines. is the rest of the exhaust ok before the cat, no leaks. if the cat is only 2 years old, that has to be very unlikely to be the problem. what engine is it?

Posted: Tue Jul 03, 2012 9:07 am

its the ABK engine, have checked for any leaks and all appears ok, nothing evident from the manifolds/downpipes or breather/vac pipes

Posted: Tue Jul 03, 2012 10:11 am

did you get a new lambda sensor when you changed the cat? the hair line crack on my exhaust manifold was only visable when i took the air filter box off and had i enough room to look around from above, i dont know if it a weak spot on the abk's, my crack is on top near 3 and 4 cylinder outlets. does it use alot of engine oil?

Posted: Tue Jul 03, 2012 11:44 am

Head was overhauled last year as valve stem seals were leaking, has only been topped up with half litre of oil in that time and must of done over 8000 miles since.

Old lamdba sensor was used, but as said, no running issues or fuel consumption issues have been noted

Posted: Tue Jul 03, 2012 7:34 pm

well i'm starting to run out of ideas, seems like you look after your car. do you have a diagnostic lead? lambda can still work but very slow to react so it will not do its job as well as it should. i know the crankcase breather pressure valve that sits on top of the engine cam cover does fail with age, but i dont know how this would effect the emissions? i would try running some injector cleaner through it and change the oil, take it for a good blast, get it hot then run at higher revs for a minute down a motorway or a long carriageway, IF YOU KNOW ITS THE ENGINES NOT GOING TO BLOW UP!! doing this just before the retest might help but as you have been taking on long runs and it was hot before the last test may not be worth pushing the car.
i would have a real good look around for any vacuum / intake / exhaust leaks aswell
what speed is your idle? is the idle steady? it should idle at 820rpm+-50, if not it may show that something is wrong.

Posted: Tue Jul 03, 2012 9:40 pm

how were the lambda readings taken? there's a "pre-heat" on them, I think? Could still possibly be a lambda issue. Have you had the timing checked?

Posted: Wed Jul 04, 2012 11:44 am

Going to change the oil and filter tonight, and try some injector cleaner as well, and also clean out the breather pipes juust in case.

The lambda reading was taken at the tail pipe, should be a pre heated sensor as theres 3 wires to it. Distributer timings never been checked but would seem ok as no running issues, valve timings spot on as all set with new belt and tensioners when head overhauled

Posted: Wed Jul 04, 2012 4:06 pm

good luck with the retest, let us know how you got on and if you found anything wrong. my lambda seemed ok and i hadn't noticed any issues but only when i got a diagnostic lead, although it didn't show a fault it was working very slowly, was i able to have a good look and see how all the sensors where working and sort out a few issues.

Posted: Thu Jul 05, 2012 9:25 am

Changed oil and filter last night, old oil did have a smell of fuel to it so may be being drawn through breathers so fingers crossed thats rectified it and it passes on Saturday.

However, the cooling fans are now coming on as soon as the ignitions turned on, so suspect switch failure. Does anyone know if this has a timer unit at all or if its just from the rad switch?

Posted: Thu Jul 05, 2012 9:40 am

Its the first thing i do before putting any older car through for an mot is change the oil and filters, but run some injector cleaner in about 1/4 of a tank of fuel thru it, and give it a good hard run before taking back in for the emmissions test

Posted: Thu Jul 05, 2012 6:34 pm

the fan thermoswitch is mounted on the bottom of the rad on the drivers side, if you also have a warning light then the temp sender/overheat sensor could have failed, this is the grey sensor on the coolant flange that is on the passenger side of the engine that joins to the top rad pipe. you can unplug the rad thermoswitch and this will show whats at fault

Posted: Sat Jul 07, 2012 8:07 am

Passed this morning, advisory on 2 rear tyres low, not bad at 18 years old, just need to sort fans now, thanks to all for advice

Posted: Mon Jul 16, 2012 7:21 pm

If the valve stems were leaking oil after you had the new cat you've probarbly knackered it.
Mine had a top end overhaul due to the stems. still struggled to pass MOT until i got a new cat welded in.

Best thing to do is to get a 50quid bosal cat job, think mine was bosail and find an ehaust place willing to weld it in for about 20quid. Sorted Very Happy

http://www.catalogue.bosal.com/pages/exh_system_list.php?query_nr=2&make=audi&model=coupe

I had to get him to modify the v6 cat to fit on mine Very Happy

Was a few year back now mind you but HTH