Could exhaust leak cause increased MOT fast idle CO reading

[AlVal]

Skoda Superb petrol 2.0 tsi 280bhp.

remapped stage 1 by superchips. blew a turbo, turbo spat some stuff down into the downpipe/cat, so had to replace downpipe/cat when replacing turbo, went for a scorpion sports cat downpipe.

passed a couple of mots since then.

car has been lumpy at idle for a long time, always blamed it on the remap etc, but been throwing more misfire codes and recently a camshaft position code.

failed mot on CO emissions (0.9), given the fault codes they said take it to a specialist like vagtech, its most likely all the timing issues causing the emissions problems

vagtech had to change the timing chain , tensioners and a load of other related timing chain stuff, £2k bill.

car now idles buttery smooth, which is nice.

back in for mot retest, still failing, now 0.7 CO (needs to be below 0.3)

so I thought maybe at this time of year it's just that the garage didnt heat it up enough, so booked another mot.

did some reading on catalytic converter cleaning , and additives like cataclean and other fuel system additives to help clean out the cat.
turns out they're just a mixture of isopropyl, acetone and a little bit of parrafin.

So did some research, I threw a litre of acetone, a litre of isopropyl, and 60ml of parrafin mixed up into a tank with about 40 litres in it.

drove around a lot to run that through the system as quick as i could so as not to leave the acetone solvent hanging around the various plastic and rubber parts of fuel pumps etc too long, and got it back in for mot, this time making sure the garage were ready for me coming in hot.

brought the thing in blazing hot and now emissions are down to 0.41, but still need to get below 0.3

there's a small exhaust leak between the front downpipe/cat and the centre section. MOT garage says that could be affecting the test results.

here's my question: how could fixing a small exhaust leak there reduce CO emissions when they put their testing probe into the tailpipe? surely all that fixing the leak is going to do is send even more CO down to the tailpipes where they test?

[cuprabob]

The exhaust leak could be causing the pre-cat sensors to under read the fuel / air ratio and as a consequence the ECU will increase the fuel / air ratio to try and correct.

Edited by cuprabob on Monday 18th November 14:09

[AlVal]

cuprabob said:

The exhaust leak could be causing the pre-cat sensors to under read the fuel / air ratio and as a consequence the ECU will increase the fuel / air ratio to try and correct.

Edited by cuprabob on Monday 18th November 14:09

Thanks a lot for the reply. I just dont understand how a pre - cat sensor could know anything about what's happening if the leak is further down the system past that sensor?

[fbwinston]

You have an exhaust leak, I'd get it fixed anyway then re-test. If it passes you have your answer.

[stevieturbo]

post the full results.

But if there was an exhaust leak, it should be an automatic fail regardless.

[cuprabob]

AlVal said:

Thanks a lot for the reply. I just dont understand how a pre - cat sensor could know anything about what's happening if the leak is further down the system past that sensor?

Sorry, I misread your post, I read it as the leak was before the cat but you actually said it's after the cat and before the centre silencer, in which case I don't understand either.

[GreenV8S]

You need all four gas readings to diagnose it properly. Excess CO suggests incomplete combustion, which can be caused by it running slightly rich, which can be caused by a small exhaust leak upstream of the sensor. The fact you have found a leak downstream doesn't mean there isn't another upstream.

This is the sort of problem that can eventualy damage the cat and could be masked temporarily by replacing the cat - but unless your resolve the underlying problem this is just a very expensive temporary fix.

Before you throw any more money at it, get the full gas report with all four readings and make sure you fully understand the problem. If you diagnosis concludes that you have an exhaust leak, a smoke machine is a good way to find it.

[AlVal]

stevieturbo said:

post the full results.

But if there was an exhaust leak, it should be an automatic fail regardless.

nope. advisory

"exhaust has a minor leak of exhaust gases [6.1.2(a)]"

[AlVal]

GreenV8S said:

You need all four gas readings to diagnose it properly. Excess CO suggests incomplete combustion, which can be caused by it running slightly rich, which can be caused by a small exhaust leak upstream of the sensor. The fact you have found a leak downstream doesn't mean there isn't another upstream.

This is the sort of problem that can eventualy damage the cat and could be masked temporarily by replacing the cat - but unless your resolve the underlying problem this is just a very expensive temporary fix.

Before you throw any more money at it, get the full gas report with all four readings and make sure you fully understand the problem. If you diagnosis concludes that you have an exhaust leak, a smoke machine is a good way to find it.

