W124 just failed MOT on CO emissions...

MercStraight6

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1995 W124 E280 estate
I took the E280 (140k miles) in for it's MOT today, it wasn't too bad and only failed on 2 things:

-Excessive play in nearside front wheel bearing
-Exhaust CO content after 2nd fast idle excessive

I presume the wheel bearing is a simple matter of replacement, but I'm puzzled by the emissions.

The figures were:

Fast Idle #1 (2397rpm):

CO - 0.64%, limit 0.30 FAIL
Lambda - 1.02 PASS
HC Level - 132ppm, limit 200 PASS

Fast Idle #2 (2190 rpm):

CO - 0.53% FAIL
Lambda - 1.01 PASS
HC Level - 117ppm PASS

Natural Idle (694rpm):

CO - 0.17% PASS

The garage said the catalytic converter was probably worn out, or that the fuel system may just need adjustment.

I looked through the old MOTs and found that it has failed on CO before in 2007, but later passed without any explanation of what was done.

The car has been recently serviced and runs very well, it should have been warm for the test aswell because I had taken it for a fast 10 mile drive on the motorway.

I don't really want to replace the cat and I'm hoping it's something simple.
 

turbopete

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ive come across this a few times and theres 2 alternatives id try before changing the cat. first id try an italian tune up (thrash it for a couple of miles) and if that fails, have the lambda sensor (o2 sensor) checked for operation as they usually stop switching correctly
 
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MercStraight6

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ive come across this a few times and theres 2 alternatives id try before changing the cat. first id try an italian tune up (thrash it for a couple of miles) and if that fails, have the lambda sensor (o2 sensor) checked for operation as they usually stop switching correctly

Funnily enough, I had the Lambda sensor changed in April this year because it was very reluctant to rev and drove quite sluggishly

The new Lambda was a cheap (but brand new) universal one from ebay, but it fixed the problem and now it revs freely and drives well.

Do you think a genuine Bosch sensor might help?

Otherwise I might just try the Italian tune up; It's done a lot of city driving lately, rarely going much above 2200rpm or 30mph.
 

47p2

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Put a bottle of Forte petrol treatment into the tank before giving it an Italian tune-up, it works wonders
 

turbopete

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Funnily enough, I had the Lambda sensor changed in April this year because it was very reluctant to rev and drove quite sluggishly

The new Lambda was a cheap (but brand new) universal one from ebay, but it fixed the problem and now it revs freely and drives well.

Do you think a genuine Bosch sensor might help?

Otherwise I might just try the Italian tune up; It's done a lot of city driving lately, rarely going much above 2200rpm or 30mph.

it may be better with a bosch sensor. some universal ones are ok, but i remember they gave many problems on early 16valve fords by failing within a few months. and as has been said, a forte fuel treatment may help.
 

Alex Crow

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right, it looks like a cat fault. as pete says, a really hard drive may help.

the info you have shows clearly that the lambda sensor was working fine during the test, the values being bang in the middle of the range. also HC is at a good level suggesting that the engine is in fair mechanical condition.
 

television

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I used to have to lock my 124 300TE in 2nd and take it for a fast run to get it through the test when it was 2 years old
 
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MercStraight6

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Thanks for the help; am I right in thinking the front Wheel bearing should be fairly simple / cheap?
 

television

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Thanks for the help; am I right in thinking the front Wheel bearing should be fairly simple / cheap?

You can do this your self, it only needs tightening up a touch.

Remove the end cap, undo the lock nut and while spinning the wheel tighten the nut till the wheel slows, the back off a quarter turn and you will be fine.

I always spin the wheel when doing this so the the rollers do not dent the cage that can lead to bearing failure
 

kth286

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also consider the condition of the spark plugs if old.

proper ones from merc are only 2 pounds each BUT be very careful when changing if wring loom old.
 
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MercStraight6

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also consider the condition of the spark plugs if old.

proper ones from merc are only 2 pounds each BUT be very careful when changing if wring loom old.

Genuine Mercedes / Bosch ones were fitted this April, about 3000 miles ago.
 
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MercStraight6

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You can do this your self, it only needs tightening up a touch.

Remove the end cap, undo the lock nut and while spinning the wheel tighten the nut till the wheel slows, the back off a quarter turn and you will be fine.

I always spin the wheel when doing this so the the rollers do not dent the cage that can lead to bearing failure

Seems strange that they mention the wheel bearing; I usually thought that they made a noise when they had worn out, mine seems to be completely silent.
 

turbopete

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usually play can be felt long before they get noisy. when they are noisy, they are worn beyond adjustment as a rule and therefore get replaced. if they get noisy and DO adjust up ok, they should NOT fail the mot (or that was the case when i last worked in a garage at least)
 


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