W124 MoT Emissions Failure

[S80]

Hi Guys,

Unfortunately, my '93 W124 280E (M104 engine) failed its MoT emissions test last weekend :-(

Results were:

Fast Idle Test

CO 2.301% (limit 0.300%) FAIL

HC 139ppm (limit 200ppm) PASS

Lambda 0.945 (limit 0.97 - 1.03) FAIL



Second Fast Idle Test

CO 2.208% (limit 0.300%) FAIL

HC 140ppm (limit 200ppm) PASS

Lambda 0.945 (limit 0.97 - 1.03) FAIL


Natural Idle Test

CO 2.748% (limit 0.500%) FAIL


Which I assume means the engine was running rich.


On getting home, a quick check underneath revealed a half-broken lambda sensor - "A-ha!" thought I

I must shamefully admit that I already had a spare £13 Ebay-sourced no-name 'universal' 4-wire zirconia sensor - so that was duly spliced in. The one removed looked like it too was not OE either - but that was done by a previous owner...

The car was driven for half a dozen 'trips' to give the self-adaptation mechanism a chance to 'settle down', and I then called in at a local garage (who did not do the original MoT) to do another emissions test.

I didn't get a print-out of the results because they very kindly did a quick test at no charge, but the CO was down to virtually zero, with a HC figure of 8 to 12 ppm.

The lambda value at natural idle was, however, about 1.08. I don't recall seeing the figure at fast idle because I realised this was being done very much as a favour.

This time, it looks like the engine is running too lean - at natural idle at least.

The mixture has clearly gone from too rich to too lean, so the £13 sensor is at least doing something!

My questions are:

Would an OE sensor give a lambda value at natural idle nearer to 1.00 (not actually part of the MoT test)?

Will things improve even with the cheapie lambda sensor if the self-adaptation is given some more time, over a variety of driving conditions?

Are these cheap 'universal' sensors just a waste of effort?

Do the results indicate that the catalytic converter may still be living?

It was convenient for me to get the original MoT done at a testing station I trust, but that is about 40 miles away (we've moved house recently) - so I'd like to be reasonably sure of passing the emissions before taking it back there. There were no other problems or even 'advisories' so I guess I owe it to the old girl to get her through for another year

 

[Ellsy Tanners]

To my knowledge, the limit for cars made before 1996, is 3.5% co. I haven't worked in a mot centre for 8 years however.

 

[kth286]

Good plugs of the correct type as supplied by Mercedes at around £2 each is essential, plus clean air filter and that new lambda sensor (I would be getting the Mercedes one (not cheap) but you will know it is correct.

You need to make sure the heater element is working on it with no feed wires to it being open circuit.

You also need to check if there are any diagnostic fault codes stored. For this, find an Indie that has proven experience of your generation of car.

 

[S80]

Thanks for the reply, David.

The plugs are Champion C10YCC non-resistor types, and together with the air filter are about 3,000 miles old. Maybe that's the problem - the car has only done that mileage in two years:-|

I did a code check with an 'LED Flasher' tool on pin 8 of the 16-pin socket, which had been showing a lambda sensor heating element fault. On fitting the new probe and clearing the code, no further faults have been seen (just the one flash).

I'll stick an oscilloscope on the lambda probe signal wire to check that the fuelling is running closed-loop. If it is, perhaps this means there's a small leak into the exhaust, post-lambda.

Cheers,
David

 

[pikeydave]

Always helps to give the car a ****** good thrash getting it nice and hot before taking it in for test.

 

[S80]

£13 Well spent!

Just a quick update: took the W124 for its re-test this morning and.... :razz:

Results were:

FAST IDLE TEST:

CO 0.005% (limit 0.2% funny, it was 0.3% last time!) PASS

HC 2ppm (limit 200ppm) PASS

Lambda 1.019 (limit 0.97 - 1.03) PASS

NATURAL IDLE TEST:

CO 0.006% (limit 0.3% - and that was 0.5% last time...) PASS


Pikey - I did indeed give the old girl a chance to explore the upper rev range on the way there (I think we both rather enjoyed it!), and filled the tank last night with Shell V-Power (may have helped, maybe not).

Couldn't help thinking on the way back what great cars these really are - I might even wash the darned thing

When the clocks go forward I hope to do a bit more therapeutic 'evening fettling'....!

 

[mikestrivens]

Good result.

 

[pikeydave]

Well done. Got to keep these straight 6s on the road!