I have the print from the first test (before i had timing chain replaced)

https://i.imgur.com/FU5GGsK.jpeg

after timing chain the co went down from 0.9 to 0.7, and after i threw my homebrew fuel system/cat cleaner in there and testing it blazing hot, got it down to 0.41

[GreenV8S]

AlVal said:

I have the print from the first test (before i had timing chain replaced)

Can you get the current figures? Presumably it's the current state you want to get diagnosed.

[AlVal]

They only told me the CO, didn't get the print of all, but I'm sure lambda was close to 1. Have another test Wednesday, I'm going to clean the air filter as best I can, try and figure out how to make sure there's no leaks around the air intake side, and consider whether I use my vag diag software (I have odis-e) and flash my stage 1 remap back to factory to get it through mot. I wouldn't be able to flash it back to remapped though so I'd lose my remap.

I'm kicking myself because while warming it up I put it in sport mode which they probably left it in, I'm not sure if that affects fuelling at all. I'd like to think it just changes rev limit at which gear changes happen

Edited by AlVal on Monday 18th November 18:28

[stevieturbo]

AlVal said:

They only told me the CO, didn't get the print of all, but I'm sure lambda was close to 1. Have another test Wednesday, I'm going to clean the air filter as best I can, try and figure out how to make sure there's no leaks around the air intake side, and consider whether I use my vag diag software (I have odis-e) and flash my stage 1 remap back to factory to get it through mot. I wouldn't be able to flash it back to remapped though so I'd lose my remap.

I'm kicking myself because while warming it up I put it in sport mode which they probably left it in, I'm not sure if that affects fuelling at all. I'd like to think it just changes rev limit at which gear changes happen

Any fail sheet should give the full test result. Giving you no information on the fail is unlikely, and wrong.

Without the info, it is nigh on impossible to diagnose other than just random guesses

And different ecu modes should not affect idle at all.

[AlVal]

stevieturbo said:

And different ecu modes should not affect idle at all.

Thanks, I do know it's the fast idle it's failing on though

[stevemcs]

Fix the exhaust leak, even if you bodge it (exhaust bandage) and see if it goes through if it’s a sports cat then you have to get them stinking hot.

[stevieturbo]

AlVal said:

Thanks, I do know it's the fast idle it's failing on though

the full results sheet which they should give you, will offer vital info needed

[mk2 24v]

Most likely not got hot enough for the sports cat to work.

They are usually a swine to get up to temp for emissions testing, especially at this cold time of year

[AlVal]

Latest test :



I stressed to them : after fixing exhaust leak, make sure it's blazing hot before testing.

Of course When I come to collect it:
"so minimum is 80 deg, we got it all the way up to 86" I'm thinking ffs I brought it in here at 103 deg and phoned you in advance to have the emissions test ready I'm coming in hot.

Also as I expected, fixing exhaust leak a total waste of time. Was minor on mot, and all it's going to do is make sure more crappy gas makes it to the tailpipe where they test, since leak was in the midsection after all sensors.

I think next step will have to be:

Option 1 taking the downpipe with cat out to clean it manually, which I bet will be a bh to get at.

Option 2 maybe there's a way to pull the pre-cat lambda sensor and use the hole to spray some sort of cleaning stuff down into cat

Option 3 I have the tools to put the ecu remap back to stock, although I wouldn't be able to put the remap back on after

Option 4. Dodgy garage mot. Not my preferred way really

Option 5. Replace downpipe with cat. Ouch on cost.

Option 6 take a rubber hammer and start tapping the cat with it to loosen out any carbon buildup, but could risk the ceramic inside cracking.

Option 7 maybe pull spark plugs and learn how to tell if they're still optimal (only changed 7 months ago)

Option 8 learn how to look for air intake leaks (things seem OK in as far as the air box and pipe from it don't show any cracking

Options I haven't thought of?

Oil was changed days ago with timing chain. Paper filter 7 months old and still looks fairly like new. Hoovered the dirty side just for the heck of it, but was nothing really there anyway.

Edited by AlVal on Wednesday 20th November 22:12

[E-bmw]

There almost certainly will be no carbon build up in the cat.

Sports cats & cheap ceramic cats invariably only last a year or 2, if you genuinely believe the cat to be the issue get a metallic substrate cat.

[Rockets7]

E-bmw said:

There almost certainly will be no carbon build up in the cat.

Sports cats & cheap ceramic cats invariably only last a year or 2, if you genuinely believe the cat to be the issue get a metallic substrate cat.

This ……

[AlVal]

E-bmw said:

There almost certainly will be no carbon build up in the cat.

Sports cats & cheap ceramic cats invariably only last a year or 2, if you genuinely believe the cat to be the issue get a metallic substrate cat.

Thanks for the response. I wonder what about them degrades then?