MOT failure why?

[phil993]

My car just failed its MOT on Lamda very slightly over the limit. Car is Sept 03 996tt with X50, Milltek exhaust and cats, and previous owner appears to have had a remap but I do not know who did it. The OPC who did the MOT suggested it was due to slight exhaust leak between the 2 silencer boxes as this was the only thing they could find. no faults at all on the diagnostics which also did not pick up non standard map. Went to my independent who does everything I need bar the basic service stamp and he also found Lamda slightly out of range and no faults showing up. He said the exhaust leak was not an issue - is very slight.

Has anyone had any experience of this?

If it is remap any suggestions as to who will check it without charging me £2k for their own remap? (eg DMS, Revo etc) Don't want to put it back to standard anyway as the power is addictive....

 

[sawood12]

A local (non Porsche) mechanic i've used for years and trust has a good trick for cars who's emissions are just slightly out of limits. He will instruct the driver to take the car out to a peice of road that he knows well and give the car a jolly good thrashing (within the allowable speed limits of course). The problem is that sometimes the Lambda sensor can get sooted up especially on cars that who's rev counters don't see the second half of the rev counter very often and as a result give dodgy readings - sometimes high, sometimes low. In his experience this has failed rarely and he has only rarely had to replace Lambda sensors. If the Lambda sensor is truly malfunctioning then you'll get a warning in your dash.

The exhaust leak wont have an effect on this as the thing with exhaust leaks is that you tend to get exhaust gasses leaking out of the exhaust rather than air leaking into the exhaust, therefore the lambda sensor should still be seeing pure exhaust gasses and not a mixture of exhaust gasses and air drawn into the system. However on a 996TT if the exhaust leak is before the turbo's you'll be getting more lag so is worth fixing!

 

[NIL98]

For the sake of £40 I'd go somewhere else for a re-test. MOT's are all far too subjective.

Regards
Neil

 

[David M]

Did you get them to try both tail pipes?

Don't know about other remaps but if it is a Revo then you can change the map or even remove it yourself with the Select Plus for Porsche serial port switch. I have one of these so if you do take the car to an OPC you can reset the car to stock so they can't see the higher boost etc. If you are local you are welcome to try it.

I would also echo the above.....go for a long hard drive before the MOT and get them to do the Lamda test first. A very hot cat will help with the test.

Have you spoken to the previous owner?

 

[dparslow]

A leak on the exhaust system will contribute to a high lambda reading as can a lazy or faulty lambda sensor. A diagnostic doesn`t always show a faulty lambda unless its completely failed, you need to see live data for the full picture of how it is functioning.

As already mentioned re-seal the exhaust and a lengthy "spirited" roadtest is the first thing to do, then submit for a re-test while the exhaust is still creaking with the heat..

Mapping mods can produce a higher CO and HC on the emission test due to the higher fueling requirements when running higher boost levels, they don`t generally effect lambda readings that much.

Mot`s and diagnostics are part of my business and generally Porsches, even old ones like my own have extremely low emissions. It`s usually the chav chariots that fail miserably when they are chipped for ridiculously high boost levels, the cat has been removed or they are modded very poorly.

 

[phil993]

Thanks for responses. I have tried the hot retest and still failed on Lambda although other readings well within range. I will sort the very minor leak but so small I cant believe its the culprit. As you say, Dave, perhaps a Lambda sensor is dodgy but not failed fully so not showing on diagnostics?

Am I correct in thinking that a high Lambda reading may indicate running rich? The inside of the exhaust tips do seem quite sooty......

Is it a big job to replace the sensors in case one or more are on the way out?

 

[dparslow]

Phil, the fact that the other readings CO and HC are within limits would suggest that there is efficient combustion and the cats are working ok. If CO and HC are very low, close to zero on fast idle is the norm for our cars. I still suspect the exhaust leak could be your problem rather than anything else.

As a comparison the 968 and 993 manage 0 on CO and between 4 and 8 HC readings on the fast idle test, without all the electronic complexity of the later cars.