Failed MOT CO emissions (1997, 2.5)

[Norbert]

Hi,

To my surprise, my car failed the MOT due to high CO emissions. I must say, this caught me off guard because the engine seems to be running smoothly with no noticeable issues. There's no excessive smoking, the fuel economy is good  and no audible leaks in the exhaust. Here is the report:

Natural Idle Test :

   CO (%): 0.359 (max 0.5) PASS

Fast Idle Test:

   CO (%): 0599 (max 0.3) FAIL

   HC (PPM): 94 (max 200) PASS

   Lambda: 1.01   (0.97 and 1.03) PASS

Notes:

   Engine speed measurement was bypassed.

   Engine oil temperature measurement was bypassed (Temp Gauge checked, assuming coolant).

I'm a bit puzzled by this result, and I was wondering if anyone else has encountered a similar situation. The lambda value does not point to any direction. I assume that the typical things can be ruled out like bad MAF, o2 sensors, fuel delivery issue, ignition issue since the air fual ratio is good. In my logic that leaves to culprits 1) bad cat, 2) cold engine when tested. 

Am I looking the right direction?

Edited December 19, 2023 by Norbert
unfinished post

[Norbert]

1) Bad catalytic converter: How can I test the cat? Wouldn't be the HC value also high if the cat failed?

2) Cold engine when tested: They tested 2 hours after I dropped the car. Could an italian tuneup (+high RON petrol and warm engine) halve the CO value?

Am I looking the right direction? Any support welcome,I am pretty disappointed considering how much love, time and energy this car I spent on my car.

Thank you!

[BBB]

Catalyst’s definitely work better when hot, so a cold car would not have helped. It’s unusual for a Porsche cat to fail, so I would go for your Italian tune up option and insist on a test when you’ve just arrived.

[K.I.T.T.]

As above, make sure the cats are hot.

Worth taking the car for a spirited run (using the whole rev range) en route to the station, straight for retest. 

IE if it's passed on everything else, get them to redo the emissions as soon as you rock up pre-warmed cats.

Cataclean / etc is also known to work wonders - make sure you follow the fuel / cataclean ratio on the bottle.

[Fat Rat]

As above. Loads of people do it. Me being one. A mate just had his 4 year old merc fail badly on emissions. MOT station said to put some cleaner in the fuel a bat it about a bit. He went to a merc garage and the mechanic said the same. He did this and flew through emissions. Test station said it was the best they had seen for a week. 😂🤙🏻

[½cwt]

I was running late form my last MOT so it was not fully warm.  Failed drastically on first emissions attempt. I they let me warm it up on site running it a around 5,000 rpm for 30 to 40 seconds to get it really warmed through rather than just idling before the test.  Then it flew through.

[jonogt6]

How are the clamps on the exhaust system. If you've got any leaks the emissions values will be thrown out.

[K.I.T.T.]

2 hours ago, jonogt6 said:

How are the clamps on the exhaust system. If you've got any leaks the emissions values will be thrown out.

That would affect lambda, but not (at least not significantly) HC / CO.

[Norbert]

Thanks a lot for the support! It's good to know i am not alone.

Retest is booked. This time I will replace the air filter, put some nice high octave fuel in it, bught some cat clean too. I will trash it before the test.

If it fails again, i think the cats are going to be the next to check. 

[Norbert]

1 hour ago, K.I.T.T. said:

That would affect lambda, but not (at least not significantly) HC / CO.

Yes, my lambda was spot on and the car drives amazing. Would a leak after the o2 sensor affect the lambda?  My backbox is not in the best condition. I am running stock backbox with questinable seals around (like every stock exhaust you can find). 

[Menoporsche]

42 minutes ago, Norbert said:

This time I will replace the air filter, put some nice high octave fuel in it

And go screaming around some B roads just before?

[Norbert]

There has been a new development. The engine is now losing power under heavy acceleration around 4k prm. No CEL. I open another post for this.

[Norbert]

Car just passed the emissions:
lambda: 1 (yes, bang on)

HC: 20 (max 200)

CO: 0.06 (max 0.3)

Surprisingly good results. I used cata clean, good petrol and trashed before the test but I can confidently say these were not the issues. The MOT tester told me today that I had a coolant leak during the previous test. My impression was that he didn't wanted to run the engine long becouse I was losing coolant fast. More than likley that the previous emission values were coming form a cold engine. Anyway, today the enegine was already hot before the test so it passed. He showed me where the coolant was coming form. It is  from the new expension tank I installed few months ago. Probably a loose clamp... fixable. I guess I never noticed becouse it was dripping on the exhaust manifold and cat. 

[½cwt]

Sounds like a good result.

Moral of the story, Italian Tune Ups immediately prior to MOTs are a good thing!!

Edited December 22, 2023 by ½cwt

[Norbert]

1 hour ago, ½cwt said:

Sounds like a good result.

Moral of the story, Italian Tune Ups immediately prior to MOTs are a good thing!!

Yes... it's something everyone will tell you but you only learn it when made the mistake... Anyway they found a leak i have been hunting for weeks so I am happy. 

[Rev Ken]

I always did an Italian tune-up and my 20 year old Boxster never failed. Now I've got a little electric car, when it comes to an MOT in three years time, should I still do an Italian tune-up before going for an MOT?

[½cwt]

3 hours ago, Rev Ken said:

I always did an Italian tune-up and my 20 year old Boxster never failed. Now I've got a little electric car, when it comes to an MOT in three years time, should I still do an Italian tune-up before going for an MOT?

You can if you wish, not sure it will make the difference between pass and fail though. 😉