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Exhaust Emissions Report

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#1 Killingtime



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Posted 16 January 2021 - 05:53 PM


My MK4 GT TDI failed it's emissions today. 185K miles.

I asked the tester on the type of failure and he wouldn't say, other than 'it was beyond manufacturer limits, and it the exhaust smelled really bad when he stood behind the car'.

I have to agree on the smell. It can be absolutely acrid on startup, but it's been like this for years and passed before. It's a similar smell to burning car tyres. No joke. Lingers for ages if the smell gets on your clothes. Gets slightly better after the engine has warmed up (10-20 miles). Others have reported this issue with high mileage VW TDIs. The car is still returning 50+ mpg, so the fueling can't be that much out.

So I asked for the emissions print out and it can be found below.



I don't understand what it's saying about emissions other than it's failed. Don't they monitor different gasses and particulates? These are normally listed. This will give me an idea of where to start fault finding.

I gave the car an short high RPM run the day before the test, to clear the soot from the exhaust and turbo. This normally does the trick, but not this time.

The car has been sat for about 9 months on a battery maintenance charger to stop it sulphating.

The next step is to run half a tank of fuel cleaner through the engine, and hand it over when the engine is hot (it wasn't during the test - just warm).

It would be nice to understand the emissions report though.


#2 Killingtime



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Posted 16 January 2021 - 06:58 PM

After a bit more research, the plated smoke limit on my car is 0.7 - it's on a sticker under the bonnet (I checked), and that matches the figure shown on the smoke test report.

So it's just a case of too much smoke when being revved. The report does say 'Smoke Test' and 'Smoke Reading'. A bit of a give away. I don't think any other gasses were measured.

I found some more information here:





I read something on a forum for MOT testers that they're meant to rev it to the limiter, but not all do out of respect for the engine. So this would explain why so many public transport busses are still on the road. It's not just a myth then that the same car could fail at one MOT station but pass at another on the same day with a different tester.


I'll run a bottle of diesel cleaner through it and report back after another smoke test with a hot engine.

#3 adam-


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Posted 16 January 2021 - 07:07 PM

Check for boost leaks first, this will cause an overboost.  When they're tested they're basically put to 100% throttle until the revs get to about 2500 and they'll let off.  So like a pump of the pedal.  


Any excess smoke is a fail really.  These shouldn't smoke once up to temp.

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#4 Killingtime



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Posted 21 January 2021 - 04:09 PM

Went for another test today and it failed again.


I added Wynns Stop Smoke to the oil and Wynns Clean Burn diesel treatment. Filled the tank with Shell Vpower diesel. Also changed the air filter and then took it out for a 50 mile run with some high revs. The car was run right up to the test so the engine was hot.


There is a lot less smoke on start now, and when idling it runs clean. No visible smoke at all when running or accelerating. I revved it a couple of times before the test as well. If you rev the engine reasonably slowly (2 seconds to 4500rpm) then there is no smoke.


I watched the guy do the emissions test. He floored it from idle 5 times untile the rpm hit about 4500 (guestimate from the engine sound), and waited about 10 seconds between each. The smoke at high rpm was almost nothing at first but worsened each time.


The latest emission report is worse than the 1st one....


It could be so many different things. Boost leaks, vacuum leaks, EGR choked up, air mass sensor, oil leak on the turbo...


Got it booked into another garage next week.

#5 Killingtime



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Posted 26 January 2021 - 02:08 PM

OK, I found the problem. Take a look at the EGR and inlet manifold below.




I'd say the inlet was down to about 1/5 of the original volume. That's 185k miles of fouling.


After cleaning both the EGR and manifold, and blanking the exhaust port to the EGR off, the car went back for an emissions test and passed.




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#6 northpole


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Posted 26 January 2021 - 04:06 PM

it is one of the problems with the egr on the diesels...  now you've blocked it the intake will stay clean and to be fair...  most diesels with a blocked egr will pass the MOT test even if you decat the car...  but have a dirty sooted up intake  and it will fail as it will inject enough fuel but it doesn't get enough air to burn = more particals and a reading that is way to high... also it is bad for your cat( if that is still on your car)

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