Fixed emissions MOT failure

124coupe

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Hi all, might be useful to someone else.....


My E320 coupe failed the MOT on a broken front spring and on emissions :shock:.

The car passed at idle.

At 2,500rpm it passed the calculated lambda and HC tests but failed on CO - about .5% where the limit is .3%.

Came as a shock as the car drives very well and idles (almost) perfectly....

"You need a new cat mate" - hmmmmm.....seen too many new cats that "fix" this kind of problem, only to fail in a year or so because the underlying cause is still there.


Back to basics:

High CO means incomplete combustion at the tailpipe.

Cat needs sufficient oxygen to combust HC and CO (the reason why O2 sensors are used to rapidly swing the mixture and so supply the needed O2).

Working cat will be hotter at the tail end than at the front end because of the heat generated by combusting the HC and CO.


Tests:

Quick infra-red check of the cat - its NOT much hotter at the back than the front (so could be dead/dying) OR could be short on O2 to combust the pollutants.

So, its an M104/HFM car - what influences mixture?

O2 sensor measured - cycling correctly and cycles speed up at higher revs (not broken!).

MAF cleaned (it was clean already) and then measured - correct voltages and quick voltage rise with a throttle blip (not broken!).

Intake air temp sensor - correct resistance at 20C (not broken)

Coolant temp sensor - correct resistance (pins 1 and 4) at 20C and 80C (not broken)

Car "knows" when its in Park and when the A/C clutch kicks in so those parameters should be OK.


Careful listen to the idle at the tailpipe - its hunting a bit - weak mixture?


Could be injector(s) or could be an air leak after the MAF (unmetered air).
(As a side-note, I wish I had access to an HHT to be able to see live date from this engine.....).

Taped a drinking straw to my soldering torch and start (carefully) feeding the inlet manifold with propanefrom the MAF forwards...

Found a leak! Where the inlet cross-over pipe sits on the cylinder had cover there are 2 crankase vent connections sealed by rubber bungs - no2 admits the propane and causes a change in idle. Whipped off, cleaned up and replaced - leak gone.

Found a second leak! Under the inlet manifold are 2 rubber vent pipes - one for the main crankcase vent and one that links upper and lower manifold. The PCV one has hardened and cracked.

Used some self-amalgamating tape as a botch/temp repair while new one on order 3 or 4 days....(why is nothing in stock any more??)


Idle now smooth so looks like that might have done it....CAT now 100C hotter at the back than before!! (not a scientific test at the conditions were not the same as the first test).


Re-test. CO now .11% - total cost less than £5......and no unnecessary CAT (a fresh one would probably have been able to deal with the CO but I would rather fix the issue).

Sometimes this is fun......
 

Rory

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I couldn't in a million years imagine an MB dealer solving that!
 

pomm001

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Hi all, might be useful to someone else.....


My E320 coupe failed the MOT on a broken front spring and on emissions :shock:.

The car passed at idle.

At 2,500rpm it passed the calculated lambda and HC tests but failed on CO - about .5% where the limit is .3%.

Came as a shock as the car drives very well and idles (almost) perfectly....

"You need a new cat mate" - hmmmmm.....seen too many new cats that "fix" this kind of problem, only to fail in a year or so because the underlying cause is still there.


Back to basics:

High CO means incomplete combustion at the tailpipe.

Cat needs sufficient oxygen to combust HC and CO (the reason why O2 sensors are used to rapidly swing the mixture and so supply the needed O2).

Working cat will be hotter at the tail end than at the front end because of the heat generated by combusting the HC and CO.


Tests:

Quick infra-red check of the cat - its NOT much hotter at the back than the front (so could be dead/dying) OR could be short on O2 to combust the pollutants.

So, its an M104/HFM car - what influences mixture?

O2 sensor measured - cycling correctly and cycles speed up at higher revs (not broken!).

MAF cleaned (it was clean already) and then measured - correct voltages and quick voltage rise with a throttle blip (not broken!).

Intake air temp sensor - correct resistance at 20C (not broken)

Coolant temp sensor - correct resistance (pins 1 and 4) at 20C and 80C (not broken)

Car "knows" when its in Park and when the A/C clutch kicks in so those parameters should be OK.


Careful listen to the idle at the tailpipe - its hunting a bit - weak mixture?


Could be injector(s) or could be an air leak after the MAF (unmetered air).
(As a side-note, I wish I had access to an HHT to be able to see live date from this engine.....).

Taped a drinking straw to my soldering torch and start (carefully) feeding the inlet manifold with propanefrom the MAF forwards...

Found a leak! Where the inlet cross-over pipe sits on the cylinder had cover there are 2 crankase vent connections sealed by rubber bungs - no2 admits the propane and causes a change in idle. Whipped off, cleaned up and replaced - leak gone.

Found a second leak! Under the inlet manifold are 2 rubber vent pipes - one for the main crankcase vent and one that links upper and lower manifold. The PCV one has hardened and cracked.

Used some self-amalgamating tape as a botch/temp repair while new one on order 3 or 4 days....(why is nothing in stock any more??)


Idle now smooth so looks like that might have done it....CAT now 100C hotter at the back than before!! (not a scientific test at the conditions were not the same as the first test).


Re-test. CO now .11% - total cost less than £5......and no unnecessary CAT (a fresh one would probably have been able to deal with the CO but I would rather fix the issue).

Sometimes this is fun......

Well done !
I read with interest , but not to sure about the propane
i guess this is similar to using 'easy start' spray it around any joints when you find the leak the engine sucks it in and engine note changes ?
I would have never have thought of that !
 
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124coupe

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Well done !
I read with interest , but not to sure about the propane
i guess this is similar to using 'easy start' spray it around any joints when you find the leak the engine sucks it in and engine note changes ?
I would have never have thought of that !

Yes - it just works as extra fuel so you can see the effect in a change in idle (or by having a meter on the O2 sensor lead at the same time).

Using the straw means its accurately aimed to help if its a gasket leak (my M104 has 6 separate gaskets on the inlet manifold - sort of like big o-rings).

Talking of easy start - this one makes me chuckle http://www.nulon.com.au/products.php?productName=Start_Ya_Bastard_Instant_Engine_Starter
 

turbopete

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Well done !
I read with interest , but not to sure about the propane
i guess this is similar to using 'easy start' spray it around any joints when you find the leak the engine sucks it in and engine note changes ?
I would have never have thought of that !

wd40 also works for this!
 
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124coupe

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wd40 also works for this!

Yes, WD40/carb cleaner/contact cleaner all work - I just like the way a small gas flow is controllable and doesn't leave a mess behind.

Just make sure not to use anything that doesn't list ingredients in case there's silicone in there (fatal to O2 sensors).
 


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